How do astronomers know how heavy stars are?

Astronomers can figure out a star’s mass using a formula based on Kepler’s Law. They can also deduce mass by analyzing a star’s color.

Astronomers know that stars range in mass from about a tenth the mass of our sun – up to about 100 times the sun’s mass. But how do they know?

Astronomers estimate that half of all the stars in the universe may be binary systems – two stars orbiting each other. Looking out into the universe, astronomers can observe these double star systems. They can estimate the masses of the two stars by using what’s called Kepler’s Law – a mathematical relationship between the distance of the stars from each other – and how long it takes the stars to complete a single mutual orbit.

It’s also possible to estimate the masses of some stars by determining the colors of light they emit. For stars on what’s called the “main sequence” – stars like our sun that are still, more or less, in the middle portion of their lives – it’s possible to observe a star’s color to estimate its temperature.

Astronomers then plug this temperature – and the age of the star – into computer models that estimate the star’s mass. The model is based on the way that astronomers think a star evolves over time. In other words, there’s a relationship between how much matter a star is born with – and its temperature and therefore color – and the way in which it ages.

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