Neutron Stars in Globular Clusters

a r X i v :a s t r o -p h /0012548v 1 29 D e c 2000A&A manuscript no.(will be inserted by hand later)1.Validity of General Relativity General Relativity (GR)is the correct description of gravity and space-time.The phe-nomena verified with three classic tests of GR are so well established that they are now used as tools in every-day astronomical practice and even in technological applications.The gravitational bending of light,famously detected in Eddington’s solar eclipse expedition is today used to determine the stellar content of our Galaxy and the Mag-ellanic Clouds (from stellar micro-lensing events detected by the OGLE,MACHO and EROS experiments).Lensing of distant galaxies by intervening galaxy clusters is used to determine the (dark)matter distribution in the latter.Gravitational redshift,first observed in spectra of the white dwarf Sirius B in 1925,has since been detected in the laboratory (Pound-Rebka experiment)and is now of necessity taken into account in surveying practice (the GPS system).The effect is also essential in timing radio pulsars—when compared to some millisecond pulsars,terrestrial clocks clearly run slower at full moon than at new moon.The magnitude of precession of the perihelion of Mercury is dwarfed by the same effect in the Hulse-Taylor pulsar,where the periastron shifts by 4.2◦per year.A similar system,Wolszczan’s binary pulsar,allows a confirmation of the Shapiro delay.Of course,GR also provides the framework for understanding the evolution of our expanding Universe.All these successes allow us to confidently use general relativity,even in domains where its validity has not yet been strictly proven.Observations of certain X-ray binaries (e.g.,Cygnus X-1and the so called X-ray novae),as well as of stellar motions in our Galaxy,and of velocities in the inner cores2Klu´z niak:Neutron stars and general relativityof other galaxies,strongly suggest the existence of black holes.However,the laws of GR have not yet been truly tested in the strongfield regime.1.1.Why neutron starsThe strength of gravity is conveniently parametrized by the mass to size ratio, (M/R)(G/c2).For black holes,of course,GM/(Rc2)∼1,as for the Schwarzschild radius R Sch=2MG/c2.For the Sun,GM⊙/c2≈1.5km,while the solar radius R⊙≈300000km,which yields M⊙/R⊙∼10−5(in units of c2/G).A similar value is obtained for mass/distance in the binary Hulse-Taylor pulsar,where relativistic effects in the or-bital motion are so clearly detected(because the pulsar period is so short≈0.06s,and known to10significantfigures).For white dwarfs,M/R∼10−3.But for neutron stars, M/R∼10−1,and GR effects just outside their surface are about as important as near the black hole surface.As a testbed for GR,neutron stars have one great advantage over black holes—they have a tangible surface which can support magneticfields and can emit X-rays and other radiation.A great deal can be learned about neutron stars without assuming the validity of GR.Hence,a great deal can be learned about GR by observing neutron stars.Today, about1000radio pulsars are known and about100X-ray binaries containing neutron stars,so also in sheer numbers neutron stars have an advantage over black holes.1.2.Basic referencesThe narrative presented in Sections1and2,to a large extent relies on well established observations and theories,which have made their way into excellent textbooks,where detailed references can be found to the literature.Among those,particularly useful in the context of these lectures are the ones by Shapiro and Teukolsky(1983),Lipunov(1992), M´e sz´a ros(1992),Glendenning(1997),and Frank,King and Raine(1985).2.A brief history of neutron stars.Before discussing in detail the properties of rapidly rotating,(at most)weakly mag-netized,compact stars—which are ideal astrophysical objects for testing strong-field pre-dictions of General Relativity—let us recount how they were identified.Klu´z niak:Neutron stars and general relativity3 2.1.Key datesThe basic chronology of the discovery of neutron stars can be found,together with the references,e.g.,in the text by Shapiro and Teukolsky.The following selection reflects my bias of what seems particularly important with the hindsight of today.1914:Adams discovered that the rather dim,L≈3×10−3L⊙,star Sirius B(orbiting Sirius),whose mass had been determined to be M≈0.85±0.10M⊙,has the spectrum of a“white”star—hence the name white dwarf.The unusual combination of low luminosity and high temperature implied a small radius,R≈2×104km.This conclusion was based on an application of the black-body formulaL=4πσB R2T4.(1)1925:Adams measures the redshift,z,of certain lines in Sirius B.Applying general relativity,one can infer the value of M/R from z,and from the known mass a value of the stellar radius,R∼104km.The agreement with the spectroscopically determined value was a great triumph of GR.1926:The Fermi-Dirac statistic is discovered.1926(December):Fowler identifies the agent holding up white dwarfs against gravity—it is the degeneracy pressure of electrons.1930:Chandrasekhar discovers theoretical models of white dwarfs,from which the maximum value for white dwarf mass follows,the famous1.4M⊙.Incidentally,M∼1M⊙and R∼few×103km imply a densityρ∼106g/cm3,which in turn implies a minimum period of possible rotation or vibration of a few seconds:(Gρ)−1/2∼3s.1932:Chadwick discovers the neutron.1932:Landau discusses cold,degenerate stars composed of neutrons.1934:Baade and Zwicky write:“With all reserve we advance the view that supernovae represent the transition from ordinary star to neutron stars.”This remains a remarkable contribution—two years after the discovery of neutrons,Baade and Zwicky correctly explain the mechanism of Supernovae(type II)explosions,find the correct value for the gravitational binding energy released in the creation of a neutron star,∼1053erg,and even identify a site where a neutron star is present(and was discovered35years later!): the Crab nebula.1938:Landau discusses the energy released inside ordinary stars with neutron-star cores(a theoretical precursor of what is now known as a Thorne-˙Zytkow object).At the time,the energy source of the Sun was not known.The great contribution here is the pointing out of the enormous energy released in accretion onto neutron stars.1939:Oppenheimer and Volkoffsolve the relativistic equations of stellar structure for a fermi gas of neutrons,and thus construct thefirst detailed model of a neutron star.4Klu´z niak:Neutron stars and general relativityTheyfind a maximum mass(≈0.7M⊙,lower than the one for modern equations of state), above which the star is unstable to collapse.Thus the road to the theoretical discovery of black holes is paved.1940’s are lost to the Second World War.1950’s:The basic physics of the interior of neutron stars is worked out by the Soviet school,including a detailed understanding of the superfluid phase.1962:Giacconi et al.discover thefirst extrasolar source of X-rays,Sco X-1.1967:Shklovsky derives a model for Sco X-1,in which the X-ray source is an accreting neutron star in a binary system.1967:Pacini points out that neutron stars should rotate with periods P<<1s,and may have magneticfields of surface value B∼1012G.The ensuing dipole radiation is not directly observable,as its frequency2π/P is below the plasma frequency of interstellar space.1967:Radio pulsars with P≤3s discovered by Hewish,Bell et al.1968:Gold gives the“lighthouse”model of radio pulsars.1968:Spin-down of radio pulsars is measured,˙P>0.From this moment,it is clear that pulsars are rotating,compact objects,ultimately powered by the kinetic energy of their rotation.1971:Giacconi et al.discover thefirst of accreting counterparts of radio pulsars,the X-ray pulsar Cen X-3,of period4.84s.Today,many are known,in the period range 0.7s≤P≤10000s.1978:Tr¨u mper et al.discover the∼40keV cyclotron line in the spectrum of the accreting X-ray pulsar Her X-1.From the formula hν=1keV×(B/108G),the inferred value of the magneticfield at the stellar surface is B p=few×1012G,in agreement with the estimates of the dipole strength of ordinary radio pulsars.1982:The discovery of millisecond pulsars by Backer,Kulkarni et al.1996:The discovery of kHz quasi-periodic oscillations(QPOs)in the X-rayflux of low-mass X-ray binaries(LMXBs).1998:The discovery of2.5ms pulsar in the transient LMXB SAX J1808.4-3658by Wijnands and van der Klis.2.2.The physics of identifying neutron starsIt should be apparent from the above review,that the basic physics behind identifying neutron stars is fairly simple.Of course,the discovery was possible only after decades of sustained technological development,particularly in thefield of radio and X-ray detectors, as well as much observational effort.Also,the existence of neutron stars would not have been so readily accepted without the solid theoretical foundations laid down over a periodKlu´z niak:Neutron stars and general relativity5 of many years.But the basic,incontrovertible,observational arguments are really based on two or three simple formulae.Let us accept the theoretical result,that a neutron star is a body of mass M∼1M⊙and radius R∼10km,hence of mean density¯ρ>1014g/cm3.How can we be certain that such bodies have been discovered?a)The mass can be determined directly in some binary systems by methods of classi-cal astronomy(as developed for spectroscopic binaries),essentially by an application of Kepler’s laws.For the binary X-ray pulsars,the errors are rather large,but it is clear that one or two solar masses is the right value.For the binary radio pulsars(the Hulse-Taylor and Wolszczan pulsars),where the pulse phase can be determined very precisely and relativistic effects give much redundancy,the mass has been measured very accurately (to0.01M⊙)and is close to1.4M⊙.For binary(millisecond)radio pulsars with white dwarf companions,the mass function is always consistent with these values.b)In bright,steady,X-ray sources,and especially in X-ray bursters(where the X-ray flux briefly saturates at a certain peak value),one can assume that the radiativeflux is limited,at the so called Eddington value,by a balance between radiation pressure on electrons and gravitational pull on protons.Since both forces are proportional to (distance)−2,there is a direct relation betweenflux and mass.Again,M∼1M⊙is obtained,for L X≈1038erg/s.c)The radius can be determined whenever a thermal spectrum is detected,by a combination of the black-body formula,eq.(1),and of Wien’s law giving the characteristic temperature of a body emitting the thermal spectrum.Thus,for X-ray pulsars,such as Her X-1,the spectrum gives a characteristic temperature of T∼10keV∼108K,which in combination with the luminosity L X∼1037erg/s gives an area of∼1010cm2,consistentwith the area of a“polar cap.”This is the area through which open magneticfield lines pass for a R∼10km star,rotating at P=1.24s,with a B∼1012Gfield.For the non-pulsating bright X-ray source Sco X-1,T∼1keV,L∼1038erg/s,i.e., R∼10km directly,as expected if the accreting material is spread over the whole surface.d)For pulsars,an upper limit to the stellar radius follows from causality,ωR<c, hence R<cP/(2π).For millisecond pulsars,this gives R<100km.e)The moment of inertia of certain pulsars(if they are powered by rotation)can be measured directly in“cosmic calorimeters.”If the luminosity of the Crab nebula (≈5×1038erg/s)is equated to Iω˙ω,for the known period(P=33ms)and its derivative of the Crab pulsar(or the known age of the nebula),the value I≈1045g·cm2is obtained.A similar,but less secure,argument can be given for the famous eclipsing pulsar PSR 1957+20(P=1.6ms,˙P≈10−19).It is thought that the power needed to ablate the6Klu´z niak:Neutron stars and general relativity0.02M⊙companion is∼1038erg/s(assuming isotropic emission from the pulsar).Again, I∼1045g·cm2is obtained.f)Finally,a lower limit to the density can at once be derived for rotating objects from Newton’s formula for keplerian orbital motion:ωK= 4π¯ρ/3.Since 2π/P=ω≤ωK,for any star rotating at a period P,the mean density satisfies¯ρ≥3πG−1P−2.With the known value of Newton’s constant,this gives directly¯ρ>2×1014g/cm3,for SAX J1808.4-3658(P=2.5ms)or the millisecond pulsars,such as PSR 1957+20(P=1.6ms).These basic results are subject to many consistency checks,which in all cases support the basic result that objects with a solid orfluid surface(i.e.,they are not black holes!) have been identified of dimensions M∼1M⊙and R∼10km:i)The gravitational energy released in accretion L∼GM˙M/R is consistent(for the discussed values M∼M⊙and R∼10km)with the mass accretion rate inferred from theoretical studies of binary evolution.ii)In some X-ray bursters,the photosphere clearly expands.Again,spectralfits for the temperature and for the radius of the photosphere(eq.[1]),assuming Eddington luminosity,constrain the M–R relationship,in a manner consistent with the values discussed above.iii)The surface magneticfield measured from the cyclotron line in X-ray pulsars agrees,to an order of magnitude(B p∼1012±1G),with the one inferred for radio pulsars, by applying the notion that the spin down in the latter sources is obtained through balancing the energy loss in the simple dipole formula˙E=−2|¨m|2/(3c2),where|m|= B p R3/2,with the kinetic energy loss of a body of moment of inertia I=1045g·cm2. Incidentally,for millisecond pulsars,the value inferred from spin-down,B p∼109±1G, is consistent with the absence of polar cap accretion(and of associated pulsations)in X-ray bursters and other LMXBs.Thus,as far as the magneticfield is concerned,two or three classes of neutron stars are known—ordinary radio pulsars and accreting X-ray pulsars(B∼1012±1G),millisecond radio pulsars(B∼109±1G),and low-mass X-ray binaries,where there is no evidence for such strong magneticfields(i.e.,B<109G).iv)The observed long-term spin-up and spin-down of accreting X-ray pulsars is also consistent with a moment of inertia I∼1045g·cm2,for torques which are expected at the mass-accretion rates derived from the observed X-rayflux,assumed to be L X∼GM˙M/R∼0.1˙Mc2,and the assumption that the lever arm corresponds to an Alfvenic radius,obtained by balancing the ram pressure with the dipole magnetic pressure,i.e., B2/(8π)∼ρv2r at r=r A,˙M=ǫ4πr2ρv r,B=B p R3/r3,whereǫ∼1is a geometric factor.Klu´z niak:Neutron stars and general relativity7 3.The maximum mass of compact stars3.1.Neutron stars or quark stars?It is clear that radio pulsars and some accreting X-ray sources contain compact objects of properties closely resembling those known from theoretical models of neutron stars. Specifically,there can be no doubt that rotating stars of M∼M⊙and R∼10km exist. However their internal constitution is not yet known.The expected mass and radius of “strange”(quark)stars is similar,the main difference being in that quark stars of small masses would have small radii—unlike neutron stars whose radius generally grows with decreasing mass—(Alcock et al.1986).The observed“neutron stars”could be made up mostly of neutrons,but some of them could also be composed partly,or even mostly,of quark matter.From the point of view of testing GR,the internal constitution of static(non-rotating) stars would matter little,as their external metric,directly accessible to observations, would be independent of their nature—the only parameter in the unique static,spher-ically symmetric,asymptoticallyflat solution(the Schwarzschild metric)is the gravita-tional mass,M,of the central body.However,for rapidly rotating stars,the metric does vary with properties of the body other than its mass,and it would be good to know the precise form of the equation of state(e.o.s.)of matter at supranuclear density.As we have seen,at least some low-mass X-ray binaries(LMXBs)contain stellar rem-nants of extremely high density,exceeding1014g cm−3,and many of them are not black holes because they exhibit X-ray bursts of the type thought to result from a thermonu-clearflash on the surface of an ultra-compact star.Further,in these long-lived accreting systems the mass of the compact star is thought to have increased over time by several tenths of a solar mass above its initial value,and in the process the stars should have been spun up to short rotational periods.The compact objects in the persistent LMXBs are expected to be the most massive stellar remnants other than black holes,hence the most stringent limits on the e.o.s.of dense matter is expected to be derived from the mass of the X-ray sources in low-mass X-ray binaries.Before we discuss how this can be done,let us turn to the maximum mass.3.2.The maximum mass of neutron starsOne quantity that depends sensitively on the e.o.s.is the maximum mass of afluid configuration in hydrostatic equilibrium.For neutron stars this maximum mass,and in general the mass–radius relationship,is known from integrating the TOV equations for a wide variety of e.o.s.(Arnett and Bowers1977).The mass of rotating configurations is also known(Cook et al.1994).Here,I will only briefly review the basic physics behind8Klu´z niak:Neutron stars and general relativitythe existence of the maximum mass and then give an example for strange stars,where the e.o.s.is so simple that the variation of mass with the parameter describing the interactions can be determined analytically.As we know from the work of Chandrasekhar and others,the maximum mass is reached when the adiabatic index reaches a sufficiently low value that the star becomes unstable to collapse.In the Newtonian case,this critical index is4/3,corresponding to the extreme relativistic limit for fermions supplying the degeneracy pressure,when the formula for kinetic energy of a particle E=d r =−Gmρd r=4πr2ρ,i.e.,if the pressure and density scale with somefiducial density,P∝ρ∝ρ0,thenm∝r∝ρ−1/2.Such scalings allow some general statements to be made about the maximum mass,such as the Rhoads-Ruffini limit:M<3M⊙,ifρ≥ρ0>2×1014g/cm3.3.3.Quark starsConversion of some up and down quarks into strange quarks is energetically favorable in bulk quark matter(because the Fermi energy is so high)and it has been suggested that at large atomic number,matter in its ground state is in the form of“collapsed nuclei”Klu´z niak:Neutron stars and general relativity9 with strangeness about equal to the baryon number(Bodmer1971).On this assumption, Witten(1984)discussed the possible transformation of neutron stars to stars made up of matter composed of up,down,and strange quarks in equal proportions,and found the maximum mass of such quark stars as a function of the density of(self-bound)quark matter at zero pressure isρ0≥4×1014g/cm3.Detailed models of these“strange”starshave been constructed(Alcock et al.1986,Haensel et al.1986).Here,I discuss only the maximum mass of such stars.Following Alcock(1991),take a gas of any relativistic particles—the e.o.s.is P g=ρg c2/3.If these are moving in a background of vacuum with uniform energy density ρv c2=B,i.e.,negative pressure p v=−B,then the e.o.s.connecting the total pressure p=p g+p v,with the total densityρ=ρg+ρv,isp=(ρ−ρ0)c2/3,(2) withρ0c2=4B.Witten(1984)showed that for this simple e.o.s.the maximum mass from the TOV equation is M=2M⊙10Klu´z niak:Neutron stars and general relativityequivalently,that quark matter composed of up and down quarks in1:2ratio is unstable to emission of neutrons through the reaction u+2d→n.This implies that the baryonic chemical potential at zero pressure of such quark matter satisfies(Haensel1996)µu,d(0)>939.57MeV.(3)As we neglect the masses of up and down quarks in our considerations,the baryonic chemical potential at pressure P is given by the expression(Chapline and Nauenberg 1976)µ(P)=(P+ρc2)/n=4(A/3)3/4(P+B)1/4,(4) where n is the baryon number density,andρc2=An4/3+B is the energy density.For matter(not in beta equilibrium)composed of deconfined up and down quarks in1:2 ratio,n=n u=n d/2and hence A=(1+24/3)(3¯h c/4)π2/3C−1/3,i.e.,µ(0)∝(B/C)1/4, where C≡1−2αc/πandαc is the QCD coupling constant.Inequality(1)then becomesBπ ρ0(0).Thus,through lowest order in the QCD interaction,thefiducial density is changed,but,this implies that the least upper not the e.o.s.Since the stellar mass scales asρ−1/2bound on the mass of the star as a function of the QCD coupling constant is given for non-rotating strange stars byM max(αc)= 1−2αc4.Measuring the mass of accreting neutron(or strange)starsFinally,we have to confront the question how the mass of the compact objects in LMXBs may be determined.Hopefully,a mass will be measured which will eliminate a class o equations of state of dense matter.Unfortunately,application to X-ray bursters of stan-dard methods for determining the mass function of the binary—and hence constraining the mass of the compact X-ray source—is exceedingly difficult,as the optical emission is usually dominated by that of the accretion disk(e.g.van Paradijs et al.,1996).However, reliable mass values obtained by this method may soon become available,particularly for transient sources,such as the accreting millisecond pulsar SAX J1808.4–3658.The mass of the compact object in an X-ray binary may also be determined by study-ing the time variability of the radiationflux formed in the accretionflow.Specifically,for sufficiently weakly magnetized stars,a maximum frequency is expected corresponding to the presence of the innermost(marginally)stable circular orbit allowed in general relativity(Klu´z niak,Michelson and Wagoner,1990).It has been reported that such a maximum frequency may have been observed,at least in one system where quasi periodic oscillations(QPOs)in the X-rayflux saturate at a particular value(Zhang et al.1998). In this manner,several e.o.s.were excluded(Klu´z niak1998)on the understanding that the maximum observed kHz QPO frequency implies a mass in excess of2M⊙;see also Kaaret et al.(1997).Similar considerations(Bulik et al.1999)exclude static(or slowly rotating)quark stars if the minimum density of quark matter isρ0>4.2×1014g/cm3, and the quark matter is taken to be described by the MIT bag model.The overall conclusion(Klu´z niak1998)is that neutron-star matter may be composed simply of neutrons with some protons,electrons and muons,as models of more exotic neutron-star matter(including hyperons or pion and kaon condensates)do not agree with the simplest interpretation of the kHz QPO data,namely that the maximum frequency observed in the low-mass X-ray binary4U1820-30,i.e.,1066Hz(Zhang et al.1998), is attained in the marginally stable orbit around a neutron star.If the compact stellar remnants in these systems are slowly rotating,the same conclusion would apply to ultra-dense matter in general,at densities greater than4.2×1014g/cm3,as matter composed of massless quarks would also be excluded for such densities(Bulik et al.1999).However, as we have seen,minimum densities smaller than4.2×1014g/cm3seem possible for more realistic models of self-bound quark matter,and this would change the conclusion.For rapidly rotating strange stars the conclusion may be drastically different,as the metric is greatly modified by a pronouncedflattening of the star(this effect is less im-portant for neutron stars).In general,the marginally stable orbit is pushed out by this effect,and a fairly low orbital frequency can be obtained for a low mass star.This is illus-trated in Fig.1(taken from Stergioulas et al.1999)which exhibits the frequency in theinnermost(marginally)stable circular orbit of general relativity(ISCO)as a function of stellar mass,M,for the Schwarzschild metric[the hyperbola f+=2.2kHz(M⊙/M)],as well as the ISCO frequency for strange stars rotating at Keplerian frequencies(i.e.,max-imally rotating,at the equatorial mass-shedding limit),for various values of the density at zero pressure,ρ0of eq.(2).It turns out that for these maximally rotating models,the ISCO is always at1.7to1.8km above the stellar surface,the increase of the ISCO orbital√frequency for these models can then be understood in terms of Kepler’s law:2πf∼Fig.1.The frequency of the co-rotating innermost stable circular orbit as a function of mass for static models(thin,continuous line)and for strange stars rotating at the equa-torial mass-shedding limit(thick lines,in the style of Fig.1).For the static models,this frequency is given by the keplerian value at r=6GM/c2,i.e.,by f+=2198Hz(M⊙/M), and the minimum ISCO frequency corresponds to the maximum mass,denoted by afilled circle,an empty circle,and a star,respectively forρ0/(1014g cm−3)=4.2,5.3,and6.5. Note that the ISCO frequencies for rapidly rotating strange stars can have much lower values,and f+<1kHz can be achieved for strange stars of fairly modest mass,e.g.1.4M⊙,if the star rotates close to the equatorial mass-shedding limit.Thisfigure is from Stergioulas et al.1999.to the marginally stable orbit(Kaaret1997,Zhang1998,Klu´z niak1998).But with the data gathered to date,it seems easier to constrain the e.o.s.of dense matter,on the assumption that the QPO frequency saturates in the ISCO,than to show that this as-sumption is indeed correct.One difficulty is that the physics of accretion disks is still very poorly understood.New data is being gathered daily and new experiments are planned which may lead to a break-through in thisfield.I thank the organizers of this School for their wonderful hospitality in Guanajuato. 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跟天文知识有关的英语作文全文共3篇示例，供读者参考篇1The Wonders of the CosmosEver since I was a young child, I have always been fascinated by the night sky. I vividly remember lying in the backyard on warm summer nights, gazing up at the twinkling stars in awe and wonder. My parents would point out the constellations to me - Orion the Hunter, the Big Dipper, and more. Even at that young age, I was captivated by the vastness of the universe and the idea that those tiny pinpricks of light were enormous balls of gas millions of miles away.As I grew older, my curiosity about space only deepened. I devoured books on astronomy, mesmerized by the tales of ancient astronomers like Galileo and Copernicus who revolutionized our understanding of the cosmos. I learned about the birth and death of stars, the foreign landscapes of alien planets, and the mysteries that still baffle scientists to this day, like dark matter and black holes.In school, astronomy quickly became my favorite subject. While my friends zoned out during lessons on planetary motion and the life cycles of stars, I was hanging on every word. I loved learning about how stars are formed from massive clouds of dust and gas, burning bright for billions of years before eventually running out of fuel and collapsing in on themselves. Some go out with a whimper, shrinking into dense white dwarfs. But the largest stars meet a more spectacular demise, exploding in a brilliant supernova before their remnants form either a neutron star or a black hole from which not even light can escape.I found the concept of black holes particularly mind-bending. These gravitational behemoths, with their powerful tidal forces capable of spaghettifying any object that strays too close, really put into perspective the puniness of humanity in the grand scheme of things. At the same time, their sheer strangeness sparked my imagination. What lies beyond the event horizon, the point of no return? Is it possible to travel through a black hole's wormhole and emerge in another part of the universe? Or do they lead, as some scientists theorize, to entirely separate realities or dimensions?Learning about faraway exoplanets orbiting distant stars stirred my sense of wonder as well. For most of human history,we assumed our solar system was the only one of its kind. But in recent decades, scientists have identified thousands of exoplanets using cutting-edge telescopes and detection methods. Some are similar to the rocky inner planets like Earth and Mars, while others are gas giants akin to Jupiter and Saturn. But we've also discovered planets utterly unlike anything in our cosmic backyard, from scalding hot Jupiters orbiting precariously close to their suns to diamond planets crystallized by intense heat and pressure. Each new discovery deepens the mystery of how these strange worlds formed and whether any could potentially harbor life.On a cloudless night, I'll often set up my telescope and spend hours scanning the skies, hunting for distant galaxies and stellar phenomena. I've witnessed meteor showers that lit up the heavens like celestial fireworks displays. I've spotted wispy nebulae where new stars are being forged and crystal-clear globular clusters comprised of hundreds of thousands of ancient suns. The Andromeda Galaxy, our nearest major galactic neighbor, appears as a faint, fuzzy blob to the naked eye, but through my telescope's lens it transforms into a majestic spiral of glowing stars and dust.When I reflect deeply on how our sun is just one of billions of stars in the Milky Way, itself one of countless galaxies strewn across the vast ocean of the universe, my mind is overwhelmed by the immensity and age of it all. Our solar system, our galaxy, emerged from the ashes of the Big Bang nearly 14 billion years ago. The cosmos has been in a perpetual state of creation and destruction for eons beyond our comprehension. Stars are born, they live, they die, and from their remnants new ones eventually take form. This grand, unending cosmic ballet has played out over infinite time and space in a cycle as inexorable as it isawe-inspiring.To gaze upon the night sky and grasp, even for a moment, the epic scale of time and distance inherent in those ancient points of light is humbling in the most profound way. It lays bare our true insignificance in the fabric of the universe. For all our species' achievements and capabilities, we are but temporary wayfarers on a pale blue dot orbiting an utterly average star. We emerged from the cosmos, and to the cosmos we will one day return, our entire lineage a mere blip in the cosmic calendar.And yet, this existential perspective fills me not with dread, but an immense appreciation for the rarity and preciousness of our sliver of existence. We are profoundly fortunate to be here,alive, and able to look up on a clear night and bear witness to the majesty of creation. Out of the infinite cosmic void, we have developed the capacity to observe, to wonder, to have our souls stirred by the beauty and splendor of something as simple as a starry sky. We are small, yes, but we areers capable of no less than experiencing and revering the very universe itself.So I continue to keep watching the skies, pushing the boundaries of my understanding while simultaneously celebrating the grand mystery of it all. Perhaps one day we'll uncover insights that will reveal some deeper cosmic truths and unravel timeless riddles. But I don't mind either way. Because in this interminable dance of celestial spheres, the greatest gift is simply the journey of being able to embrace the unknown.篇2The Wonders of the Night SkyEver since I was a little kid, I've been fascinated by the night sky. There's just something magical about looking up at the twinkling stars and planets on a clear night. It makes you feel so small and insignificant in the grand scheme of the universe, yet also filled with a sense of awe and wonderment.I can still vividly remember one night when I was about 8 years old. It was a warm summer evening, and my dad had taken me out to our backyard after bedtime to go stargazing. We laid out an old blanket on the grass and just gazed upwards into the inky blackness. My dad pointed out the constellations - Orion with his distinctive belt of three bright stars, the Big Dipper hanging like a ladle in the northern sky, and Cassiopeia looking like a W made of stars. I was completely transfixed.That night sparked my lifelong interest in astronomy. As I grew older, I devoured books about the planets, stars, galaxies, and the mind-boggling vastness of space. I learned that a light year, the distance light travels in a vacuum in one year, is almost 6 trillion miles. Our entire solar system, as huge as it seems to us, would be just a tiny speck from that distance away. And our Milky Way galaxy contains over 100 billion stars! The numbers and distances involved in the study of the cosmos are truly beyond human comprehension.In school, my favorite units were always the ones on astronomy. I loved learning about how stars are born from massive clouds of dust and gas, fusing hydrogen atoms into helium and radiating their brilliant light for millions or billions of years. Eventually, stars like our Sun will run out of fuel, expandinto a red giant that could potentially swallow the Earth, and then shrink down into a smoldering white dwarf. The Universe's largest stars have even more dramatic fates, exploding as supernovas so bright they can outshine entire galaxies for weeks at a time.I found the history of astronomy and space exploration just as fascinating as the science itself. Ancient cultures like the Babylonians, Greeks, and indigenous Americans all studied the night sky and devised complex systems for tracking the motions of the planets and stars across the heavens. When Galileo turned his new telescope towards the heavens in 1610, he revolutionized our understanding by glimpsing craters on the Moon, spots on the Sun, and moons orbiting Jupiter.The 20th century will forever be remembered as the era when humanity first sent machines and then people out into space. The early satellites, manned spaceflights and Moon landings captured the imagination of the entire world. I'll never forget the iconic words of Neil Armstrong as he took his first steps onto the lunar surface in 1969: "That's one small step for man, one giant leap for mankind." Just 40 years later, we had landed rovers on Mars and captured stunning images of that rusty red world's ancient river valleys and extinct volcanoes.Looking ahead, the future of space exploration and astronomy is mind-boggling. NASA's new James Webb Space Telescope has already begun capturing incredible images of some of the oldest and most distant galaxies in the observable universe, shortly after the Big Bang. New super-powerful ground-based telescopes may someday find definitive evidence of Earth-like planets orbiting other stars where life could potentially exist. And Elon Musk's SpaceX is working towards establishing permanent human settlements on Mars within our lifetimes. Who knows what other wonders await to be uncovered in the depths of space?For my part, I plan to keep pursuing my passion for astronomy throughout my education and hopefully even make a career out of it someday. There's still so much about the cosmos left to explore and understand. Whether I end up as a researcher studying dark matter and black holes, an engineer helping design new space telescopes and rockets, or a science writer sharing the majesty of the heavens with the public, I know the night sky will keep filling my life with a sense of infinite possibility.To me, the greatest miracle of astronomy is forcing us to confront our own tiny place in this incomprehensibly vastuniverse. As the pioneering cosmologist Carl Sagan wrote, "We are a way for the cosmos to know itself." The atoms that make up our bodies were literally forged in the nuclear furnaces of ancient stars. We are made of star-stuff, both intimately connected to the cosmos yet somehow able to study and understand it through science. That humbling realization is what astounds me most of all.篇3The Wonders of the Cosmos: An Astronomical JourneyEver since I was a young child, I have always been fascinated by the night sky. There was something magical about gazing up at the twinkling stars and the glowing moon that filled me with a sense of wonder and curiosity. As I grew older and began learning about astronomy, my enchantment with the cosmos only deepened. The universe is a vast, mysterious realm that continues to astound scientists and amateur stargazers alike with its grandeur and complexity.One of the first astronomical concepts that captured my imagination was the life cycle of stars. These celestial beacons, which appear as tiny pinpricks of light in the night sky, are actually massive, blazing spheres of gas undergoing continuouscycles of birth, life, and death. Stars are formed from massive clouds of dust and gas known as nebulae. Over millions of years, the gravitational forces within these clouds cause the material to condense and form a protostar. As the protostar continues to contract, its core becomes incredibly hot and dense, setting off the nuclear fusion reactions that mark the birth of a new star.A star's life is a delicate balance between the inward pull of gravity and the outward push of the nuclear fusion occurring in its core. For most of its life, a star will exist in a stable equilibrium, fusing hydrogen into helium and radiating energy in the form of light and heat. However, as the star ages and exhausts its supply of hydrogen fuel, it enters into the final stages of its life cycle.Depending on the star's mass, it may undergo various transformations, such as expanding into a red giant or even a supergiant. Massive stars may end their lives in spectacular fashion, exploding as supernovae and briefly outshining entire galaxies. The remnants of these stellar explosions can form exotic objects like neutron stars or black holes, whose intense gravitational fields warp the very fabric of space and time.Another aspect of astronomy that has always captivated me is the study of galaxies. These vast, gravitationally bound systems of stars, gas, dust, and dark matter are the buildingblocks of the universe. Our own Milky Way galaxy is a spiral galaxy, containing hundreds of billions of stars and spanning over 100,000 light-years in diameter. Yet, it is but one of countless galaxies that populate the observable universe.The sheer scale and diversity of galaxies are trulymind-boggling. Some, like elliptical galaxies, are smooth and featureless, while others, such as spiral galaxies, display intricate patterns of dust lanes and stellar nurseries. Some galaxies even exist in clusters, bound together by the immense gravitational forces at play. And at the heart of many galaxies, including our own, lie supermassive black holes, objects so dense that not even light can escape their gravitational pull.Beyond the realm of individual galaxies lies the cosmic web, the large-scale structure of the universe itself. This vast, interconnected network of galaxies, galaxy clusters, and filamentary structures is the result of the intricate interplay between matter and the mysterious forces of dark matter and dark energy. These unseen components, which make up the bulk of the universe's mass and energy, continue to perplex astronomers and challenge our fundamental understanding of the cosmos.One of the most profound realizations in modern astronomy is that the universe itself had a beginning – the Big Bang. This cataclysmic event, which occurred approximately 13.8 billion years ago, marked the birth of space, time, and all matter and energy in the observable universe. The afterglow of this primordial explosion, known as the cosmic microwave background radiation, is a powerful piece of evidence supporting the Big Bang theory and provides a glimpse into the earliest moments of the universe's existence.As our knowledge of the cosmos continues to expand, new frontiers of exploration emerge. The search for exoplanets, or planets orbiting stars other than our Sun, has become a major focus of modern astronomy. With the advent of powerful telescopes and advanced detection techniques, thousands of exoplanets have been discovered, ranging from gas giants to rocky, Earth-like worlds. The possibility of finding habitable exoplanets and the potential for extraterrestrial life has ignited the imaginations of scientists and the public alike.Moreover, the study of dark matter and dark energy, which together make up approximately 95% of the universe's total mass and energy, remains one of the greatest unsolved mysteries in modern cosmology. Unraveling the nature of theseelusive components could revolutionize our understanding of the fundamental laws of physics and the ultimate fate of the universe.As a student of astronomy, I am constantly in awe of the vast and complex universe we inhabit. From the intricate dance of celestial bodies within our own solar system to the cosmic choreography of galaxies and clusters on the grandest scales, the cosmos is a tapestry of wonder and mystery waiting to be explored.The pursuit of astronomical knowledge is not merely an academic endeavor but a profound journey of self-discovery and existential questioning. By studying the heavens, we gain a deeper appreciation for our place in the cosmos and the interconnectedness of all things. We are reminded of our own insignificance in the grand scheme of the universe, yet simultaneously elevated by the knowledge that we are part of something much larger and more wondrous than ourselves.As I continue my studies and gaze up at the night sky, I am filled with a sense of humility and awe. The universe is a vast, ever-evolving canvas, and we are but tiny brushstrokes in its grand cosmic tapestry. Yet, it is through our curiosity, our thirst for knowledge, and our unwavering pursuit of understandingthat we can unravel the mysteries of the cosmos, one star, one galaxy, one cosmic epoch at a time.。

a r X i v :h e p -p h /0105158v 1 16 M a y 20011Probing Quark Matter In Neutron Stars M.Prakash a aDepartment of Physics &Astronomy,State University of New York at Stony Brook,Stony Brook,New York-11794-3800,U.S.A.The presence of quark matter in neutron star interiors may have distinctive signatures in basic observables such as (i)masses and radii [1],(ii)surface temperatures versus age [2],(iii)spin-down rates of milli-second pulsars [3],and (iv)neutrino luminosities from future galactic core collapse supernovae [4].I highlight recent developments in some of these areas with a view towards assessing how theory may be confirmed by ν−signals from future galactic supernovae in detectors like SuperK,SNO and others under consideration,including UNO [5],and by multi-wavelength photon observations with new generation satellites such as the HST,Chandra,and XMM.1.NEUTRINO SIGNALS A proto-neutron star (PNS)is born following the gravitational collapse of the core of a massive star,in conjunction with a successful supernova explosion.During the first tens of seconds of evolution,nearly all (∼99%)of the remnant’s binding energy is radiated away in neutrinos of all flavors [6–8].The ν−luminosities and the evolutionary timescale are controlled by several factors,such as the total mass of the PNS and the ν−opacityat supranuclear density,which depends on the composition and equation of state (EOS).Collins and Perry [9]noted that the superdense matter in neutron star cores might consist of weakly interacting quarks rather than of hadrons,due to the asymptotic freedom of QCD.The appearance of quarks causes a softening of the EOS which leads to a reduction of the maximum mass and radius [1].In addition,quarks would alter ν−emissivities and thereby influence the surface temperature of a neutron star [2]during the hundreds of thousands or millions of years that they might remain observable with such instruments as HST,Chandra,and XMM.Many calculations of dense matter predict the appearance of other kinds of exotic matter in addition to quarks:for example,hyperons or a Bose (pion,kaon)condensate[10,and references therein].An important question is whether or not νobservations from a supernova could reveal the presence of such exotic matter,and further could unambiguously point to the appearance of quarks.The detection of quarks in neutron stars would go a long way toward the delineation of QCD at finite baryon density which would be complementary to current Relativistic Heavy Ion Collider experiments,which largely address the finite temperature,but baryon-poor regime.An important consequence of the existence of exotic matter in neutron stars (in what-ever form,as long as it contains a negatively charged component),is that a sufficiently2massive PNS becomes metastable[10,11].After a delay of up to100s,depending upon which component appears,a metastable PNS collapses into a black hole[7,8].Such an event should be straightforward to observe as an abrupt cessation ofν−flux when the instability is triggered.In Ref.[4]we provide a benchmark calculation with quarks by solving the general relativisticν−transport and hydrostatic equations(see[7,8])with the EOS of[12]and ν-opacities of[13]as microphysical ingredients.In the left panel of Fig.1,we compare ν−signals observable with different detectors for stars containing nucleons and quark matter(npQ stars).The two upper shaded bands correspond to estimated SN1987A (50kpc distance)detection limits with KII and IMB,and the lower bands correspond to estimated detection limits set to a count rate dN/dt=0.2Hz[8]in SNO,SuperK, and UNO,for a Galactic supernova(8.5kpc distance).It is possible that this limit is too conservative and could be lowered with identifiable backgrounds and knowledge of the direction of the signal.The width of the bands represents the uncertainty in the ¯νe average energy due to theflux-limited diffusion approximation[7,8].We conclude that it should be possible to distinguish between stable and metastable stars,since the luminosities when metastability is reached are always above conservative detection limits. Our quark EOS[12],in conjunction with the baryonic EOS we used,was motivated to maximize the extent of the quark matter phase in a cold neutron star,and was limited by the necessity of producing a maximum mass cold star in line with accurate observational constraints(M G=1.444M⊙).Use of an alternative quark EOS that otherwise produces a larger maximum mass,delays the appearance of quarks and raises the metastability window to larger stellar masses[12].Necessarily,this results in an increased timescale for metastability for a given mass,and a lowerν−luminosity when metastability occurs. Fig.1shows the relation between time to instability and M B for the original case(thick solid curve)and a case(thin solid curve),in which the maximum gravitational mass of a cold neutron star is about1.85M⊙.For the latter case,the metastability timescales lie in a narrow range40–45s.In the right panel of Figure1,we show the metastability time-M B relation found for matter containing hyperons(npH,dashed lines[7])or matter with kaons(npK,dotted line[8])instead of quarks.All three types of strange matter are suppressed by trapped neutrinos[10,12],but hyperons always exist in npH matter atfinite temperatures and the transition to quark matter can occur at lower densities than that for very optimistic kaon cases[8].Thus,the metastability timescales for npH matter can be very short,and those for npK matter are generally larger than for npQ matter.Note the relatively steep dependence of the metastability time with M B for npH stars,which decreases to very small values near the maximum mass limit of hot,lepton-rich,stars.The thick npH and npQ lines,as well as the npK line,represent minimum metastability times for a given M B as discussed above.The thin npQ and npH lines are for EOSs with larger cold,catalyzed maximum mass.Clearly,the observation of a single case of metastability,and the determination of the metastability time alone,will not necessarily permit one to distinguish among the various possibilities.Only if the metastability time is less than10–15s,could one decide on this basis that the star’s composition was that of npH matter.3Figure1.Left panel:The totalν−luminosity for npQ stars of various baryon masses. Shaded bands illustrate the limiting luminosities corresponding to count rates of0.2Hz for the indicated supernovae in some detectors.Right panel:Lifetimes of metastable stars versus the PNS M B for various assumed compositions.Thick lines denote cases in which the maximum masses of cold,catalyzed stars are near M G≃1.45M⊙,which minimizes the metastability lifetimes.The thin lines for the npQ and npH cases are for EOSs with larger maximum masses(M G=1.85and1.55M⊙,respectively).Our conclusions are that(1)the metastability and subsequent collapse to a black hole of a PNS containing quark matter,or other types of matter including hyperons or a Bose condensate,are observable in current and plannedνdetectors,and(2)discriminating among these compositions may require more than one such observation.This highlights the need for breakthroughs in lattice simulations of QCD atfinite baryon density in order to unambiguously determine the EOS of high density matter.In the meantime,intriguing possible extensions of PNS simulations with npQ matter include the consideration of heterogenoeus structures[14],quark matter superfluidity[15],and coherentν−scattering on droplets[16].2.MULTI-WAVELENGTH PHOTON OBSERVATIONSIn Ref.[2],the prospects of detecting baryon and quark superfluidity from neutron stars during their long-term(up to106years)cooling epoch was studied.Our assessment is that,from future photon observations of neutron star cooling,(1)one could constrain the smaller of the n−orΛ−pairing gaps and the star’s mass,(2)deducing the sizes of quark4gaps will be difficult,(3)large q−gaps render quarks invisible,and(4)vanishing q−gaps lead to cooling behaviors which are indistinguishable from those of np or npH stars. However,think this titillating thought!The observation of a neutron star older than 106year and hotter than∼107o K signals quarks with large gaps in neutron stars!It is a pleasure to thank James ttimer,Dany Page,Jose A.Pons,and Andrew W.Steiner with whom the work reported here was performed.This work was supported by the U.S.Department of Energy under contract number DOE/DE-FG02-88ER-40388. REFERENCESttimer and M.Prakash,Astrophys.J.550,426(2001).2. D.Page,M.Prakash,ttimer,and A.W.Steiner,Phys.Rev.Lett.85,2048(2000).3.N.K.Glendenning,S.Pei,and F.Weber,Phys.Rev.Lett.79,1603(1997).4.J.A.Pons,A.W.Steiner,M.Prakash,and ttiner,Phys.Rev.Lett.(2001),submitted;astro-ph/0102015.5. C.K.Jung,in Next Generation Nucleon Decay and Neutrino Detector,AIP Confer-ence Proceedings No.533,edited by M.V.Diwan and C.K.Jung(AIP,New York, 2000),p.29.6. 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Observation of Gravitational Waves from a Binary Black Hole MergerB.P.Abbott et al.*(LIGO Scientific Collaboration and Virgo Collaboration)(Received21January2016;published11February2016)On September14,2015at09:50:45UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal.The signal sweeps upwards in frequency from35to250Hz with a peak gravitational-wave strain of1.0×10−21.It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole.The signal was observed with a matched-filter signal-to-noise ratio of24and a false alarm rate estimated to be less than1event per203000years,equivalent to a significance greaterthan5.1σ.The source lies at a luminosity distance of410þ160−180Mpc corresponding to a redshift z¼0.09þ0.03−0.04.In the source frame,the initial black hole masses are36þ5−4M⊙and29þ4−4M⊙,and the final black hole mass is62þ4−4M⊙,with3.0þ0.5−0.5M⊙c2radiated in gravitational waves.All uncertainties define90%credible intervals.These observations demonstrate the existence of binary stellar-mass black hole systems.This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.DOI:10.1103/PhysRevLett.116.061102I.INTRODUCTIONIn1916,the year after the final formulation of the field equations of general relativity,Albert Einstein predicted the existence of gravitational waves.He found that the linearized weak-field equations had wave solutions: transverse waves of spatial strain that travel at the speed of light,generated by time variations of the mass quadrupole moment of the source[1,2].Einstein understood that gravitational-wave amplitudes would be remarkably small;moreover,until the Chapel Hill conference in 1957there was significant debate about the physical reality of gravitational waves[3].Also in1916,Schwarzschild published a solution for the field equations[4]that was later understood to describe a black hole[5,6],and in1963Kerr generalized the solution to rotating black holes[7].Starting in the1970s theoretical work led to the understanding of black hole quasinormal modes[8–10],and in the1990s higher-order post-Newtonian calculations[11]preceded extensive analytical studies of relativistic two-body dynamics[12,13].These advances,together with numerical relativity breakthroughs in the past decade[14–16],have enabled modeling of binary black hole mergers and accurate predictions of their gravitational waveforms.While numerous black hole candidates have now been identified through electromag-netic observations[17–19],black hole mergers have not previously been observed.The discovery of the binary pulsar system PSR B1913þ16 by Hulse and Taylor[20]and subsequent observations of its energy loss by Taylor and Weisberg[21]demonstrated the existence of gravitational waves.This discovery, along with emerging astrophysical understanding[22], led to the recognition that direct observations of the amplitude and phase of gravitational waves would enable studies of additional relativistic systems and provide new tests of general relativity,especially in the dynamic strong-field regime.Experiments to detect gravitational waves began with Weber and his resonant mass detectors in the1960s[23], followed by an international network of cryogenic reso-nant detectors[24].Interferometric detectors were first suggested in the early1960s[25]and the1970s[26].A study of the noise and performance of such detectors[27], and further concepts to improve them[28],led to proposals for long-baseline broadband laser interferome-ters with the potential for significantly increased sensi-tivity[29–32].By the early2000s,a set of initial detectors was completed,including TAMA300in Japan,GEO600 in Germany,the Laser Interferometer Gravitational-Wave Observatory(LIGO)in the United States,and Virgo in binations of these detectors made joint obser-vations from2002through2011,setting upper limits on a variety of gravitational-wave sources while evolving into a global network.In2015,Advanced LIGO became the first of a significantly more sensitive network of advanced detectors to begin observations[33–36].A century after the fundamental predictions of Einstein and Schwarzschild,we report the first direct detection of gravitational waves and the first direct observation of a binary black hole system merging to form a single black hole.Our observations provide unique access to the*Full author list given at the end of the article.Published by the American Physical Society under the terms of the Creative Commons Attribution3.0License.Further distri-bution of this work must maintain attribution to the author(s)and the published article’s title,journal citation,and DOI.properties of space-time in the strong-field,high-velocity regime and confirm predictions of general relativity for the nonlinear dynamics of highly disturbed black holes.II.OBSERVATIONOn September14,2015at09:50:45UTC,the LIGO Hanford,W A,and Livingston,LA,observatories detected the coincident signal GW150914shown in Fig.1.The initial detection was made by low-latency searches for generic gravitational-wave transients[41]and was reported within three minutes of data acquisition[43].Subsequently, matched-filter analyses that use relativistic models of com-pact binary waveforms[44]recovered GW150914as the most significant event from each detector for the observa-tions reported here.Occurring within the10-msintersite FIG.1.The gravitational-wave event GW150914observed by the LIGO Hanford(H1,left column panels)and Livingston(L1,rightcolumn panels)detectors.Times are shown relative to September14,2015at09:50:45UTC.For visualization,all time series are filtered with a35–350Hz bandpass filter to suppress large fluctuations outside the detectors’most sensitive frequency band,and band-reject filters to remove the strong instrumental spectral lines seen in the Fig.3spectra.Top row,left:H1strain.Top row,right:L1strain.GW150914arrived first at L1and6.9þ0.5−0.4ms later at H1;for a visual comparison,the H1data are also shown,shifted in time by this amount and inverted(to account for the detectors’relative orientations).Second row:Gravitational-wave strain projected onto each detector in the35–350Hz band.Solid lines show a numerical relativity waveform for a system with parameters consistent with those recovered from GW150914[37,38]confirmed to99.9%by an independent calculation based on[15].Shaded areas show90%credible regions for two independent waveform reconstructions.One(dark gray)models the signal using binary black hole template waveforms [39].The other(light gray)does not use an astrophysical model,but instead calculates the strain signal as a linear combination of sine-Gaussian wavelets[40,41].These reconstructions have a94%overlap,as shown in[39].Third row:Residuals after subtracting the filtered numerical relativity waveform from the filtered detector time series.Bottom row:A time-frequency representation[42]of the strain data,showing the signal frequency increasing over time.propagation time,the events have a combined signal-to-noise ratio(SNR)of24[45].Only the LIGO detectors were observing at the time of GW150914.The Virgo detector was being upgraded, and GEO600,though not sufficiently sensitive to detect this event,was operating but not in observational mode.With only two detectors the source position is primarily determined by the relative arrival time and localized to an area of approximately600deg2(90% credible region)[39,46].The basic features of GW150914point to it being produced by the coalescence of two black holes—i.e., their orbital inspiral and merger,and subsequent final black hole ringdown.Over0.2s,the signal increases in frequency and amplitude in about8cycles from35to150Hz,where the amplitude reaches a maximum.The most plausible explanation for this evolution is the inspiral of two orbiting masses,m1and m2,due to gravitational-wave emission.At the lower frequencies,such evolution is characterized by the chirp mass[11]M¼ðm1m2Þ3=5121=5¼c3G596π−8=3f−11=3_f3=5;where f and_f are the observed frequency and its time derivative and G and c are the gravitational constant and speed of light.Estimating f and_f from the data in Fig.1, we obtain a chirp mass of M≃30M⊙,implying that the total mass M¼m1þm2is≳70M⊙in the detector frame. This bounds the sum of the Schwarzschild radii of thebinary components to2GM=c2≳210km.To reach an orbital frequency of75Hz(half the gravitational-wave frequency)the objects must have been very close and very compact;equal Newtonian point masses orbiting at this frequency would be only≃350km apart.A pair of neutron stars,while compact,would not have the required mass,while a black hole neutron star binary with the deduced chirp mass would have a very large total mass, and would thus merge at much lower frequency.This leaves black holes as the only known objects compact enough to reach an orbital frequency of75Hz without contact.Furthermore,the decay of the waveform after it peaks is consistent with the damped oscillations of a black hole relaxing to a final stationary Kerr configuration. Below,we present a general-relativistic analysis of GW150914;Fig.2shows the calculated waveform using the resulting source parameters.III.DETECTORSGravitational-wave astronomy exploits multiple,widely separated detectors to distinguish gravitational waves from local instrumental and environmental noise,to provide source sky localization,and to measure wave polarizations. The LIGO sites each operate a single Advanced LIGO detector[33],a modified Michelson interferometer(see Fig.3)that measures gravitational-wave strain as a differ-ence in length of its orthogonal arms.Each arm is formed by two mirrors,acting as test masses,separated by L x¼L y¼L¼4km.A passing gravitational wave effec-tively alters the arm lengths such that the measured difference isΔLðtÞ¼δL x−δL y¼hðtÞL,where h is the gravitational-wave strain amplitude projected onto the detector.This differential length variation alters the phase difference between the two light fields returning to the beam splitter,transmitting an optical signal proportional to the gravitational-wave strain to the output photodetector. To achieve sufficient sensitivity to measure gravitational waves,the detectors include several enhancements to the basic Michelson interferometer.First,each arm contains a resonant optical cavity,formed by its two test mass mirrors, that multiplies the effect of a gravitational wave on the light phase by a factor of300[48].Second,a partially trans-missive power-recycling mirror at the input provides addi-tional resonant buildup of the laser light in the interferometer as a whole[49,50]:20W of laser input is increased to700W incident on the beam splitter,which is further increased to 100kW circulating in each arm cavity.Third,a partially transmissive signal-recycling mirror at the outputoptimizes FIG. 2.Top:Estimated gravitational-wave strain amplitude from GW150914projected onto H1.This shows the full bandwidth of the waveforms,without the filtering used for Fig.1. The inset images show numerical relativity models of the black hole horizons as the black holes coalesce.Bottom:The Keplerian effective black hole separation in units of Schwarzschild radii (R S¼2GM=c2)and the effective relative velocity given by the post-Newtonian parameter v=c¼ðGMπf=c3Þ1=3,where f is the gravitational-wave frequency calculated with numerical relativity and M is the total mass(value from Table I).the gravitational-wave signal extraction by broadening the bandwidth of the arm cavities [51,52].The interferometer is illuminated with a 1064-nm wavelength Nd:Y AG laser,stabilized in amplitude,frequency,and beam geometry [53,54].The gravitational-wave signal is extracted at the output port using a homodyne readout [55].These interferometry techniques are designed to maxi-mize the conversion of strain to optical signal,thereby minimizing the impact of photon shot noise (the principal noise at high frequencies).High strain sensitivity also requires that the test masses have low displacement noise,which is achieved by isolating them from seismic noise (low frequencies)and designing them to have low thermal noise (intermediate frequencies).Each test mass is suspended as the final stage of a quadruple-pendulum system [56],supported by an active seismic isolation platform [57].These systems collectively provide more than 10orders of magnitude of isolation from ground motion for frequen-cies above 10Hz.Thermal noise is minimized by using low-mechanical-loss materials in the test masses and their suspensions:the test masses are 40-kg fused silica substrates with low-loss dielectric optical coatings [58,59],and are suspended with fused silica fibers from the stage above [60].To minimize additional noise sources,all components other than the laser source are mounted on vibration isolation stages in ultrahigh vacuum.To reduce optical phase fluctuations caused by Rayleigh scattering,the pressure in the 1.2-m diameter tubes containing the arm-cavity beams is maintained below 1μPa.Servo controls are used to hold the arm cavities on resonance [61]and maintain proper alignment of the optical components [62].The detector output is calibrated in strain by measuring its response to test mass motion induced by photon pressure from a modulated calibration laser beam [63].The calibration is established to an uncertainty (1σ)of less than 10%in amplitude and 10degrees in phase,and is continuously monitored with calibration laser excitations at selected frequencies.Two alternative methods are used to validate the absolute calibration,one referenced to the main laser wavelength and the other to a radio-frequencyoscillator(a)FIG.3.Simplified diagram of an Advanced LIGO detector (not to scale).A gravitational wave propagating orthogonally to the detector plane and linearly polarized parallel to the 4-km optical cavities will have the effect of lengthening one 4-km arm and shortening the other during one half-cycle of the wave;these length changes are reversed during the other half-cycle.The output photodetector records these differential cavity length variations.While a detector ’s directional response is maximal for this case,it is still significant for most other angles of incidence or polarizations (gravitational waves propagate freely through the Earth).Inset (a):Location and orientation of the LIGO detectors at Hanford,WA (H1)and Livingston,LA (L1).Inset (b):The instrument noise for each detector near the time of the signal detection;this is an amplitude spectral density,expressed in terms of equivalent gravitational-wave strain amplitude.The sensitivity is limited by photon shot noise at frequencies above 150Hz,and by a superposition of other noise sources at lower frequencies [47].Narrow-band features include calibration lines (33–38,330,and 1080Hz),vibrational modes of suspension fibers (500Hz and harmonics),and 60Hz electric power grid harmonics.[64].Additionally,the detector response to gravitational waves is tested by injecting simulated waveforms with the calibration laser.To monitor environmental disturbances and their influ-ence on the detectors,each observatory site is equipped with an array of sensors:seismometers,accelerometers, microphones,magnetometers,radio receivers,weather sensors,ac-power line monitors,and a cosmic-ray detector [65].Another∼105channels record the interferometer’s operating point and the state of the control systems.Data collection is synchronized to Global Positioning System (GPS)time to better than10μs[66].Timing accuracy is verified with an atomic clock and a secondary GPS receiver at each observatory site.In their most sensitive band,100–300Hz,the current LIGO detectors are3to5times more sensitive to strain than initial LIGO[67];at lower frequencies,the improvement is even greater,with more than ten times better sensitivity below60Hz.Because the detectors respond proportionally to gravitational-wave amplitude,at low redshift the volume of space to which they are sensitive increases as the cube of strain sensitivity.For binary black holes with masses similar to GW150914,the space-time volume surveyed by the observations reported here surpasses previous obser-vations by an order of magnitude[68].IV.DETECTOR VALIDATIONBoth detectors were in steady state operation for several hours around GW150914.All performance measures,in particular their average sensitivity and transient noise behavior,were typical of the full analysis period[69,70]. Exhaustive investigations of instrumental and environ-mental disturbances were performed,giving no evidence to suggest that GW150914could be an instrumental artifact [69].The detectors’susceptibility to environmental disturb-ances was quantified by measuring their response to spe-cially generated magnetic,radio-frequency,acoustic,and vibration excitations.These tests indicated that any external disturbance large enough to have caused the observed signal would have been clearly recorded by the array of environ-mental sensors.None of the environmental sensors recorded any disturbances that evolved in time and frequency like GW150914,and all environmental fluctuations during the second that contained GW150914were too small to account for more than6%of its strain amplitude.Special care was taken to search for long-range correlated disturbances that might produce nearly simultaneous signals at the two sites. No significant disturbances were found.The detector strain data exhibit non-Gaussian noise transients that arise from a variety of instrumental mecha-nisms.Many have distinct signatures,visible in auxiliary data channels that are not sensitive to gravitational waves; such instrumental transients are removed from our analyses [69].Any instrumental transients that remain in the data are accounted for in the estimated detector backgrounds described below.There is no evidence for instrumental transients that are temporally correlated between the two detectors.V.SEARCHESWe present the analysis of16days of coincident observations between the two LIGO detectors from September12to October20,2015.This is a subset of the data from Advanced LIGO’s first observational period that ended on January12,2016.GW150914is confidently detected by two different types of searches.One aims to recover signals from the coalescence of compact objects,using optimal matched filtering with waveforms predicted by general relativity. The other search targets a broad range of generic transient signals,with minimal assumptions about waveforms.These searches use independent methods,and their response to detector noise consists of different,uncorrelated,events. However,strong signals from binary black hole mergers are expected to be detected by both searches.Each search identifies candidate events that are detected at both observatories consistent with the intersite propa-gation time.Events are assigned a detection-statistic value that ranks their likelihood of being a gravitational-wave signal.The significance of a candidate event is determined by the search background—the rate at which detector noise produces events with a detection-statistic value equal to or higher than the candidate event.Estimating this back-ground is challenging for two reasons:the detector noise is nonstationary and non-Gaussian,so its properties must be empirically determined;and it is not possible to shield the detector from gravitational waves to directly measure a signal-free background.The specific procedure used to estimate the background is slightly different for the two searches,but both use a time-shift technique:the time stamps of one detector’s data are artificially shifted by an offset that is large compared to the intersite propagation time,and a new set of events is produced based on this time-shifted data set.For instrumental noise that is uncor-related between detectors this is an effective way to estimate the background.In this process a gravitational-wave signal in one detector may coincide with time-shifted noise transients in the other detector,thereby contributing to the background estimate.This leads to an overestimate of the noise background and therefore to a more conservative assessment of the significance of candidate events.The characteristics of non-Gaussian noise vary between different time-frequency regions.This means that the search backgrounds are not uniform across the space of signals being searched.To maximize sensitivity and provide a better estimate of event significance,the searches sort both their background estimates and their event candidates into differ-ent classes according to their time-frequency morphology. The significance of a candidate event is measured against the background of its class.To account for having searchedmultiple classes,this significance is decreased by a trials factor equal to the number of classes [71].A.Generic transient searchDesigned to operate without a specific waveform model,this search identifies coincident excess power in time-frequency representations of the detector strain data [43,72],for signal frequencies up to 1kHz and durations up to a few seconds.The search reconstructs signal waveforms consistent with a common gravitational-wave signal in both detectors using a multidetector maximum likelihood method.Each event is ranked according to the detection statistic ηc ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2E c =ð1þE n =E c Þp ,where E c is the dimensionless coherent signal energy obtained by cross-correlating the two reconstructed waveforms,and E n is the dimensionless residual noise energy after the reconstructed signal is subtracted from the data.The statistic ηc thus quantifies the SNR of the event and the consistency of the data between the two detectors.Based on their time-frequency morphology,the events are divided into three mutually exclusive search classes,as described in [41]:events with time-frequency morphology of known populations of noise transients (class C1),events with frequency that increases with time (class C3),and all remaining events (class C2).Detected with ηc ¼20.0,GW150914is the strongest event of the entire search.Consistent with its coalescence signal signature,it is found in the search class C3of events with increasing time-frequency evolution.Measured on a background equivalent to over 67400years of data and including a trials factor of 3to account for the search classes,its false alarm rate is lower than 1in 22500years.This corresponds to a probability <2×10−6of observing one or more noise events as strong as GW150914during the analysis time,equivalent to 4.6σ.The left panel of Fig.4shows the C3class results and background.The selection criteria that define the search class C3reduce the background by introducing a constraint on the signal morphology.In order to illustrate the significance of GW150914against a background of events with arbitrary shapes,we also show the results of a search that uses the same set of events as the one described above but without this constraint.Specifically,we use only two search classes:the C1class and the union of C2and C3classes (C 2þC 3).In this two-class search the GW150914event is found in the C 2þC 3class.The left panel of Fig.4shows the C 2þC 3class results and background.In the background of this class there are four events with ηc ≥32.1,yielding a false alarm rate for GW150914of 1in 8400years.This corresponds to a false alarm probability of 5×10−6equivalent to 4.4σ.FIG.4.Search results from the generic transient search (left)and the binary coalescence search (right).These histograms show the number of candidate events (orange markers)and the mean number of background events (black lines)in the search class where GW150914was found as a function of the search detection statistic and with a bin width of 0.2.The scales on the top give the significance of an event in Gaussian standard deviations based on the corresponding noise background.The significance of GW150914is greater than 5.1σand 4.6σfor the binary coalescence and the generic transient searches,respectively.Left:Along with the primary search (C3)we also show the results (blue markers)and background (green curve)for an alternative search that treats events independently of their frequency evolution (C 2þC 3).The classes C2and C3are defined in the text.Right:The tail in the black-line background of the binary coalescence search is due to random coincidences of GW150914in one detector with noise in the other detector.(This type of event is practically absent in the generic transient search background because they do not pass the time-frequency consistency requirements used in that search.)The purple curve is the background excluding those coincidences,which is used to assess the significance of the second strongest event.For robustness and validation,we also use other generic transient search algorithms[41].A different search[73]and a parameter estimation follow-up[74]detected GW150914 with consistent significance and signal parameters.B.Binary coalescence searchThis search targets gravitational-wave emission from binary systems with individual masses from1to99M⊙, total mass less than100M⊙,and dimensionless spins up to 0.99[44].To model systems with total mass larger than 4M⊙,we use the effective-one-body formalism[75],whichcombines results from the post-Newtonian approach [11,76]with results from black hole perturbation theory and numerical relativity.The waveform model[77,78] assumes that the spins of the merging objects are alignedwith the orbital angular momentum,but the resultingtemplates can,nonetheless,effectively recover systemswith misaligned spins in the parameter region ofGW150914[44].Approximately250000template wave-forms are used to cover this parameter space.The search calculates the matched-filter signal-to-noiseratioρðtÞfor each template in each detector and identifiesmaxima ofρðtÞwith respect to the time of arrival of the signal[79–81].For each maximum we calculate a chi-squared statisticχ2r to test whether the data in several differentfrequency bands are consistent with the matching template [82].Values ofχ2r near unity indicate that the signal is consistent with a coalescence.Ifχ2r is greater than unity,ρðtÞis reweighted asˆρ¼ρ=f½1þðχ2rÞ3 =2g1=6[83,84].The final step enforces coincidence between detectors by selectingevent pairs that occur within a15-ms window and come fromthe same template.The15-ms window is determined by the10-ms intersite propagation time plus5ms for uncertainty inarrival time of weak signals.We rank coincident events basedon the quadrature sumˆρc of theˆρfrom both detectors[45]. To produce background data for this search the SNR maxima of one detector are time shifted and a new set of coincident events is computed.Repeating this procedure ∼107times produces a noise background analysis time equivalent to608000years.To account for the search background noise varying acrossthe target signal space,candidate and background events aredivided into three search classes based on template length.The right panel of Fig.4shows the background for thesearch class of GW150914.The GW150914detection-statistic value ofˆρc¼23.6is larger than any background event,so only an upper bound can be placed on its false alarm rate.Across the three search classes this bound is1in 203000years.This translates to a false alarm probability <2×10−7,corresponding to5.1σ.A second,independent matched-filter analysis that uses adifferent method for estimating the significance of itsevents[85,86],also detected GW150914with identicalsignal parameters and consistent significance.When an event is confidently identified as a real gravitational-wave signal,as for GW150914,the back-ground used to determine the significance of other events is reestimated without the contribution of this event.This is the background distribution shown as a purple line in the right panel of Fig.4.Based on this,the second most significant event has a false alarm rate of1per2.3years and corresponding Poissonian false alarm probability of0.02. Waveform analysis of this event indicates that if it is astrophysical in origin it is also a binary black hole merger[44].VI.SOURCE DISCUSSIONThe matched-filter search is optimized for detecting signals,but it provides only approximate estimates of the source parameters.To refine them we use general relativity-based models[77,78,87,88],some of which include spin precession,and for each model perform a coherent Bayesian analysis to derive posterior distributions of the source parameters[89].The initial and final masses, final spin,distance,and redshift of the source are shown in Table I.The spin of the primary black hole is constrained to be<0.7(90%credible interval)indicating it is not maximally spinning,while the spin of the secondary is only weakly constrained.These source parameters are discussed in detail in[39].The parameter uncertainties include statistical errors and systematic errors from averaging the results of different waveform models.Using the fits to numerical simulations of binary black hole mergers in[92,93],we provide estimates of the mass and spin of the final black hole,the total energy radiated in gravitational waves,and the peak gravitational-wave luminosity[39].The estimated total energy radiated in gravitational waves is3.0þ0.5−0.5M⊙c2.The system reached apeak gravitational-wave luminosity of3.6þ0.5−0.4×1056erg=s,equivalent to200þ30−20M⊙c2=s.Several analyses have been performed to determine whether or not GW150914is consistent with a binary TABLE I.Source parameters for GW150914.We report median values with90%credible intervals that include statistical errors,and systematic errors from averaging the results of different waveform models.Masses are given in the source frame;to convert to the detector frame multiply by(1þz) [90].The source redshift assumes standard cosmology[91]. Primary black hole mass36þ5−4M⊙Secondary black hole mass29þ4−4M⊙Final black hole mass62þ4−4M⊙Final black hole spin0.67þ0.05−0.07 Luminosity distance410þ160−180MpcSource redshift z0.09þ0.03−0.04。

天文学专业词汇CAMC, Carlsberg Automatic Meridian 卡尔斯伯格自动子午环Circlecannibalism 吞食cannibalized galaxy 被吞星系cannibalizing galaxy 吞食星系cannibalizing of galaxies 星系吞食carbon dwarf 碳矮星Cassegrain spectrograph 卡焦摄谱仪Cassini 〈卡西尼〉土星探测器Cat's Eye nebula （ NGC 6543 ）猫眼星云CCD astronomy CCD 天文学CCD camera CCD 照相机CCD photometry CCD 测光CCD spectrograph CCD 摄谱仪CCD spectrum CCD 光谱celestial clock 天体钟celestial mechanician 天体力学家celestial thermal background 天空热背景辐射celestial thermal background radiation 天空热背景辐射central overlap technique 中心重迭法Centaurus arm 半人马臂Cepheid distance 造父距离CFHT, Canada-Franch-Hawaii Telecope 〈CFHT〉望远镜CGRO, Compton Gamma-Ray Observatory 〈康普顿〉γ射线天文台chaos 混沌chaotic dynamics 混沌动力学chaotic layer 混沌层chaotic region 混沌区chemically peculiar star 化学特殊星Christmas Tree cluster （ NGC 2264 ）圣诞树星团chromosphere-corona transition zone 色球-日冕过渡层chromospheric activity 色球活动chromospherically active banary 色球活动双星chromospherically active star 色球活动星chromospheric line 色球谱线chromospheric matirial 色球物质chromospheric spectrum 色球光谱CID, charge injected device CID、电荷注入器件circular solution 圆轨解circumnuclear star-formation 核周产星circumscribed halo 外接日晕circumstellar dust disk 星周尘盘circumstellar material 星周物质circumsystem material 双星周物质classical Algol system 经典大陵双星classical quasar 经典类星体classical R Coronae Borealis star 经典北冕 R 型星classical T Tauri star 经典金牛 T 型星Clementine 〈克莱芒蒂娜〉环月测绘飞行器closure phase imaging 锁相成象cluster centre 团中心cluster galaxy 团星系COBE, Cosmic Background Explorer 宇宙背景探测器coded mask imaging 编码掩模成象coded mask telescope 编码掩模望远镜collapsing cloud 坍缩云cometary burst 彗暴cometary dynamics 彗星动力学cometary flare 彗耀cometary H Ⅱ region 彗状电离氢区cometary outburst 彗爆发cometary proplyd 彗状原行星盘comet shower 彗星雨common proper-motion binary 共自行双星common proper-motion pair 共自行星对compact binary galaxy 致密双重星系天文学专业词汇compact cluster 致密星团; 致密星系团compact flare 致密耀斑composite diagram method 复合图法composite spectrum binary 复谱双星computational astrophysics 计算天体物理computational celestial mechanics 计算天体力学contact copying 接触复制contraction age 收缩年龄convective envelope 对流包层cooling flow 冷却流co-orbital satellite 共轨卫星coplanar orbits 共面轨道Copernicus 〈哥白尼〉卫星coprocessor 协处理器Cordelia 天卫六core-dominated quasar （ CDQ ）核占优类星体coronal abundance 冕区丰度coronal activity 星冕活动、日冕活动coronal dividing line 冕区分界线coronal gas 星冕气体、日冕气体coronal green line 星冕绿线、日冕绿线coronal helmet 冕盔coronal magnetic energy 冕区磁能coronal red line 星冕红线、日冕红线cosmic abundance 宇宙丰度cosmic string 宇宙弦cosmic void 宇宙巨洞COSMOS 〈COSMOS〉底片自动测量仪C-O white dwarf 碳氧白矮星Cowling approximation 柯林近似Cowling mechnism 柯林机制Crescent nebula （ NGC 6888 ）蛾眉月星云Cressida 天卫九critical equipotential lobe 临界等位瓣cross-correlation method 交叉相关法cross-correlation technique 交叉相关法cross disperser prism 横向色散棱镜crustal dynamics 星壳动力学cryogenic camera 致冷照相机cushion distortion 枕形畸变cut-off error 截断误差Cyclops project 〈独眼神〉计划D abundance 氘丰度Dactyl 艾卫dark halo 暗晕data acquisition 数据采集decline phase 下降阶段deep-field observation 深天区观测density arm 密度臂density profile 密度轮廓dereddening 红化改正Desdemona 天卫十destabiliizing effect 去稳效应dew shield 露罩diagonal mirror 对角镜diagnostic diagram 诊断图differential reddening 较差红化diffuse density 漫射密度diffuse dwarf 弥漫矮星系diffuse X-ray 弥漫 X 射线diffusion approximation 扩散近似digital optical sky survey 数字光学巡天digital sky survey 数字巡天disappearance 掩始cisconnection event 断尾事件dish 碟形天线disk globular cluster 盘族球状星团dispersion measure 频散量度dissector 析象管distance estimator 估距关系distribution parameter 分布参数disturbed galaxy 受扰星系disturbing galaxy 扰动星系Dobsonian mounting 多布森装置Dobsonian reflector 多布森反射望远镜Dobsonian telescope 多布森望远镜dominant galaxy 主星系double-mode cepheid 双模造父变星double-mode pulsator 双模脉动星double-mode RR Lyrae star 双模天琴 RR 型星double-ring galaxy 双环星系DQ Herculis star 武仙 DQ 型星dredge-up 上翻drift scanning 漂移扫描driving system 驱动系统dumbbell radio galaxy 哑铃状射电星系Du Pont Telescope 杜邦望远镜dust ring 尘环dwarf carbon star 碳矮星dwarf spheroidal 矮球状星系dwarf spheroidal galaxy 矮球状星系dwarf spiral 矮旋涡星系dwarf spiral galaxy 矮旋涡星系dynamical age 动力学年龄dynamical astronomy 动力天文dynamical evolution 动力学演化。

GRE阅读高频机经原文：蓝脱序星的两种形成过程gre阅读是许多考生难以攻克的大山，下面先来看看GRE阅读高频机经原文：蓝脱序星的两种形成过程，一起来学习吧！GRE阅读高频机经原文：蓝脱序星的两种形成过程蓝脱序星blue straggler的两种形成过程Vampires and collisions rejuvenate starsUsing the NASA/ESA Hubble Space Telescope, astronomers have uncovered two distinct kinds of "rejuvenated" stars in the globular cluster Messier 30. A new study shows that both stellar collisions and a process sometimes called vampirism are behind this cosmic "face lift". The scientists also uncover evidence that both sorts of blue stragglers were produced during a critical dynamical event (known as "core collapse") that occurred in Messier 30 a few billion years ago.Stars in globular clusters [1] are generally extremely old, with ages of 12-13 billion years. However, a small fraction of them appear to be significantly younger than the average population and, because they seem to have been left behind by the stars that followed the normal path of stellar evolution and became red giants, have been dubbed blue stragglers [2]. Blue stragglers appear to regress from "old age" back to a hotter and brighter "youth", gaining a new lease on life in the process. A team of astronomers used Hubble to study the blue straggler star content in Messier 30, which formed 13 billion years ago and was discovered in 1764 by Charles Messier. Located about 28 000 light-years away from Earth, this globular cluster — a swarm of several hundred thousand stars — is about 90 light-years across.Although blue stragglers have been known since the early 1950s, their formation process is still an unsolved puzzle in astrophysics. "It’s like seeing a few kids in the group picture of arest-home for retired people. It is natural to wonder why they are there," says Francesco Ferraro from the University of Bologna in Italy, lead author of the study that will be published this week in Nature [3]. Researchers have been studying these stars for many years and knew that bluestragglers are indeed old. They were thought to have arisen in a tight binary system [4]. In such a pair, the less massive star acts as a "vampire", siphoning fresh hydrogen from its more massive companion star. The new fuel supply allows the smaller star to heat up, growing bluer and hotter — behaving like a star at an earlier stage in its evolution.The new study shows that some of the blue stragglers have instead been rejuvenated by a sort of "cosmic facelift", courtesy of cosmic collisions. These stellar encounters are nearlyhead-on collisions in which the stars might actually merge, mixing their nuclear fuel andre-stoking the fires of nuclear fusion. Merged stars and binary systems would both be about twice the typical mass of individual stars in the cluster."Our observations demonstrate that blue stragglers formed by collisions have slightly different properties from those formed by vampirism. This provides a direct demonstration that the two formation scenarios are valid and that they are both operating simultaneously in this cluster," says team member Giacomo Beccari from ESA.Using data from the now-retired Wide Field Planetary Camera 2 (WFPC2) aboard Hubble, astronomers found that these "straggling" stars are much more concentrated towards the centre of the cluster than the average star. "This indicates that blue stragglers are more massive than the average star in this cluster," says Ferraro. "More massive stars tend to sink deep into the cluster the way a billiard ball would sink in a bucket of honey."The central regions of high density globular clusters are crowded neighbourhoods where interactions between stars are nearly inevitable. Researchers conjecture that one or two billion years ago, Messier 30 underwent a major "core collapse" that started to throw stars towards the centre of the cluster, leading to a rapid increase in the density of stars. This event significantly increased the number of collisions among stars, and favoured the formation of one of the families of blue stragglers. On the other hand, the increase of stellar crowding due to the collapse of the core also perturbed the twin systems, encouraging the vampirism phenomenon and thus forming the other family of blue stragglers. "Almost ten percent of galactic globular clusters have experienced core collapse, but this is the first time that we see the effect of the core collapse imprinted on a stellar population," says Barbara Lanzoni, University of Bologna."The two distinct populations of blue stragglers discovered in Messier 30 are the relics of the collapse of the core that occurred two billion years ago. In a broad context our discovery is direct evidence of the impact of star cluster dynamics on stellar evolution. We should now try to see if other globular clusters present this double population of blue stragglers," concludes Ferraro.GRE阅读词汇精选之渗透douse v.把…浸入水中，用水泼drenched adj.湿透的soak v.浸泡，渗透soaked adj.湿透的sodden adj.浸透了的soggy adj.湿透的，濡湿的souse v.浸在水中，使湿透steep v.浸泡，浸透logged adj笨重的，湿透的immerse v.浸入，沉浸于immersion n.沉入，浸入macerate v.浸软，消瘦GRE阅读表示选择的逻辑词汇总逻辑词条词性例句选择otherwiseadv.You need to work hard. Otherwise, you will fail.选择or (else)conj.You need to work hard, or (else) you will fail.选择lestconj.You need to work hard, lest you fail the exam.选择in caseconj.You need to work hard, in case the exam is hard. GRE阅读表示转折的逻辑词汇总逻辑词条词性例句转折butconj.I worked hard, but I failed.转折howeveradv.I worked hard. However, I failed.转折neverthelessadv.I worked hard. Nevertheless, I failed.转折stilladv.I worked hard. Still, I failed.转折nonethelessadv.I worked hard. Nonetheless, I failed.转折thoughadv.I worked hard. I failed, though.。

a r X i v :0712.1036v 2 [a s t r o -p h ] 16 A p r 2008Draft version April 16,2008Preprint typeset using L A T E X style emulateapjTHE LOWEST-MASS STELLAR BLACK HOLES:CATASTROPHIC DEATH OF NEUTRON STARS INGAMMA-RAY BURSTSK.Belczynski 1,R.O’Shaughnessy 2,V.Kalogera 3,F.Rasio 3,R.E.Taam 3,T.Bulik 41Los Alamos National Laboratory (Oppenheimer Fellow)2Penn State University 3Northwestern University 4Warsaw UniversityDraft version April 16,2008ABSTRACTMergers of double neutron stars are considered the most likely progenitors for short gamma-ray bursts.Indeed such a merger can produce a black hole with a transient accreting torus of nuclear matter (Lee &Ramirez-Ruiz 2007,Oechslin &Janka 2006),and the conversion of a fraction of the torus mass-energy to radiation can power a gamma-ray burst (Nakar 2006).Using available binary pulsar observations supported by our extensive evolutionary calculations of double neutron star formation,we demonstrate that the fraction of mergers that can form a black hole –torus system depends very sensitively on the (largely unknown)maximum neutron star mass.We show that the available observations and models put a very stringent constraint on this maximum mass under the assumption that a black hole formation is required to produce a short gamma-ray burst in a double neutron star merger.Specifically,we find that the maximum neutron star mass must be within 2−2.5M ⊙.Moreover,a single unambiguous measurement of a neutron star mass above 2.5M ⊙would exclude a black hole –torus central engine model of short gamma-ray bursts in double neutron star mergers.Such an observation would also indicate that if in fact short gamma-ray bursts are connected to neutron star mergers,the gamma-ray burst engine is best explained by the lesser known model invoking a highly magnetized massive neutron star (e.g.,Usov 1992;Kluzniak &Ruderman 1998;Dai et al.2006;Metzger,Quataert &Thompson 2007).1.INTRODUCTIONGamma-ray bursts (GRBs)have been separated into two classes:long-soft bursts,and short bursts (Nakar 2006,Gehrels et al.2007).The origin of long-soft bursts has been connected to the death of low-metallicity massive stars (Piran 2005,Gehrels et al.2007).However,while ob-servations support a binary merger origin for short bursts (Nakar 2006,Gehrels et al.2007),the exact nature of the progenitor remains uncertain:they could be either dou-ble neutron stars (NS–NS)or black hole –neutron star (BH–NS)binaries.The number of BH–NS binaries that both merge and produce GRBs is hard to estimate since (i)no such system has yet been observed;(ii)formation models are rather uncertain and predict very small BH–NS merger rates (likely too small to explain most of the short bursts);and (iii)theory suggests that the fraction of BH–NS mergers producing bursts depends sensitively on the black hole spin and spin–orbit orientation (Belczyn-ski et al.2007b),but black hole birth spins are not well constrained observationally or theoretically.On the other hand,NS–NS binaries are only observed in the Milky Way,but their properties and numbers are also in agreement with theoretical models,and their merger rate is sufficient to explain the present-day short burst population (Nakar 2006,Belczynski et al.2007a).We have performed an extensive theoretical study of high-mass binary stars (potential progenitors of NS–NS systems)using StarTrack ,a population synthesis code in-corporating the most up-to-date and detailed input physicsfor massive stars (Belczynski et al 2008).The code em-ploys state-of-the-art predictions for neutron star and black hole masses based on hydrodynamic core collapse simulations (Fryer &Kalogera 2001)and detailed stel-lar structure and evolution calculations for massive stars (Timmes,Woosley,&Weaver 1996).Our models predict a Galactic NS–NS merger rate in the range ∼10−100Myr −1(Belczynski et al.2007a),in good agreement with the em-pirical estimate of ∼3−190Myr −1(Kim,Kalogera,&Lorimer,2006).The spread in our predicted rates orig-inates from including the most significant model uncer-tainties associated with the treatment of dynamical mass transfer episodes (common envelope phases),which are in-volved in the formation of most double compact objects (Belczynski et al.2007a).2.RESULTSIn Figure 1we compare short GRB rates with NS–NS merger rates in the present-day (redshift 0)universe.Ex-trapolating the NS–NS merger rates to the local universe by assuming a star-forming density of 10−2Milky Way-equivalents per Mpc 31,we estimate the local universe NS–NS merger rate to be in the range ∼100−1000Gpc −3yr −1.By comparison,the estimated conservative lower limit on the short GRB rate is ∼10Gpc −3yr −1,based on the BATSE/SWIFT sample (Nakar 2006).This estimate re-lies on very conservative assumptions:(i)there is no colli-mation and (ii)there are no bursts dimmer than we have already observed,thus providing a true lower limit on the 1Introductionof different modes of star formation and redistribution of progenitors over old elliptical and younger spiral galaxies does notincrease the predicted merger rates by more than a factor of ∼3O’Shaughnessy et al.2007.12rate.Therefore,even adopting the most optimistic predic-tions for the NS–NS merger rate and the most pessimistic bound on the local short GRB event rate,the fraction of NS–NS mergers f grb that produce GRBs must be greater than at least10−2to explain the majority of known short bursts.In this paper we start with the assumption that all short GRBs are connected with NS-NS system mergers that pro-duce a black hole.We discuss the implications of relaxing this stringent assumption at the end of the paper.From our models we also derive physical properties of double neutron stars,with individual masses of neutron stars being of particular interest.Figure2shows the rela-tion between progenitor(single star)mass andfinal rem-nant mass used in our evolutionary calculations.Mass transfer and other binary interactions change this simple picture,through both accretion and mass loss,which can either increase or decrease an individual binary compo-nent mass.However,(Belczynski et al.2007b)argues that we do not expect significant mass accretion onto the components of NS–NS binaries.The population model we adopt for our discussion here produces NS mass distri-butions that appear consistent with the current observed NS-NS sample,at least in the extent of the mass ranges (Fig.3).While mass transfer does influence the remnant masses(e.g.,smearing the narrowly peaked mass distribu-tion implied by Fig.2),the qualitative structure is largely preserved,as one would expect from isolated stellar evo-lution combined with an initial mass function that falls steeply with increasing initial mass.The predicted neu-tron star mass distribution only very weakly depends on evolutionary model assumptions because the neutron star formation mass is almost independent of the progenitor mass(Timmes et al.1996)and mass accretion in NS-NS progenitor binaries is rather small(Belczynski et al. 2007a).Depending on the masses in the progenitor binary and the highly uncertain nuclear equation of state,thefinal remnant of a NS–NS merger may or may not collapse to a black hole.We estimate thefinal gravitational mass of the compact remnant asM rem=0.9(M ns,1+M ns,2−0.1M⊙),(1) where the initial neutron star masses are denoted by M ns,1,M ns,2and we have assumed that the torus mass is sufficiently large to power a GRB(i.e.,≃0.1M⊙)(Seti-awan,Ruffert,&Janka2004,Lee&Ramirez-Ruiz2007) and that10%of rest mass is lost in neutrinos(Lattimer &Yahli1989;Timmes et al.1996).Higher rest mass loss(e.g.,Metzger,Thompson&Quataert2007)would only strengthen our subsequent conclusions.Because stars more massive than18M⊙(progenitors of massive neutron stars with M ns≃1.8M⊙)are much rarer than those form-ing lighter neutron stars(M ns≃1.35M⊙)we a priori ex-pect that most remnants from NS–NS mergers will have rather low mass M rem≃2.3M⊙(see eq.1for two1.35M⊙neutron stars).Neither observations nor nuclear theory have yet pinned down the maximum neutron star mass M ns,max above which a black hole must form.Thus,the fraction of binary neutron stars which produce black holes and are able to power short GRBs is set by the fraction of mergers such thatM rem≥M ns,max.(2) We therefore calculate the fraction of our simulated NS–NS mergers that lead to black hole formation and a short GRB as a function of M ns,max;see Fig.4.Observations of the highest mass neutron stars(∼<2M⊙;(Barziv et al. 2001,Ransom et al.2005)and lowest mass black holes(∼> 3M⊙;(Orosz2003,Casares2006)only weakly constrain this parameter.Remnant masses from NS–NS mergers (M rem)obtained both from our simulations and from ob-servations all fall very close to the range2.2−2.5M⊙. Comparing Figs1and4we immediately deduce that, since the fraction f grb of NS–NS mergers that produce short GRBs must be greater than10−2(Fig.1),the neu-tron star maximum mass M ns,max must be less than2.5M⊙(Fig.4).Because we lack a robust lower bound on the mass of the residual torus surrounding the black hole,we have adopted a conservative upper limit on M ns,max obtained by assuming a negligible torus mass(i.e.,replace0.1M⊙with0in eq.1).This result has been obtained with the assumption that all(k=1.0)short GRBs are connected with NS-NS merg-ers.It is however possible that only a fraction of short GRBs is produced in NS-NS mergers.How does our result depend on the fraction k of short GRBs that are connected with NS-NS mergers?The lower limit on f grb is then de-creased by k,see Figure1.If k∼>0.1,then the limit lower limit becomes f grb>10−3,and as is clearly seen from Fig-ure3,the upper limit on the maximum mass of a neutron star remains unchanged.For the values of k∼<0.01the NS-NS mergers are not important for overall short GRB population,as the mergers would consist of only∼<1%of the short GRBs.Therefore,in this case short GRBs do not provide information about the merger product.3.DISCUSSIONOur proposed limit on the maximum neutron star mass is still above the maximum masses allowed by almost all proposed models for the nuclear equation of state(Lat-timer&Prakash2007).However,the proposed limit would remain unchanged even if a dramatic improvement in short GRB surveys led to a significantly larger lower bound on the local short GRB rate:because of the sharp decrease in f grb with M ns,max shown in Fig.4.If,however, electromagnetic observations could constrain the least lu-minous short GRBs and thus provide an upper bound on the short GRB rate,with gravitational wave obser-vations at the same time accurately determining the NS–NS merger rate,then f grb could also be constrained from above.If only a fraction of NS–NS mergers produce short bursts,because f grb depends so sensitively on M ns,max,the combination of upper and lower limits would constrain the maximum neutron star mass extremely tightly,even if the assumptions going into eq.1are relaxed.While our limit on the effective maximum neutron star mass is entirely empirical,detailed merger models in-cluding realistic relativistic dynamics,neutrino transport, magneticfields,and potentially even energy extraction from thefinal black hole remain under intense investi-gation(Janka&Ruffert1996;Oechslin&Janka2006). Many merger remnants are expected to be(temporarily) rotationally supported against collapse(Morrison,Baum-3garte&Shapiro2004),with a“hypermassive”remnant neutron star eventually spinning down and collapsing to a black hole(Faber et al.2006;Duez et al.2007;Shibata& Taniguchi2006).Our model only relies upon the current consensus on double neutron star mergers,as summarized by Oechslin&Janka(2006):sufficiently massive binary mergers produce a black hole and only mergers that pro-duce a black hole extract enough energy to power short GRBs.Known Galactic black holes extend in mass up to 10−15M⊙(Casares2006),while two recently discovered black hole candidates in other galaxies(Orosz et al.2007, Prestwich et al.2007)have even higher masses of≃16 and∼>24M⊙.Clearly black holes can form with rather high masses in different types of environments.The lower mass limit is not well constrained observationally,as the highest-mass neutron stars barely reach2M⊙,while the lowest-mass black holes are above3M⊙.In order to ex-plain the observed short GRBs with NS–NS mergers,un-der the assumption of black hole–torus central engine model,we have shown that the maximum neutron star mass must be lower than2.5M⊙.However,pulsar surveys (Ransom et al.2005)have discovered increasingly more massive neutron stars.So far in our analysis we have in-cluded only NS–NS systems formed in thefield.There is one known relativistic double neutron star system in the Galactic globular cluster M15,that has probably formed through dynamical interactions.This binary consists of two low-mass neutron stars(1.36and1.35M⊙;Jacoby et al.2006)very similar to those in the Galacticfield,so the results of our analysis are not changed by this isolated ob-servation.Moreover,it was estimated that no more than 10−30%short GRBs can originate from mergers of dou-ble neutron stars formed in globular clusters(Grindlay, Portegies Zwart,&McMillan2006).If any observation can be made that establishes unam-biguously a pulsar mass(either in thefield or a globular cluster)over2.5M⊙,this would exclude a black hole–torus short GRB central engine model for double neutron star mergers.We note that a tentative mass measurement for a pulsar of2.74±0.21M⊙was recently reported by (Freire et al.2007).If this measurement is confirmed,the double neutron star mergers may still be possible progen-itors for short GRBs.However,the central engine model will need to be reexamined.In particular,it was proposed that a merger of two neutron stars may lead to the forma-tion of a magnetar;a rapidly rotating highly magnetized and high mass neutron star(with or without a torus)that can lead to a short GRB(e.g.,Usov1992;Kluzniak&Ru-derman1998;Dai et al.2006;Metzger et al.2007).Grav-itational wave observatories(LIGO,VIRGO)may provide the direct evidence that NS-NS merger can produce a short GRB if there is a coincidence of the burst and the inspiral gravitational wave signal.It may also be possible to dis-tinguish a merger product(NS versus BH)from the shape of the merger and ringdown signal or from radio pulses if a magnetar was formed in a nearby gamma-ray burst event.We would like to thank Neil Gehrels,Duncan Lorimer, Scott Ransom,Ben Owen,Jerome Orosz,Chris Stanek, John Beacom,Paulo Freire,Chunglee Kim,Todd Thomp-son and an anonymous referee for very useful discussions.REFERENCESBarziv,O.,Kaper,L.,Van Kerkwijk,M.H.,Telting,J.H.,&Van Paradijs,J.2001,A&A,377,925Belczynski,K.,Kalogera,V.,Rasio,F.,Taam,R.,Zezas,A.,Mac-carone,T.,&Ivanova,N.2008,ApJS,174,223Belczynski,K.,Taam,R.E.,Kalogera,V.,Rasio,F.A.,&Bulik,T.2007,ApJ,662,504Belczynski,K.,Taam,R.E.,Rantsiou,E.,&Sluys,M.v.d.2007, ApJ,submitted(astro-ph/0703131)Casares,J.2006,IAU Symposium238:”Black Holes:From Stars to Galaxies-Across the Range of Masses”,in press (astro-ph/0612312)Dai,Z.,Wang,X.,Wu,X.,&Zhang,B.2006,Science,311,1127 Duez,M.D.,Liu,Y.T.,Shapiro,S.L.,Shibata,M.,&Stephens,B.C.2007,Proceedings of the Eleventh Marcel Grossmann Meet-ing,in 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short GRB event rate (Nakar 2006),based solely on the rate of detected bursts.Depending on the amount of beaming and the fraction of distant faint short GRBs that are missed,the true event rate is often estimated to be at least 10times larger (Nakar 2006).This lower limit is smaller than the double neutron star merger rate estimated for the Milky Way both from (i)observations of Galactic binary pulsars (filled blue region)and (ii)our population synthesis simulations (filled red region),when these two estimates are extrapolated to cosmological scales.Based on the maximum plausible double neutron star merger rate with the minimum plausible short GRB event rate,the fraction f grb of binary mergers that lead to short GRBs should be greater than 10−2if double neutron stars are the progenitors of short GRBs.557.51012.51517.52022.5Initial star mass MFig.2.—Initial(Zero Age Main Sequence)mass tofinal compact object mass relation for single stars.This represents our current understanding of compact object formation.Stars below about7.5M⊙form white dwarfs;stars in the narrow range around8M⊙can potentially form very light neutron stars through electron capture supernovae(Podsiadlowski et al.2004).More massive stars show a well defined bifurcation caused by different modes of energy transport in the stellar core:stars below18M⊙form light neutron stars(≃1.35M⊙),while stars above this mass form heavy neutron stars (≃1.8M⊙).Above≃20M⊙stars experience partial fallback of material that can turn nascent neutron stars into black pact objects originating from stars of∼20−22M⊙form either very heavy neutron stars or low-mass black holes depending on the unknown limiting mass between these two remnant types(expected to lie around2−3M⊙).6Fig. 3.—Predicted mass distribution for neutron stars in merging double neutron star binaries.First born neutron stars are slightly heavier as they can accrete some matter from their unevolved binary companions.Population synthesis models(red and blue lines)are shown along with measured neutron star masses for the known double neutron star binaries.Although more observations are needed to constrain the shape of this distribution,the mass ranges of observed and predicted systems are in agreement.We use direct mass estimates for B1913+16,B1534+12,J0737−3039and J1756−2251(O’Shaughnessy et al.2008),while for J1906+0746we assume that both neutron stars have masses of1.3M⊙(total system mass is2.6M⊙;Lorimer et al.2006).The few compact objects found in our simulations with masses as high as≃2.5M⊙may well be low-mass black holes(see also Fig.1).7Fig. 4.—Gamma Ray Burst production efficiency as a function of the maximum neutron star mass in the framework of the double neutron star model for GRBs involving the formation of a black hole.The mass of the merger product is plotted here as the blue (observations)and red (theory)lines.There is a sharp drop in number of NS–NS systems that can form a merger with mass over 2.5M ⊙.For example,only 1in 103NS–NS mergers can form a remnant with a mass of 2.5M ⊙or higher.Therefore,if short GRBs are connected to NS–NS mergers,the maximum neutron star mass is required to be M ns ,max <2.5M ⊙.For comparison we show observed masses of the lowest-mass black holes (GRO J0422−32,GRS 1009−45;Casares 2006)and highest-mass neutron stars (Vela X-1,Terzan 5I,and Terzan 5J;Barziv et al.2001,Ransom et al.2005).These observations,along with our findings,constrain the maximum mass of a neutron star to lie in the narrow range of 2−2.5M ⊙.。

银河系漫游指南英文版pdfHere is the English essay with a word count of over 1000 words, as requested:The Milky Way Galactic OdysseyEmbark on a captivating journey through the vast expanse of the Milky Way Galaxy, a celestial wonder that has captivated the human imagination for millennia. As we delve into the mysteries and marvels of this galactic realm, prepare to be awestruck by the sheer scale and beauty of the cosmos that lies beyond our earthly confines.Let us begin our odyssey by venturing to the heart of the Milky Way, where the supermassive black hole known as Sagittarius A* resides. This gravitational behemoth, nearly 4 million times the mass of our Sun, anchors the center of our galaxy and exerts a powerful influence on the surrounding stars and stellar matter. As we approach this enigmatic cosmic phenomenon, we will witness the intricate dance of stars and gas clouds as they are drawn inexorably towards the event horizon, their fate forever sealed within the crushing grip of the black hole.Venturing outwards from the galactic center, we will encounter the diverse and vibrant neighborhoods that make up the Milky Way. Spiral arms, such as the Orion Arm in which our Solar System resides, are vast regions of star formation, with newborn stars and stellar nurseries dotting the landscape. We will marvel at the brilliant nebulae, glowing clouds of gas and dust that serve as the birthplaces of these young celestial bodies, their ethereal hues and intricate structures a testament to the dynamic processes that shape the galaxy.As we traverse the spiraling arms, we will come across the globular clusters – ancient, densely packed collections of stars that orbit the galactic center. These spherical assemblages, some of the oldest objects in the Milky Way, harbor valuable insights into the early history and evolution of our galaxy, their stars dating back to a time when the universe was a mere fraction of its current age.Amidst the stellar tapestry, we will discover the diverse array of stellar populations that call the Milky Way home. From the towering red giants, their brilliant crimson hues a testament to their advanced age and increased size, to the compact and enigmatic neutron stars, the collapsed remnants of once-mighty suns. Each type of star, with its unique properties and life cycle, contributes to the rich tapestry of the galactic landscape.But the Milky Way is not merely a collection of stars – it is a dynamic and ever-changing system, influenced by the complex interplay of gravity, stellar evolution, and the ever-present threat of cosmic catastrophes. We will explore the regions where massive stars meet their explosive demise, supernovae that briefly outshine entire galaxies and leave behind the dense, spinning neutron stars known as pulsars. These cataclysmic events not only shape the galactic environment but also provide the building blocks for new generations of stars and planets.As we venture deeper into the Milky Way, we will encounter the harrowing regions where the fabric of space-time is stretched and distorted by the intense gravitational fields of neutron stars and black holes. Here, we will witness the bizarre and mind-bending phenomena predicted by Einstein's theory of general relativity, from the warping of spacetime to the accretion disks that feed these cosmic monsters.Throughout our journey, we will be in awe of the sheer scale and majesty of the Milky Way. The galaxy, spanning nearly 100,000 light-years in diameter, is home to an estimated 200 to 400 billion stars, each one a unique and fascinating world unto itself. We will ponder the possibility of life elsewhere in this vast cosmic tapestry, wondering if intelligent civilizations have arisen on distant worlds and if they, too, gaze up at the night sky, marveling at the splendorof our shared galactic home.As our odyssey draws to a close, we will reflect on the profound impact that the study of the Milky Way has had on our understanding of the universe. From the groundbreaking work of pioneering astronomers to the cutting-edge research conducted with the most advanced observational tools, the Milky Way has been a constant source of fascination and discovery. And as we look to the future, we know that there are countless more secrets and mysteries waiting to be unveiled, beckoning us to continue our exploration of this awe-inspiring celestial realm.So let us embark on this Milky Way galactic odyssey, armed with a sense of wonder and a thirst for knowledge. For in unraveling the mysteries of our galactic home, we may just find the answers to some of the most profound questions that have puzzled humanity since the dawn of time.。