Editing Stellar evolution (section)

==Stellar remnants==<!-- This section is linked from [[Planet]] -->
[[File:star life cycles red dwarf en.svg|thumb|upright=1.5|Stellar evolution of low-mass (left cycle) and high-mass (right cycle) stars, with examples in italics]]
After a star has burned out its fuel supply, its remnants can take one of three forms, depending on the mass during its lifetime.

===White and black dwarfs===
{{Main|White dwarf|Black dwarf}}
For a star of {{Solar mass|1}}, the resulting white dwarf is of about {{Solar mass|0.6}}, compressed into approximately the volume of the Earth. White dwarfs are stable because the inward pull of gravity is balanced by the [[degeneracy pressure]] of the star's electrons, a consequence of the [[Pauli exclusion principle]]. Electron degeneracy pressure provides a rather soft limit against further compression; therefore, for a given chemical composition, white dwarfs of higher mass have a smaller volume. With no fuel left to burn, the star radiates its remaining heat into space for billions of years.

A white dwarf is very hot when it first forms, more than 100,000 K at the surface and even hotter in its interior. It is so hot that a lot of its energy is lost in the form of neutrinos for the first 10 million years of its existence and will have lost most of its energy after a billion years.<ref>{{cite web| url = http://www.vectorsite.net/tastgal_05.html| title = Fossil Stars (1): White Dwarfs}}</ref>

The chemical composition of the white dwarf depends upon its mass. A star that has a mass of about 8-12 solar masses will ignite [[Carbon burning process|carbon fusion]] to form magnesium, neon, and smaller amounts of other elements, resulting in a white dwarf composed chiefly of oxygen, neon, and magnesium, provided that it can lose enough mass to get below the [[Chandrasekhar limit]] (see below), and provided that the ignition of carbon is not so violent as to blow the star apart in a supernova.<ref>{{cite journal |author=Ken'ichi Nomoto |title=Evolution of 8–10 {{Solar mass}} stars toward electron capture supernovae. I – Formation of electron-degenerate O + Ne + Mg cores |volume=277 |journal=Astrophysical Journal |date=1984 |pages=791–805 |bibcode=1984ApJ...277..791N |doi=10.1086/161749|doi-access=free }}</ref> A star of mass on the order of magnitude of the Sun will be unable to ignite carbon fusion, and will produce a white dwarf composed chiefly of carbon and oxygen, and of mass too low to collapse unless matter is added to it later (see below). A star of less than about half the mass of the Sun will be unable to ignite helium fusion (as noted earlier), and will produce a white dwarf composed chiefly of helium.

In the end, all that remains is a cold dark mass sometimes called a [[black dwarf]]. However, the universe is not old enough for any black dwarfs to exist yet.

If the white dwarf's mass increases above the [[Chandrasekhar limit]], which is {{Solar mass|1.4}} for a white dwarf composed chiefly of carbon, oxygen, neon, and/or magnesium, then electron degeneracy pressure fails due to [[electron capture]] and the star collapses. Depending upon the chemical composition and pre-collapse temperature in the center, this will lead either to collapse into a [[neutron star]] or runaway ignition of carbon and oxygen. Heavier elements favor continued core collapse, because they require a higher temperature to ignite, because electron capture onto these elements and their fusion products is easier; higher core temperatures favor runaway nuclear reaction, which halts core collapse and leads to a [[Type Ia supernova]].<ref>{{cite journal |author=Ken'ichi Nomoto |author2=Yoji Kondo |name-list-style=amp |title=Conditions for accretion-induced collapse of white dwarfs |journal=Astrophysical Journal |date=1991 |volume=367 |pages=L19–L22 |bibcode=1991ApJ...367L..19N |doi=10.1086/185922}}</ref> These supernovae may be many times brighter than the Type II supernova marking the death of a massive star, even though the latter has the greater total energy release. This instability to collapse means that no white dwarf more massive than approximately {{Solar mass|1.4}} can exist (with a possible minor exception for very rapidly spinning white dwarfs, whose [[centrifugal force]] due to rotation partially counteracts the weight of their matter). Mass transfer in a [[binary system (astronomy)|binary system]] may cause an initially stable white dwarf to surpass the Chandrasekhar limit.

If a white dwarf forms a close binary system with another star, hydrogen from the larger companion may accrete around and onto a white dwarf until it gets hot enough to fuse in a runaway reaction at its surface, although the white dwarf remains below the Chandrasekhar limit. Such an explosion is termed a [[nova]].

===Neutron stars===
{{Main|Neutron star}}
[[Image:CygnusLoopSmall.jpg|thumb|right|Bubble-like shock wave still expanding from a supernova explosion 15,000 years ago]]
Ordinarily, atoms are mostly electron clouds by volume, with very compact nuclei at the center (proportionally, if atoms were the size of a football stadium, their nuclei would be the size of dust mites). When a stellar core collapses, the pressure causes electrons and protons to fuse by [[electron capture]]. Without electrons, which keep nuclei apart, the neutrons collapse into a dense ball (in some ways like a giant [[atomic nucleus]]), with a thin overlying layer of [[degenerate matter]] (chiefly iron unless matter of different composition is added later). The neutrons resist further compression by the [[Pauli exclusion principle]], in a way analogous to electron degeneracy pressure, but stronger.

These stars, known as neutron stars, are extremely small—on the order of radius 10&nbsp;km, no bigger than the size of a large city—and are phenomenally dense. Their period of rotation shortens dramatically as the stars shrink (due to [[conservation of angular momentum]]); observed rotational periods of neutron stars range from about 1.5 milliseconds (over 600 revolutions per second) to several seconds.<ref>{{cite journal | author=D'Amico, N. | author2=Stappers, B. W. | author3=Bailes, M. | author4=Martin, C. E. | author5=Bell, J. F. | author6=Lyne, A. G. | author7=Manchester, R. N. | journal=Monthly Notices of the Royal Astronomical Society | date=1998 | doi=10.1046/j.1365-8711.1998.01397.x | volume=297 | issue=1 | pages=28–40|bibcode = 1998MNRAS.297...28D | title=The Parkes Southern Pulsar Survey - III. Timing of long-period pulsars | doi-access=free }}</ref> When these rapidly rotating stars' magnetic poles are aligned with the Earth, we detect a pulse of radiation each revolution. Such neutron stars are called [[pulsar]]s, and were the first neutron stars to be discovered. Though electromagnetic radiation detected from pulsars is most often in the form of radio waves, pulsars have also been detected at visible, X-ray, and gamma ray wavelengths.<ref>{{cite news|author=Courtland, Rachel |url=https://www.newscientist.com/article/dn14968-first-pulsar-identified-by-its-gamma-rays-alone.html |title=Pulsar Detected by Gamma Waves Only |work=New Scientist |date=17 October 2008 |url-status=dead |archive-url=https://web.archive.org/web/20130402053638/http://space.newscientist.com/article/dn14968-first-pulsar-identified-by-its-gamma-rays-alone.html |archive-date=April 2, 2013 }}</ref>

===Black holes===
{{Main|Black hole}}
If the mass of the stellar remnant is high enough, the neutron degeneracy pressure will be insufficient to prevent collapse below the [[Schwarzschild radius]]. The stellar remnant thus becomes a black hole. The mass at which this occurs is not known with certainty, but is currently estimated at between 2 and {{Solar mass|3}}.

Black holes are predicted by the theory of [[general relativity]]. According to classical general relativity, no matter or information can flow from the interior of a black hole to an outside observer, although [[quantum mechanics|quantum effect]]s may allow deviations from this strict rule. The existence of black holes in the universe is well supported, both theoretically and by astronomical observation.

Because the core-collapse mechanism of a supernova is, at present, only partially understood, it is still not known whether it is possible for a star to collapse directly to a black hole without producing a visible supernova, or whether some supernovae initially form unstable neutron stars which then collapse into black holes; the exact relation between the initial mass of the star and the final remnant is also not completely certain. Resolution of these uncertainties requires the analysis of more supernovae and supernova remnants.