Editing Star (section)

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As a star's core shrinks, the intensity of radiation from that surface increases, creating such [[radiation pressure]] on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly {{Solar mass|1.4}}, it shrinks to a relatively tiny object about the size of Earth, known as a [[white dwarf]]. White dwarfs lack the mass for further gravitational compression to take place.<ref>
{{cite journal |last1=Liebert |first1=James |title=White dwarf stars |journal=Annual Review of Astronomy and Astrophysics |date=1980 |volume=18 |issue=2 |pages=363–398 |bibcode=1980ARA&A..18..363L |doi= 10.1146/annurev.aa.18.090180.002051}}
</ref> The [[electron-degenerate matter]] inside a white dwarf is no longer a plasma. Eventually, white dwarfs fade into [[black dwarf]]s over a very long period of time.<ref>{{cite news |last1=Mann |first1=Adam |title=This is the way the universe ends: not with a whimper, but a bang |url=https://www.science.org/content/article/way-universe-ends-not-whimper-bang |work=Science {{!}} AAAS |date=2020-08-11 |language=en}}</ref>

[[File:Crab Nebula.jpg|thumb|The [[Crab Nebula]], remnants of a supernova that was first observed around 1050 AD]]

In massive stars, fusion continues until the iron core has grown so large (more than {{Solar mass|1.4}}) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, [[neutrino]]s, and gamma rays in a burst of [[electron capture]] and [[inverse beta decay]]. The [[shock wave|shockwave]] formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.<ref name="supernova">
{{cite web
 | date=2006-04-06
 | url=http://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrstext.html
 | title=Introduction to Supernova Remnants
 | publisher=Goddard Space Flight Center
 | access-date=2006-07-16}}
</ref>

A supernova explosion blows away the star's outer layers, leaving a [[supernova remnant|remnant]] such as the Crab Nebula.<ref name="supernova" /> The core is compressed into a [[neutron star]], which sometimes manifests itself as a [[pulsar]] or [[X-ray burster]]. In the case of the largest stars, the remnant is a black hole greater than {{Solar mass|4}}.<ref>{{cite journal |last1=Fryer |first1=C. L. |title=Black-hole formation from stellar collapse |journal=Classical and Quantum Gravity |date=2003 |volume=20 |issue=10 |pages=S73–S80 |doi= 10.1088/0264-9381/20/10/309 |bibcode=2003CQGra..20S..73F|s2cid=122297043 |url=https://zenodo.org/record/1235744 }}</ref> In a neutron star the matter is in a state known as [[neutron-degenerate matter]], with a more exotic form of degenerate matter, [[QCD matter]], possibly present in the core.<ref>{{cite journal |bibcode=2019NuPhA.982...36V |title=Neutron stars and stellar mergers as a laboratory for dense QCD matter |last1=Vuorinen |first1=Aleksi |journal=Nuclear Physics A |year=2019 |volume=982 |page=36 |doi=10.1016/j.nuclphysa.2018.10.011 |arxiv=1807.04480 |s2cid=56422826 }}</ref>

The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.<ref name="supernova" />