Editing Galaxy (section)

=== Future trends ===
{{Main|Future of an expanding universe}}

Spiral galaxies, like the [[Milky Way]], produce new generations of stars as long as they have dense [[molecular cloud]]s of interstellar hydrogen in their spiral arms.<ref>{{cite journal
 |last1=Kennicutt |first1=Robert C. Jr.
 |last2=Tamblyn |first2=Peter
 |last3=Congdon |first3=Charles E.
 |date=1994
 |title=Past and future star formation in disk galaxies
 |journal=[[The Astrophysical Journal]]
 |volume=435 |issue=1 |pages=22–36
 |bibcode=1994ApJ...435...22K
 |doi=10.1086/174790
}}</ref> Elliptical galaxies are largely devoid of this gas, and so form few new stars.<ref>{{cite book
 |last1=Knapp
 |first1=Gillian R.
 |author-link1=Gillian Knapp
 |date=1999
 |title=Star Formation in Early Type Galaxies
 |volume=163
 |page=119
 |publisher=[[Astronomical Society of the Pacific]]
 |bibcode=1999ASPC..163..119K
 |oclc=41302839
 |isbn=978-1-886733-84-8
 |arxiv=astro-ph/9808266
 |url=https://books.google.com/books?id=tpDvAAAAMAAJ
 }}</ref> The supply of star-forming material is finite; once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end.<ref name="cosmic_battle">{{cite web
 |last1=Adams
 |first1=Fred
 |last2=Laughlin
 |first2=Greg
 |date=July 13, 2006
 |title=The Great Cosmic Battle
 |url=http://www.astrosociety.org/pubs/mercury/0001/cosmic.html
 |publisher=[[Astronomical Society of the Pacific]]
 |access-date=January 16, 2007
 |archive-date=May 13, 2012
 |archive-url=https://web.archive.org/web/20120513053244/http://www.astrosociety.org/pubs/mercury/0001/cosmic.html
 |url-status=dead
 }}</ref><ref>{{cite web|title = Cosmic 'Murder Mystery' Solved: Galaxies Are 'Strangled to Death'|website = [[Space.com]]|last=Choi|first=Charles Q.|date = May 13, 2015|url = http://www.space.com/29398-galaxy-strangulation-death-mystery.html|access-date = 2015-05-14|archive-date = March 24, 2021|archive-url = https://web.archive.org/web/20210324071733/https://www.space.com/29398-galaxy-strangulation-death-mystery.html|url-status = live}}</ref>

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years (10<sup>13</sup>–10<sup>14</sup>&nbsp;years), as the smallest, longest-lived stars in the visible universe, tiny [[red dwarf]]s, begin to fade. At the end of the stellar age, galaxies will be composed of [[compact star|compact objects]]: [[brown dwarf]]s, [[white dwarf]]s that are cooling or cold ("[[black dwarf]]s"), [[neutron star]]s, and [[black hole]]s. Eventually, as a result of [[Relaxation (physics)#Relaxation in astronomy|gravitational relaxation]], all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.<ref name="cosmic_battle" /><ref>{{cite web
 |last1=Pobojewski
 |first1=Sally
 |date=January 21, 1997
 |title=Physics offers glimpse into the dark side of the Universe
 |url=http://www.umich.edu/~urecord/9697/Jan21_97/artcl17.htm
 |publisher=[[University of Michigan]]
 |access-date=January 13, 2007
 |archive-date=January 21, 2005
 |archive-url=https://web.archive.org/web/20050121050829/http://www.umich.edu/~urecord/9697/Jan21_97/artcl17.htm
 |url-status=dead
 }}</ref>