Black holes at the centres of galaxies

Sagittarius A*, at the centre of the Milky Way, has about 4 million times the Sun's mass.

A dark mass at the centre

NASA says most large galaxies have supermassive black holes at their centres, some with billions of times the Sun's mass. A black hole is a region where matter is packed so tightly that nothing, not even light, can escape from inside a boundary called the event horizon. The radius of that boundary grows in step with mass. For one solar mass it is about 3 km. For the Milky Way's central black hole, Sagittarius A* (said "Sagittarius A-star"), with about 4 million solar masses, it works out at about 12 million km, roughly 17 times the Sun's radius and well inside Mercury's orbit.

How can anyone weigh something that gives off no light? By watching what moves around it. For decades, two research groups, one in Germany and one in the United States, followed individual stars orbiting very close to the galactic centre in infrared light, which passes through the dust. One star, called S2, swings round the centre in about 16 years. Applying Kepler's laws to those orbits gives the mass packed inside them, about 4 million Suns in a region too small to be anything except a black hole. That work was recognised with the 2020 Nobel Prize in Physics.

Pictures of shadows

Black holes themselves are dark, but hot gas swirling around them glows. The Event Horizon Telescope links radio dishes across the world into a virtual telescope the size of Earth. In 2019 it released the first image of a black hole's silhouette, the one at the centre of the giant elliptical galaxy M87, about 53 million light-years away according to JPL. In 2022 it released an image of Sagittarius A*, which JPL describes as more than 26,000 light-years away with the mass of 4 million Suns. Both pictures show a dark centre inside a bright ring. The Sagittarius A* image spans only about 50 microarcseconds, which JPL compares to about 1/13th of a billionth of the whole sky.

Quiet and active

Sagittarius A* is quiet today: little gas is falling in, so it is faint. NASA notes that the Milky Way is not currently an active galaxy, although it likely had a burst of activity in the past few million years. In active galaxies, a central black hole is swallowing plenty of gas, which heats up in a disk and can outshine all the galaxy's stars. The most luminous are quasars. NASA says more than a million quasars have been identified; the nearest, Markarian 231, is about 600 million light-years away, and the farthest known lies about 13 billion light-years away. Because light takes time to arrive, distant quasars show black holes as they were billions of years ago.