Editing Stellar evolution (section)

===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>