Question

How do astronomers estimate the mass of the supermassive black hole at the center of our galaxy?

Answer 1

Whereas at optical wavelengths the centre of our Galaxy is obscured by dust, at radio wavelengths we are able to peer deep into its center and astronomers have discovered a very compact radio source called Sagittarius A* in the constellation Sagittarius which we believe marks the position of a super-massive black hole at the center of our Galaxy. In the same way that we calculate the mass of the Sun by knowledge of the orbital velocity of the Earth and its distance from the Sun, we can estimate the mass of Sagittarius A* by measuring the speeds of stars in orbit around it at very close distances. Consider the case in which a star and a black hole orbit around their mutual centre of gravity. Although we can't see the black hole, we can see the star. With accurate observations, we can measure the speed of the star as well as the size of the orbit. Once these have been measured, the laws of gravity tell us exactly what the black hole mass is.

For example, one of the 8-m VLT telescopes at Paranal Observatory in Chile has observed a star in the infrared as it passed just 17 light hours from the center of the Milky Way (three times the distance of Pluto from the Sun). This convincingly showed that it was under the gravitational in?uence of an object that had an enormous gravitational ?eld yet must be extremely compact – a super-massive black hole. Its mass is now thought to lie between 3.2 and 4 million solar masses con?ned within a volume one-tenth the size of the Earth’s orbit.