produces material densities sufficiently extreme to trigger star formation through gravitational collapse At what is called the star's death (when a star has consumed its supply of fuel), it will undergo a contraction that can be halted only if it reaches a new state of equilibrium. Depending on the mass during its lifetime, these
stellar remnants can take one of three forms: •
White dwarfs, in which gravity is opposed by
electron degeneracy pressure •
Neutron stars, in which gravity is opposed by
neutron degeneracy pressure and short-range repulsive neutron–neutron interactions mediated by the
strong force •
Black hole, in which there is no force strong enough to resist gravitational collapse Theoretically, there are compact
exotic stars made from forms of
exotic matter such as
quarks or
preons, but these objects remain hypothetical.
White dwarf For isolated stars that formed with one to seven times the mass of the Sun, the final stage in their evolution is a white dwarf. The collapse of the stellar core to a white dwarf takes place over tens of thousands of years, while the star blows off its outer envelope to form a
planetary nebula. A white dwarf can have a
magnetic field, which may be a fossil remnant of its original stellar magnetic field. As gravitational contraction and the remaining nuclear burning contribute only a negligible amount to the energy output of a white dwarf, nearly all of the radiated luminosity comes from stored thermal energy. Thus, over time scales of billions of years, the temperature of the white dwarf will continue to decrease. If it has a close orbiting
companion star, a white dwarf-sized object can
accrete matter from the companion, increasing the mass and potentially spinning it up. When sufficient hydrogen has been accumulated along the outer shell, it can detonate to form a
nova. Since the white dwarf is not disrupted by the explosion, this cycle can occur repeatedly. Despite the thermonuclear explosion, some of the accumulated matter can be retained, allowing the white dwarf to continue to grow in mass. The upper mass limit on the growth of a white dwarf is called the
Chandrasekhar limit. This is about one and a half times the mass of the Sun, at which point gravitational collapse would start again. Before reaching this limit, the increasing density and temperature within a carbon-oxygen white dwarf initiates a new round of nuclear fusion, which is not regulated because the star's weight is supported by degeneracy rather than thermal pressure, allowing the temperature to rise exponentially. The resulting
runaway carbon detonation completely blows the star apart in a
Type Ia supernova. An alternative scenario is the merger of two white dwarfs in a close binary system; the so-called double-degenerate model. When the combined mass exceeds the Chandrasekhar limit, a Type Ia supernova results. Neutron stars are expected to have a skin or "atmosphere" of normal matter on the order of a millimeter thick, underneath which they are composed almost entirely of closely packed neutrons called
neutron matter with a slight dusting of free electrons and protons mixed in. This degenerate neutron matter has a density of about . The appearance of stars composed of
exotic matter and their internal layered structure is unclear since any proposed
equation of state of
degenerate matter is highly speculative. Other forms of hypothetical degenerate matter may be possible, and the resulting
quark stars,
strange stars (a type of quark star), and
preon stars, if they exist, would, for the most part, be indistinguishable from a
neutron star: in most cases, the
exotic matter would be hidden under a crust of "ordinary" degenerate neutrons.
Black holes According to Einstein's theory, for objects with masses above the
Tolman–Oppenheimer–Volkoff limit (roughly double the mass of the Sun), no known form of cold (non-fusing) matter can provide the repulsive force needed to oppose gravity in a new dynamical equilibrium. Hence, the gravitational collapse continues unhindered. Once a body collapses to within its
Schwarzschild radius, it forms what is termed a black hole, meaning a spacetime region from which not even light can escape. It follows from
general relativity and the theorem of
Roger Penrose that the subsequent formation of some kind of
singularity is inevitable. Nevertheless, according to Penrose's
cosmic censorship hypothesis, the singularity will be confined within the event horizon bounding the black hole, so the spacetime region outside will still have a well-behaved geometry, with strong but finite curvature, that is expected to evolve towards a rather simple form describable by the historic
Schwarzschild metric in the spherical limit and by the more recently discovered
Kerr metric if angular momentum is present. If the precursor has a magnetic field, it is dispelled during the collapse, as black holes are thought to have no magnetic field of their own. On the other hand, the nature of the kind of singularity to be expected inside a black hole remains rather controversial. According to theories based on
quantum mechanics, at a later stage, the collapsing object will reach the maximum possible energy density for a certain volume of space or the
Planck density (as there is nothing that can stop it). This is the point at which it has been hypothesized that the known laws of gravity cease to be valid. There are competing theories as to what occurs at this point. For example
loop quantum gravity predicts that a
Planck star would form. Regardless, it is argued that gravitational collapse ceases at that stage and a singularity, therefore, does not form.
Theoretical minimum radius for a star The radii of larger mass neutron stars (about 2.8 solar mass) are estimated to be about 12 km, or about twice their equivalent Schwarzschild radius. It might be thought that a sufficiently massive neutron star could exist within its Schwarzschild radius (1.0 SR) and appear like a black hole without having all the mass compressed to a singularity at the center; however, this is probably incorrect. Within the
event horizon, the matter would have to move outward faster than the speed of light in order to remain stable and avoid collapsing to the center. No physical force, therefore, can prevent a star smaller than 1.0 SR from collapsing to a singularity (at least within the currently accepted framework of
general relativity; this does not hold for the Einstein–Yang–Mills–Dirac system). A model for the nonspherical collapse in general relativity with the emission of matter and
gravitational waves has been presented. ==See also==