How do you weigh stars? © Getty Images

How do you weigh stars?

Though they look like tiny, sparkly dots from way down here, stars are massive bodies of hot gas - so how do we do about measuring their weight?

Asked by: Ali Ersoz, by email

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The trick lies in exploiting the connection between a star’s mass and the strength of the gravity it generates. For stars with planets, the strength of gravity dictates how fast a given planet will complete an orbit of any specific size, and Newton’s law of gravity can then be used to give the star’s mass. The same technique also works for binary stars, where two stars orbit around each other. Measuring the mass of isolated stars is harder, but also possible.

Astrophysicists have found a relationship between the power output of a star and its mass, allowing measurements of a star’s luminosity to be turned into estimates of its mass. Another method is to exploit subtle effects of gravity on light. According to Einstein’s theory of gravity, mass generates gravity by distorting the fabric of space and time around it. This, in turn, affects light rays passing near masses, bending them away from their straight-line paths. In 2004, a team of astronomers in the US announced the detection of distortions in light passing by a small red star around 1800 light-years away. Using Einstein’s theory and measurements of the distortions, the team found that this isolated star had a mass of around one-tenth that of the Sun.


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