stellar mass
{{Short description|Mass of a star in astronomy}}
Stellar mass is a phrase that is used by astronomers to describe the mass of a star. It is usually enumerated in terms of the Sun's mass as a proportion of a solar mass ({{Solar mass}}). Hence, the bright star Sirius has around {{Solar mass|2.02}}. A star's mass will vary over its lifetime as mass is lost with the stellar wind or ejected via pulsational behavior, or if additional mass is accreted, such as from a companion star.
Properties
Stars are sometimes grouped by mass based upon their evolutionary behavior as they approach the end of their nuclear fusion lifetimes.
Very-low-mass stars with masses below 0.5 {{Solar mass|link=yes}} do not enter the asymptotic giant branch (AGB) but evolve directly into white dwarfs. (At least in theory; the lifetimes of such stars are long enough—longer than the age of the universe to date—that none has yet had time to evolve to this point and be observed.)
Low-mass stars with a mass below about 1.8–2.2 {{Solar mass}} (depending on composition) do enter the AGB, where they develop a degenerate helium core.
Intermediate-mass stars undergo helium fusion and develop a degenerate carbon–oxygen core.
Massive stars have a minimum mass of 5–10 {{Solar mass}}. These stars undergo carbon fusion, with their lives ending in a core-collapse supernova explosion.{{dubious|reason=Stars with 8-10 stellar masses can become oxygen-neon-magnesium white dwarfs; hence no core collapse|date=November 2024}} Black holes created as a result of a stellar collapse are termed stellar-mass black holes.
The combination of the radius and the mass of a star determines the surface gravity. Giant stars have a much lower surface gravity than main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.
Range
One of the most massive stars known is Eta Carinae, with {{Solar mass|100–200|link=yes}}; its lifespan is very short—only several million years at most. A study of the Arches Cluster suggests that {{Solar mass|150}} is the upper limit for stars in the current era of the universe.{{cite journal|last1= Kroupa|first1=P.|title= Stellar mass limited|journal= Nature|volume= 434|issue= 7030|year= 2005|pages= 148–149|doi= 10.1038/434148a|pmid=15758978 |s2cid=5186383 }}{{cite journal|last1= Figer|first1= D.F.|title=An upper limit to the masses of stars|journal= Nature|volume= 434|issue= 7030|year= 2005|pages= 192–194|doi= 10.1038/nature03293|pmid= 15758993|arxiv= astro-ph/0503193|bibcode= 2005Natur.434..192F|s2cid= 4417561}} The reason for this limit is not precisely known, but it is partially due to the Eddington luminosity which defines the maximum amount of luminosity that can pass through the atmosphere of a star without ejecting the gases into space. However, a star named R136a1 in the RMC 136a star cluster has been measured at 215 {{Solar mass}}, putting this limit into question.{{Cite journal|last1=Bestenlehner|first1=Joachim M.|last2=Crowther|first2=Paul A.|last3=Caballero-Nieves|first3=Saida M.|last4=Schneider|first4=Fabian R. N.|last5=Simon-Diaz|first5=Sergio|last6=Brands|first6=Sarah A.|last7=de Koter|first7=Alex|last8=Graefener|first8=Goetz|last9=Herrero|first9=Artemio|last10=Langer|first10=Norbert|last11=Lennon|first11=Daniel J.|date=2020-10-17|title=The R136 star cluster dissected with Hubble Space Telescope/STIS. II. Physical properties of the most massive stars in R136|journal=Monthly Notices of the Royal Astronomical Society|volume=499|issue=2|pages=1918–1936|doi=10.1093/mnras/staa2801|doi-access=free |arxiv=2009.05136 |bibcode=2020MNRAS.499.1918B |issn=0035-8711}} A study has determined that stars larger than 150 {{Solar mass}} in R136 were created through the collision and merger of massive stars in close binary systems, providing a way to sidestep the 150 {{Solar mass}} limit.LiveScience.com, [https://news.yahoo.com/mystery-monster-stars-solved-monster-mash-161251348.html?_esi=1 "Mystery of the 'Monster Stars' Solved: It Was a Monster Mash"], Natalie Wolchover, 7 August 2012
The first stars to form after the Big Bang may have been larger, up to 300 {{Solar mass}} or more, due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.
With a mass only 93 times that of Jupiter ({{Jupiter mass|link=yes}}), or .09 {{Solar mass}}, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core. For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 {{Jupiter mass}}. When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 {{Jupiter mass}}. Smaller bodies are called brown dwarfs, which occupy a poorly defined grey area between stars and gas giants.
Change
The Sun is losing mass from the emission of electromagnetic energy and by the ejection of matter with the solar wind. It is expelling about {{Solar mass|{{val|2|-|3|e=-14}}|link=yes}} per year. The mass loss rate will increase when the Sun enters the red giant stage, climbing to {{Solar mass|{{val|7|-|9|e=-14}}}} y−1 when it reaches the tip of the red-giant branch. This will rise to {{Solar mass|{{10^|-6}}}} y−1 on the asymptotic giant branch, before peaking at a rate of 10−5 to 10−4 {{Solar mass}} y−1 as the Sun generates a planetary nebula. By the time the Sun becomes a degenerate white dwarf, it will have lost 46% of its starting mass.
References
{{Reflist|refs=
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{{citation | last1=Schröder | first1=K.-P. | last2=Connon Smith | first2=Robert | title=Distant future of the Sun and Earth revisited | journal=Monthly Notices of the Royal Astronomical Society | volume=386 | issue=1 | pages=155–163 | doi=10.1111/j.1365-2966.2008.13022.x | date=2008 | doi-access=free | bibcode=2008MNRAS.386..155S|arxiv = 0801.4031 | s2cid=10073988 }}
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