Measuring Black Holes

Determining a black hole's mass takes creative methods. For stellar-mass black holes in binary systems, astronomers measure the orbital motion of the visible companion star — the faster and tighter the orbit, the heavier the unseen partner.

For supermassive black holes, the most direct method is tracking stars that orbit the galactic center. Andrea Ghez and Reinhard Genzel won the 2020 Nobel Prize for mapping stellar orbits around Sagittarius A* over decades, pinning its mass at 4 million suns.