stellar mass

stellar mass

The mass of a star is its most fundamental property, upon which its other properties depend strongly. It is usually given in terms of the Sun's mass, i.e. as some number of solar masses, MO; it ranges from about 0.08 to about 60 to 120 MO. Stars of higher mass are much less common than those of low mass. Low-mass stars have an initial mass smaller than about 2.2–2.5 MO, the exact value depending on chemical composition. Intermediate-mass stars have masses between about 2.2–8 MO; they evolve from the main sequence without developing a degenerate core, unlike low-mass stars (see stellar evolution). More massive stars evolve to a state where their core temperatures are high enough to burn carbon under nondegenerate conditions.

The symbol ψ(M) is given to the number of stars with a particular mass M in a unit volume of space, and Salpeter (1955) showed that it is related to the star mass by the formula

ψ(M) ∝ M –2.35

Because of mass loss and mass transfer, this Salpeter mass function holds strictly only for stars at the instant of birth, and it is therefore sometimes called the initial mass function (IMF). The IMF is determined by fragmentation and other little-understood processes during star formation.

The lower limit on a star's mass is the minimum amount of gas whose gravitational compression will raise the central temperature high enough for nuclear fusion to occur; less massive fragments from the initial cloud may contract directly to become a degenerate star known as a brown dwarf. It is still uncertain whether there is a theoretical upper limit to star masses, or whether the rarity of very massive stars means that a galaxy is unlikely to contain a star over about 120 MO. Indirect methods indicate that Eta Carinae and some other luminous blue variables may be as massive as 120 MO .

Most stars lie on the main sequence of the Hertzsprung–Russell diagram. Their position on the main sequence, i.e. their spectral type, depends on their mass, which varies from 0.1 MO (M stars) to over 20 MO (O stars). Supergiants are generally 10 to 20 MO but young O and B supergiants are much more massive.

The mass of a star can be determined directly if it has a significant gravitational effect on a neighboring star. Thus the combined mass and in some cases the individual masses of visual and spectroscopic binary stars have been found (see also dynamical parallax). Mass can be estimated using the mass-luminosity relation, or from a detailed study of the spectrum that indicates the star's surface gravity.

The evolutionary pattern and lifetime of a star depend on its mass. The least massive stars have the longest lifetimes of thousands of millions of years; the most massive exist only a few million years before exploding as supernovae. At the end of its life, following mass loss in a planetary nebula, etc., a star will become a white dwarf if its mass is less than about 1.4 MO; if the mass exceeds about 3 MO it is likely that the star will become a black hole; a star with intermediate mass will end up a neutron star.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

stellar mass

[′stel·ər ′mas]
The mass of a star, usually expressed in terms of the sun's mass.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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