Editing Astronomy (section)

=== Stellar astronomy ===
[[File:Ant Nebula.jpg|thumb|[[Mz 3]], often referred to as the Ant planetary nebula. Ejecting gas from the dying central star shows symmetrical patterns unlike the chaotic patterns of ordinary explosions.]]
{{Main|Star}}
{{see also|Solar astronomy}}
The study of stars and [[stellar evolution]] is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.<ref name=Amos7>Harpaz, 1994, pp. 7–18</ref> [[Star formation]] occurs in dense regions of dust and gas, known as [[Dark nebula|giant molecular clouds]]. When destabilized, cloud fragments can collapse under the influence of gravity, to form a [[protostar]]. A sufficiently dense, and hot, core region will trigger [[nuclear fusion]], thus creating a [[main-sequence star]].<ref name=Smith2004/>

Almost all elements heavier than [[hydrogen]] and [[helium]] were [[nucleosynthesis|created]] inside the cores of stars.<ref name=Amos7/>

The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it fuses its hydrogen fuel into helium in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to [[Stellar evolution|evolve]]. The fusion of helium requires a higher core temperature. A star with a high enough core temperature will push its outer layers outward while increasing its core density. The resulting [[red giant]] formed by the expanding outer layers enjoys a brief life span, before the helium fuel in the core is in turn consumed. Very massive stars can also undergo a series of evolutionary phases, as they fuse increasingly heavier elements.<ref name=Amos>Harpaz, 1994</ref>

The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse [[supernova]]e;<ref>Harpaz, 1994, pp. 173–78</ref> while smaller stars blow off their outer layers and leave behind the inert core in the form of a [[white dwarf]]. The ejection of the outer layers forms a [[planetary nebula]].<ref>Harpaz, 1994, pp. 111–18</ref> The remnant of a supernova is a dense [[neutron star]], or, if the stellar mass was at least three times that of the Sun, a [[black hole]].<ref name="Cambridge atlas">{{cite book|editor= Audouze, Jean|editor2= Israel, Guy|title=The Cambridge Atlas of Astronomy|edition=3rd|publisher=Cambridge University Press|date=1994|isbn=978-0-521-43438-6}}</ref> Closely orbiting binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova.<ref>Harpaz, 1994, pp. 189–210</ref> Planetary nebulae and supernovae distribute the "[[metallicity|metals]]" produced in the star by fusion to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.<ref>Harpaz, 1994, pp. 245–56</ref>