One of the most massive stars known is Eta Carinae,[102] with 100–150 times as much mass as the Sun; its lifespan is very short—only several million years at most. A study of the Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe.[103] 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 265 solar masses, which put this limit into question.[104] A study determined that stars larger than 150 solar masses 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.[105] The reflection nebula NGC 1999 is brilliantly illuminated by V380 Orionis (center), a variable star with about 3.5 times the mass of the Sun. The black patch of sky is a vast hole of empty space and not a dark nebula as previously thought. NASA image The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more,[106] 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, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.[107] For star

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 times the mass of Jupiter.[108][109] 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 times the mass of Jupiter.[109][110] Smaller bodies are called brown dwarfs, which occupy a poorly defined grey area between stars and gas giants. 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.[32] 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 masses do not enter the asymptotic giant branch (AGB) but evolve directly into white dwarfs. Low mass stars with a mass below about 1.8–2.2 solar masses (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 7–10 solar masses, but this may be as low as 5–6 solar masses. These stars undergo carbon fusion, with their lives ending in a core-collapse supernova explosion.