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13.8 Mass Limits for Stars

What factors determine the range of possible star masses? When astronomers carry out surveys, they find stars that span a factor of a thousand in mass. Mass is the most fundamental property of a star, determining its lifetime, method of energy production, and eventual fate. The most massive stars are about a hundred times the Sun's mass and the least massive stars are about a tenth of the Sun's mass. Yet gravity can cause the collapse of gas clouds with an enormous range of masses. Why are there not enormous stars a thousand or more times the mass of the Sun? And why are there not pint-sized stars one hundredth the mass of the Sun or less? It turns out that physics limits the range of star masses.

There are physical reasons for the mass limits on stars. If a cloud is dense and cool enough to contract but has less than about 8% of a solar mass, it will contract but never develop a high enough central pressure and temperature to reach a main sequence state (i.e. no extensive fusion of hydrogen). Protostars from about 0.01 to 0.08 solar mass may heat up temporarily due to their gravitational contraction and may even develop a few feeble nuclear reactions involving the creation of deuterium, but eventually they fade without settling on the main sequence for any appreciable time. Smaller objects, including even Jupiter-sized planets, also warm up temporarily due to their gravitational contraction; they may glow for a while in the infrared, but they fade and they don’t have enough gravity for any nuclear reactions to start.

Astropedia Image
Eta Carinae is about 100 times more massive than the Sun. Click here for original source URL.

The definition of the upper bound on star mass is a subject of active research. Since massive stars are very rare, the few examples known are very far from the Earth. In most cases, the mass is inferred from limited observations and a model for the star, so it’s quite uncertain. There are a handful of stars with inferred masses 200 to 500 times the mass of the Sun. The most massive star with a reliable mass estimate, because it is in a binary system, is HD 15558, at 150 times the mass of the Sun. Why doesn’t a star form if the mass is too high? If the cloud is dense enough to contract but is more than about 100 solar masses, the contraction is violent and produces an extremely high central temperature and pressure. Under these conditions, so much energy is generated inside the new star that the star is very luminous and may blow itself apart almost immediately without spending much time on the main sequence. This rapid destruction leads to some of the most spectacular events in the night sky. Explosions of massive stars explain many features of our starry surroundings.