How do black holes get weighed from far away?
Quick Answer
Astronomers measure the motion of nearby stars or gas and use gravity to calculate the black hole’s mass. In some cases, they also use X-ray or radio observations of material orbiting the black hole.
The Full Story
Even though a black hole itself cannot be seen directly, its gravity affects nearby stars, gas, and dust. By tracking the orbits of these objects, astronomers can infer the mass of the black hole causing the motion. For supermassive black holes at galaxy centers, observations from telescopes like the Event Horizon Telescope and space observatories have provided strong evidence and mass estimates. This approach was famously used to measure the black hole at the center of the Milky Way, known as Sagittarius A*.
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