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How do you measure the mass of a star?

9 May 2021, 11:31 UTC
How do you measure the mass of a star?
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Artist’s concept of the binary star system of Sirius A and its small blue companion, Sirius B, a hot white dwarf. The 2 stars revolve around each other every 50 years. Image via ESA/ G. Bacon.
There are lots of binary stars – two stars revolving around a common center of mass – populating the starry sky. In fact, a large majority of all stars we see (around 85%) are part of a multiple star system of two or more stars! This is fortunate for astronomers because two stars together provide an easy way to measure their respective masses.
To find the masses of stars in double systems, you need to know only two things: the mean distance between the two stars (often expressed in astronomical units, which is the average distance between the Earth and sun), and the time it takes for the two stars to revolve around one another (aka the orbital period, often expressed in Earth-years). With those two observations alone, astronomers are able to calculate the stars’ masses, which they typically do in units of solar masses (that is, a measure of how many of our suns the star “weighs”. One solar mass is 1.989 ...

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