Stellar mass is a phrase that is used by astronomers to describe themass of astar. It is usually enumerated in terms of theSun's mass as a proportion of asolar mass (M_☉). Hence, the bright starSirius has around 2.02 M_☉.^[1] A star's mass will vary over its lifetime as mass is lost with thestellar wind or ejected viapulsational behavior, or if additional mass isaccreted, such as from acompanion star.

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 0.5M_☉ do not enter theasymptotic giant branch (AGB) but evolve directly into white dwarfs. (At least in theory; the lifetimes of such stars are long enough—longer than theage 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 1.8–2.2M_☉ (depending on composition) do enter the AGB, where they develop a degenerate helium core.

Intermediate-mass stars undergohelium fusion and develop a degeneratecarbon–oxygen core.

Massive stars have a minimum mass of 5–10M_☉. These stars undergocarbon fusion, with their lives ending in a core-collapsesupernova explosion.^{[2][dubious –discuss]}Black holes created as a result of a stellar collapse are termedstellar-mass black holes.

The combination of the radius and the mass of a star determines thesurface gravity. Giant stars have a much lower surface gravity thanmain 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 theabsorption lines.^[3]

One of the most massive stars known isEta Carinae,^[4] with 100–200 M_☉; its lifespan is very short—only several million years at most. A study of theArches Cluster suggests that 150 M_☉ is the upper limit for stars in the current era of the universe.^[5][6][7] The reason for this limit is not precisely known, but it is partially due to theEddington 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 namedR136a1 in the RMC 136a star cluster has been measured at 215M_☉, putting this limit into question.^[8][9] A study has determined that stars larger than 150M_☉ inR136 were created through the collision and merger of massive stars in closebinary systems, providing a way to sidestep the 150M_☉ limit.^[10]

The first stars to form after the Big Bang may have been larger, up to 300M_☉ or more,^[11] due to the complete absence of elements heavier thanlithium 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 ofJupiter (M_J), or .09M_☉,AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.^[12] 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 75M_J.^[13][14] 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 87M_J.^[14][15] Smaller bodies are calledbrown dwarfs, which occupy a poorly defined grey area between stars andgas giants.

The Sun is losing mass from the emission of electromagnetic energy and by the ejection of matter with thesolar wind. It is expelling about(2–3)×10⁻¹⁴ M_☉ per year.^[16] The mass loss rate will increase when the Sun enters thered giant stage, climbing to(7–9)×10⁻¹⁴ M_☉ y⁻¹ when it reaches thetip of the red-giant branch. This will rise to 10⁻⁶ M_☉ y⁻¹ on theasymptotic giant branch, before peaking at a rate of 10⁻⁵ to 10⁻⁴M_☉ y⁻¹ as the Sun generates aplanetary nebula. By the time the Sun becomes a degeneratewhite dwarf, it will have lost 46% of its starting mass.^[17]

• Stellar astronomy
• Concepts in stellar astronomy
• Mass

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