Editing Star (section)

===Main sequence===
{{Main|Main sequence}}

Stars spend about 90% of their lifetimes fusing hydrogen into helium in high-temperature-and-pressure reactions in their cores. Such stars are said to be on the main sequence and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.<ref>
{{cite journal | display-authors=1
 | last1=Mengel | first1=J. G. | last2=Demarque | first2=P.
 | last3=Sweigart | first3=A. V. | last4=Gross | first4=P. G.
 | title=Stellar evolution from the zero-age main sequence
 | journal=Astrophysical Journal Supplement Series
 | date=1979 | volume=40 | pages=733–791
 | bibcode=1979ApJS...40..733M | doi= 10.1086/190603
}}</ref>
The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6&nbsp;billion ({{val|4.6|e=9}}) years ago.<ref name=sun_future />

Every star generates a [[stellar wind]] of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses {{val|e=−14|u=Solar mass}} every year,<ref>
{{cite journal | display-authors=1
 | last1=Wood | first1=B. E. | last2=Müller | first2=H.-R.
 | last3=Zank | first3=G. P. | last4=Linsky | first4=J. L.
 | title=Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity
 | journal=The Astrophysical Journal | date=2002
 | volume=574 | issue=1 | pages=412–425
| doi= 10.1086/340797 | bibcode=2002ApJ...574..412W
|arxiv= astro-ph/0203437| s2cid=1500425 }}
</ref> or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose {{val|e=−7}} to {{val|e=−5|u=Solar mass}} each year, significantly affecting their evolution.<ref>
{{cite journal
 | last1=de Loore | first1=C. | last2=de Greve | first2=J. P.
 | last3=Lamers | first3=H. J. G. L. M.
 | title=Evolution of massive stars with mass loss by stellar wind
 | journal=Astronomy and Astrophysics | date=1977 | volume=61
 | issue=2 | pages=251–259
 | bibcode=1977A&A....61..251D}}
</ref> Stars that begin with more than {{Solar mass|50}} can lose over half their total mass while on the main sequence.<ref>{{cite web |title=The evolution of stars between 50 and 100 times the mass of the Sun |url=http://certificate.ulo.ucl.ac.uk/modules/year_one/ROG/stellar_evolution/conWebDoc.727.html |url-status=dead |archive-url=https://web.archive.org/web/20151118161020/http://certificate.ulo.ucl.ac.uk/modules/year_one/ROG/stellar_evolution/conWebDoc.727.html |archive-date=2015-11-18 |access-date=2015-11-17 |publisher=Royal Greenwich Observatory |publication-place=England}}</ref>

[[File:H-R diagram -edited-3.gif|thumb|upright=1.6|An example of a [[Hertzsprung–Russell diagram]] for a set of stars that includes the Sun (center) (see [[#Classification|Classification]])]]

The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10&nbsp;billion ({{val|e=10}}) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than {{Solar mass|0.25}}, called [[red dwarf]]s, are able to fuse nearly all of their mass while stars of about {{Solar mass|1}} can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion ({{val|10|e=12}}) years; the most extreme of {{Solar mass|0.08}} will last for about 12&nbsp;trillion years. Red dwarfs become [[blue dwarf (red-dwarf stage)|hotter and more luminous]] as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.<ref name="adams">{{cite conference |last1=Adams |first1=Fred C. |last2=Laughlin |first2=Gregory |last3=Graves |first3=Genevieve J. M. |title=Red Dwarfs and the End of the Main Sequence |url=http://www.astroscu.unam.mx/rmaa/RMxAC..22/PDF/RMxAC..22_adams.pdf |conference= |publisher=Revista Mexicana de Astronomía y Astrofísica |pages=46–49 |bibcode=2004RMxAC..22...46A |archive-url=https://web.archive.org/web/20190711072446/http://www.astroscu.unam.mx/rmaa/RMxAC..22/PDF/RMxAC..22_adams.pdf |archive-date=11 July 2019 |access-date=2008-06-24 |book-title=Gravitational Collapse: From Massive Stars to Planets |url-status=dead}}</ref> Since the lifespan of such stars is greater than the current age of the universe (13.8&nbsp;billion years), no stars under about {{Solar mass|0.85}}<ref name="saomainseq">
{{cite encyclopedia |title=Main Sequence Lifetime |url=http://astronomy.swin.edu.au/cosmos/M/Main+Sequence+Lifetime |encyclopedia=Swinburne Astronomy Online Encyclopedia of Astronomy |publisher=Swinburne University of Technology}}
</ref> are expected to have moved off the main sequence.

Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical [[concentration]] of these elements in a star, its [[metallicity]]. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,<ref>
{{cite journal
 | display-authors=1
 | last1=Pizzolato | first1=N. | last2=Ventura | first2=P.
 | last3=D'Antona | first3=F. | last4=Maggio | first4=A.
 | last5=Micela | first5=G. | last6=Sciortino | first6=S.
 | title=Subphotospheric convection and magnetic activity dependence on metallicity and age: Models and tests
 | journal=Astronomy & Astrophysics
 | date=2001 | volume=373
 | issue=2 | pages=597–607
 | doi=10.1051/0004-6361:20010626
| bibcode=2001A&A...373..597P| doi-access=free }}
</ref> which affects the strength of its stellar wind.<ref>
{{cite web
 | date= 2004-06-18
 | url= http://www.star.ucl.ac.uk/groups/hotstar/research_massloss.html
 | archive-url= https://web.archive.org/web/20041122143115/http://www.star.ucl.ac.uk/groups/hotstar/research_massloss.html
 | archive-date= 2004-11-22
 | title= Mass loss and Evolution | publisher= UCL Astrophysics Group
 | access-date= 2006-08-26}}
</ref> Older, [[stellar population|population II]] stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their [[stellar atmosphere|atmospheres]].<ref name="Astrophysics1984">{{cite book|author=Rutherford Appleton Laboratory. Workshop on Astronomy and Astrophysics|title=Gas in the Interstellar Medium: Rutherford Appleton Laboratory Workshop on Astronomy and Astrophysics : 21–23 May, 1983, The Cosener's House, Abingdon|url=https://books.google.com/books?id=e37vAAAAMAAJ|year=1984|publisher=Science and Engineering Research Council, Rutherford Appleton Laboratory}}</ref>