Size – Stars

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How big is the biggest* star? I’m sure most people, once they learn that stars come in different sizes, will ask themselves that question. The answer, of course, is provisional. According to calculations, the biggest a star can be before the force of its radiation overcomes the strength of its gravity is 150 solar masses. (Stars are measured as multiples of our Sun.) At that size they radiate so fiercely that they blow away mass faster than they can accumulate it. However, in theory the conditions were right shortly after the Big Bang for the formation of stars of 300 solar masses. So, let’s say that the biggest star should be 150 – 300 solar masses.

That brings us naturally to the question of how small the smallest star can be. Again, we have to set the criteria. Should a star be something that glows by its own light, or should we be more strict? I prefer the requirement that a star must be heated by hydrogen fusion, as a distinct cut-off. Otherwise, really big planets heated to glowing by intense gravitational compression might be called stars, and the line would be blurred. At 93 times the mass of Jupiter, AB Doradus C is the smallest known star by that criterion. The theoretical lower limit is 75 Jupiters. Below that is the blurry area occupied by objects called brown dwarfs. Another blurry line at about 13 Jupiter masses separates brown dwarfs from gas giants like Jupiter. It would take over a thousand Jupiters to make our Sun.

For a broad picture of star sizes, here’s the Hertzsprung-Russel Diagram.

* for this article, bigness is defined by mass, not physical size.

rjb