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Gravitational Collapse and the Death of a Star

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Gravitational Collapse and the Death of a Star 24 December 1965, Volume 150, Number 3704 SCI:ENCE: squLeezes the material in its core into a smaller and smialler Vxolume and to higher and higher temperatllres. The sUbsequent fate of the star de- penids Lipon hoxy massixve it is. For a Gravitational Collapse and tbie star of less than 1 .2 solai- masses [the 'ChandrasekhCar limlit" (1)], quLasi- Death of a Stvir static contraction is halted by rising internal pressure when a central den- sitx ol' I(),; gr(Irams per clbic centi- Relativity and nuclear theory together predict the fLite mleter is reachedl. The stair then settles down into its final resting, state. a of a star which has burned all its nuclear fu e. *x;white dwxxarf' configluration (2). On the other haind. in a star ot' more than S. 'I-hor 1 .2 solar mi.asses. the stellair core is Kip squLeezed to suIch high densities dUring quLsi-static contraction that catastroph- ic nuclea;r processes occLur before rising internlail pressuLre cain halt the contrac- \\Nhat is the fate of a star wxhen it Adclitional obserx ational tests ot' these tion. These processes caIse the star to has consumned all its nuclear fuel ancl predictions. bileh extr(emelxy diflicult explodce with such violence that its -an no longer maintain the nuclear atnd perhLaps imlpossible with presenit- I1,nminosity approaches that of a g.alaxv reactions which have sustained it dav technolo,% x c uld provide for a periodl of Ihoult I O) days since its birth'? This is a question valuLable tests of the zravit.ation and IsuLpernova explosion (3)1- Wshich observational astrononmy has nucleIar theory upon xswhich the pre- The precise physical processes wxhich cione but little to answxer. The time dictions rest. minitite andtaccomparny a supernova reqLlired for a star to COnISuImle its It should be emnphasi2'ed at the oLut- explosion are probablx different in nLuclear fuel is so long (manvn billions set that theoretical sttLudies o t' the stars of' between 1.2 and abouLt 5 solar of years in nmost cases) thtat only a deaths of st(ars are still far froml coIll- masses from those in more mnassive few stars die in our galasxy per cen- plete. Such complicatiions as stellar stars. I shall first describe the mechan- tury; and the evolution of a star froml rotation. deviations fromn Ispherical svm- ism by which the less mlassive super- the end point of thermonuclear burn- metrv. and stellar maginetic fields are novae are prodliced by tracing out the inog to its final dead state is so rapid largely uinstildied aind \Kill he irnored dceth of a representative 2-solar-niass that its death throes are observable for here. Rough estimaItes indicate that star, as preclictecd hy theor-etical cal- only! a few Nrears. these phenomena, when not too pro- culations. Then I shall tLurn yiv atten- Despite the paucity of' observation- notlnced, will probably have little ef- tion to the miorem-lassive stUpernovae. al data, theoreticians are now able to fect on the quLlitative FPictul-e oLutlined A stair of 2 solar i.masses realches discuss the deaths of stars in ever in- here; for the mlost palrt, onIN qUantita- the enid point of thermonucleCar- bUrning creasing detail and w,vith a fair degree tive details are expectecLi to change as wvhen it has converted all of the hvdro- of' certainty, thanks to recent advances theoretical stuLdies becorine mlore reills- gen in its inter-ior to Fe';. the milost in computer technology andt in our tic. tightly houLnd of all nuclei. At this to-derstanding of the physics of the point the star begins to contract quaLsi- atomic nucleus. of elementary par- statically, compressing the matter in its ticles, and of Einstein's geomletrical Evolution to the D)ead State center into a smaller and smiialler theory of gravitation (general relativity). VOl LIme. The puLrpose of this article is to re- WNVhen, after hundreds of millions or Now, we know from elementary viewv the different tv pes of death and billions of years of rnormlal nLuclear quLanttlm theory that the smaller the the final resting, states of various types bLirning, a star has exhatLsted its supply region to which we confine a particle, of stars, as deduced theoretically, and of nLlclear fuel, it has only one wvay the larger the particle's zero-point ki- to point out those few direct astro- to replenish the therma energy that it netic energy beconmes. In particuilar, nomical observations wxhich have bear- is radiating: quLasi-static graxvitaitional when a particle is confined to a re- ing on the theoretical predictions. contraction. As it con tracts, the star g,ion of the order of its Comiipton Nvaye- converts its oyravitation;.1L potential en- length. its zero-point energy becomles The auLthor is a National Science Fotunidation post- into -v rest doctoral fellow at Palmer Physical t.aboratory, crgy thermiial enerr and radiates of the order of its mass; and be- Priniceton University. Princeton, New Jersey. it away, and at the Same timile it yond this point the zero-point energy 24 DECEMBER 1965 1671 collapsing core is suddenly faced with a huge central pressure, which calls its collapse to a halt and sends a shock wave propagating outward through it. In this core shock front the huge ki- netic energy of collapse is converted+ into heat, and temperatures of over 10 billion degrees are reached. At such high temperatures and densities, ele- mentary particle transformations pro- ceed at a rapid rate, and the heat pro- C() lo' duced in the core shock front is con- :320% 10 5% verted into high-energy neutrinos. The < E \ 1 \ CORE SHOCK WAVE l mean free path of the neutrinos is less than 100 meters under these extreme CK00 t - t- .98 ME CORE conditions. Hence, instead of escaping freely from the star, the neutrinos dif-, fuse outward, depositing the energy re- leased by the core's collapse in the en- NEUTRINO DEPOSITION ON NEUTRINO DEPOSITION OFF velope of the star and thereby raising the envelope to temperatures as high as 200 billion degrees. At these enor- 1.84 1.85 1.86 1.87 1.88 1.89 1.90 mous temperatures explosive nuclearv burning is initiated in the envelope, TIME (sec) with a consequent release of additional Fig. 1. The dynamics of the collapse and reexplosion of a star of 2 solar masses. thermal energy. Because of the huge Each solid curve represents the radius as a function of time for a spherical shell of thermal energies generated by neutrino matter inside which a certain fraction of the star's mass lies. Each curve is labeled deposition and by nuclear burning, the by this "mass-fraction." [Based on calculations by Colgate and White (4)] envelope of the star suddenly becomes gravitationally unbound. An exploding shock wave forms and blows the en- rises very rapidly with additional com- Colgate and White (4) have used velope away from the core with speeds pression. Since the Compton wave- computers at Lawrence Radiation Lab- approaching the speed of light; and the length of an electron is 100,000 times oratory to study the details of this huge thermal energies of the expand-* that of an Fe56 nucleus, electrons will catastrophic collapse (5). The details ing envelope are converted to radia- resist being squeezed by the contracting of the collapse of the 2-solar-mass tion so intense that the luminosity of star much sooner than will iron nu- star, as worked out by Colgate and the exploding star approaches that of clei. As the contraction proceeds, our White, are shown in Fig. 1 and are a galaxy. In addition, nuclear par- 2-solar-mass star pushes the zero-point described below. ticles are accelerated in the exploding energies of its electrons higher and shock wave in such numbers and to' higher, while the zero-point energies such high velocities as to account for of its iron nuclei remain negligible. Supernova Explosions a significant fraction of the galactic Eventually a point is reached at cosmic rays observed at the earth. which the sum of the rest mass of an Catastrophic collapse is initiated by Stars of more than about 5 solar Fe56 nucleus plus the rest mass of an electron capture when the center of masses undergo supernova explosions electron plus the electron's rising zero- the slowly contracting star reaches a similar to that described above. How- point energy exceeds the rest mass of density of about 1011 grams per cubic ever, the collapse of a massive star's a Mn56 nucleus. No longer is a free centimeter. Within a fraction of a sec- core is initiated not by electron cap- Mn56 nucleus unstable against beta ond after initiation of collapse, nearly ture, but by the sudden breakup of decay into electron plus Fe56; rather, all the electrons and Fe56 nuclei in- its Fe56 nuclei into He4 nuclei. This electrons and Fe56 nuclei are unstable side the star's core have been trans- breakup occurs when the temperature# against combining to form Mn56 and formed into highly neutron-rich nuclei in the contracting core has become so other neutron-rich nuclei (electron cap- and free neutrons, and the core is in high that there are photons present ture); and such combination begins to free fall. At 1.86 seconds after initia- with sufficient energy to disintegrate occur. At this point in the evolution tion the core has acquired a kinetic the Fe56 nuclei. The photodisintegra- of the star the high-zero-point-energy energy of collapse equivalent to a siz- tion of Fe56 reduces the temperature electrons are providing essentially all able fraction of its rest mass, and the and hence also the pressure in the the pressure that sustains the weight neutrons in the core have been com- core of the massive star, and there- of the star.
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