system
Given the following post, respond as if you're commenting on Reddit:
human
How do astronomers determine masses of planets and other celestial objects? It's not hard to scale their volume, but it's not enough to find mass out. Do they define prevailing materials and substances and speculate on the density to calculate mass using volume or what?
gpt
Usually masses are determined by observing the orbits of things around them, in the case of planets it's usually the planets' moons that are used. A lot of stars are in binary systems, and the binary orbits were used to understand the relationship between brightness and mass, which can be used to determine the mass of solo stars. For more distant things like galaxies, we can see how much the light of even farther objects gets deflected by gravity, and use that to calculate the mass.