What is a supermassive black hole?

It is a black hole of between about 100,000 and tens of billions of solar masses, found at the centre of nearly every large galaxy. The Milky Way's is Sagittarius A*, at 4.3 million solar masses.

How do we know Sagittarius A* exists?

By tracking individual stars orbiting it. The star S2 orbits an invisible 4.3-million-solar-mass point every 16 years, reaching 3% of light speed. The work earned the 2020 Nobel Prize in Physics.

What was the first image of a black hole?

The Event Horizon Telescope released an image of M87*, the 6.5-billion-solar-mass black hole in galaxy Messier 87, in April 2019. Sagittarius A* was imaged by the same collaboration in 2022.

How did supermassive black holes get so big so early?

It remains an open question. Quasars powered by billion-solar-mass black holes existed less than a billion years after the Big Bang, which is hard to reach by ordinary accretion — so heavy seeds or direct collapse of gas clouds are actively investigated.

什么是Supermassive Black Hole？

A supermassive black hole contains between a hundred thousand and tens of billions of solar masses, and sits at the gravitational centre of almost every large galaxy. The Milky Way has one: Sagittarius A*, 4.3 million solar masses, about 26,000 light-years away.

We know its mass with unusual precision because we can watch stars orbit it. Since the 1990s, teams led by Reinhard Genzel and Andrea Ghez tracked individual stars whipping around an invisible point at the galactic centre — the star S2 completes an orbit every 16 years, reaching 3% of the speed of light at closest approach. The measurement won the 2020 Nobel Prize in Physics and left no room for any explanation but a black hole.

In 2019 the Event Horizon Telescope, a planet-sized array of radio dishes acting as one instrument, produced the first direct image of a black hole's shadow: M87*, 6.5 billion solar masses, at the centre of the galaxy Messier 87. Sagittarius A* followed in 2022. Both images show the same thing this model renders — a dark shadow ringed by light bent around the hole, with the approaching side brightened by Doppler beaming.

They are not passive. The mass of a galaxy's central black hole correlates tightly with the velocity dispersion of its stars, a relationship known as M-sigma, implying that the hole and the galaxy grew together. When one feeds actively it becomes an active galactic nucleus, launching jets that can punch clean out of the galaxy and regulate star formation across hundreds of thousands of light-years.