11.3 Classification by Mass and Size

How large can the mass of a star be? Stars more massive than the Sun are rare. Most stars have less mass than the Sun. The Sun, itself, is a low-mass star, as are all stars with mass less than 1.33 times the mass of the Sun.

None of the stars within 30 light-years of the Sun has a mass greater than four times that of the Sun. Those with mass between 1.33 and 4 times the mass of the Sun are intermediate-mass stars.

Searches at large distances from the Sun have led to the discovery of a few stars with masses up to about 100 times that of the Sun, and a handful of stars (a few out of several billion) may have masses as large as 250 solar masses. Any star with mass exceeding four times that of the Sun is categorized as high-mass.

The table above shows that the most massive stars only have a few million years of stability. A star of 1 solar mass remains stable for roughly 10 billion years, while a star of about 0.4 solar mass can be stable for some 200 billion years, which is longer than the current age of the universe. A star spends most of its total lifetime (an average of about 90% of the total) as a stable star.

As far as size (radius or diameter) is concerned:

• Giant stars have radii between 10 and 100 times that of the Sun
• Dwarf stars have radii equal to, or less than, that of the Sun
• Supergiant stars have radii more than 100 times than that of the Sun

The results of many stellar size measurements over the years have shown that most nearby stars are roughly the size of the Sun, with typical diametres of a million kilometres or so. Faint stars, as we might have expected, are generally smaller than more luminous stars. However, there are some dramatic exceptions to this simple generalization.

A few of the very luminous stars, those that are also red (indicating relatively low surface temperatures), turn out to be truly enormous. These stars are called, appropriately enough, giant stars or supergiant stars. An example is Betelgeuse, the second brightest star in the constellation of Orion and one of the dozen brightest stars in our sky. Its diameter, remarkably, is greater than 10 AU (1.5 billion kilometres), large enough to fill the entire inner solar system almost as far out as Jupiter.

Attribution

“18.2 Measuring Stellar Masses“, “22.1 Evolution from the Main Sequence to Red Giants“, and “18.3 Diametres of Stars” from Douglas College Astronomy 1105 by Douglas College Department of Physics and Astronomy, is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. Adapted from Astronomy 2e.