What unit is commonly used to state a black hole’s mass relative to the Sun? The answer is the solar mass.
A solar mass is the mass of the Sun and serves as a convenient astronomical unit for comparing massive objects. It is approximately 1.989 × 10^30 kilograms. Saying that a black hole has a mass of 10 solar masses means it contains roughly ten times the Sun’s mass, not that it has the Sun’s size or composition.
Astronomers use solar masses because kilograms produce unwieldy numbers on cosmic scales. Stellar-mass black holes commonly have masses of a few to dozens of solar masses, while supermassive black holes at galaxy centers can weigh millions or billions of solar masses.
A frequent mix-up is confusing solar mass with an astronomical unit, parsec, or light-year. Those are units of distance: an astronomical unit is based on the Earth–Sun separation, and a light-year and parsec measure immense distances. Solar mass measures mass. Black-hole masses are often estimated from the orbital motion of companion stars, gas, or nearby stars.