How do scientists measure the mass of a galaxy?
Quick Answer
They look at how fast stars, gas, or satellite galaxies orbit the galaxy and apply gravity. The result usually shows far more mass than the visible stars alone can explain.
The Full Story
Galaxy mass is often estimated from orbital motion: faster rotation or higher velocity dispersions mean more gravity is needed to hold the system together. In spiral galaxies, astronomers use rotation curves to see how speed changes with radius. In many galaxies, the outer regions rotate too quickly to be explained by visible matter alone, which is one of the classic clues for dark matter. For galaxy clusters, scientists also use X-ray gas, gravitational lensing, and the motions of member galaxies to weigh the system.
Key Facts
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"How to weigh a galaxy you can’t put on a scale"