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SESK: Stuff Everyone Should Know · sesk.info/astro-binary-star-masses

How do astronomers measure the masses of stars most directly?

By measuring the orbits in binary star systems.

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  1. From the star's color alone
  2. From how fast it twinkles
  3. From binary star orbits ✔

Why: Gravity links orbit size and period to mass, just as planet orbits reveal the Sun's mass. Two stars orbiting each other give us their masses. About half of stars are in binaries.

Huh, didn't know that: A spectroscopic binary looks like one star, but its spectral lines shift back and forth as the pair orbits.

The dad joke
Binary stars always come in pairs. They hate going to the movies alone.
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References

Answer: From binary star orbits. Gravity links orbit size and period to mass, just as planet orbits reveal the Sun's mass. Two stars orbiting each other give us their masses. About half of stars are in binaries.

  1. Astronomy 2e, 18.2 Measuring Stellar Masses openstax.org/books/astronomy-2e/pages/18-2-measuring-stellar-masses
    Masses of binary stars can be calculated from measurements of their orbits
    Checked 2026-10-10.

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