Supermassive Black Holes
A supermassive black hole is the biggest kind of black hole in the universe. The smallest one we know weighs about a million times more than our Sun, and the biggest weighs 66 billion times more than our Sun. Almost every big galaxy in the universe, including our own Milky Way, has one of these giants in the middle.
- Range1 million to 66 billion SunsFar heavier than any stellar black hole
- Where are they?Galaxy centresMost big galaxies have one in the middle
- Our ownSagittarius A*4 million Suns, in the Milky Way
- Record holderTON 61866 billion Suns, a long way away
- First photographedM87*6.5 billion Suns, imaged in 2019
- Size of M87*About our Solar SystemEvent horizon roughly Pluto-sized
How heavy are the giants?
Famous supermassive black holes by mass. (one solar mass = mass of our Sun)
Sagittarius A* and the smallest supermassive black holes barely register next to monsters like TON 618. Yet even Sagittarius A* is a million times heavier than any stellar black hole.
What is a supermassive black hole?
Supermassive black holes are the same kind of thing as stellar black holes (regions of space with gravity so strong nothing escapes) just much, much bigger. The smallest stellar black hole is approx. 5 times the mass of our Sun. The smallest supermassive black hole is about a million times the mass of our Sun. There is a huge gap between the two, and astronomers are still trying to fill it in with medium-sized "intermediate" black holes.
Where do you find them?
Almost every large galaxy in the universe has a supermassive black hole at its centre. Our own galaxy, the Milky Way, has one called Sagittarius A* (pronounced "Sagittarius A-star"). It weighs approx. 4 million Suns and sits approx. 27,000 light-years away from us. We tracked it down by watching stars near the galactic centre whip around something invisible at huge speed.
The neighbouring Andromeda Galaxy has a black hole approx. 100 million Suns. The galaxy M87, around 53 million light-years from Earth, has one of 6.5 billion Suns. And the current record holder, in a quasar called TON 618, is 66 billion Suns.
How did they get so big?
This is one of the biggest unsolved puzzles in astronomy. The universe is 13.8 billion years old, and we see supermassive black holes that already weighed a billion Suns when the universe was less than a billion years old. There simply was not enough time for a normal stellar black hole to grow that fast by eating gas.
Most scientists think the seeds of supermassive black holes are direct-collapse black holes: huge clouds of gas in the early universe that skipped the star stage altogether and collapsed straight into a black hole of maybe 100,000 Suns. Then they grew by gobbling up gas, stars and even other black holes over the next billion years.
Quasars: the most powerful objects in the universe
When a supermassive black hole has plenty of gas falling into it, the gas forms a spinning disc that heats up to millions of degrees and shines brighter than the entire galaxy around it. This is called a quasar. Some quasars are so bright we can see them from billions of light-years away, across most of the universe.
Some black holes also shoot out two jets of matter from their poles at nearly the speed of light. These jets can stretch for thousands of light-years and punch through whole galaxies.
Deeper dive: active galactic nuclei, AGN feedback and the M-sigma relation
When a supermassive black hole is actively eating gas, its central region is called an active galactic nucleus, or AGN. Quasars, blazars and Seyfert galaxies are all different views of the same basic object: gas spiralling into an SMBH, forming a hot accretion disc, sometimes throwing out relativistic jets at almost the speed of light. The energy released by AGN is enormous, often more than the combined output of all the stars in the host galaxy put together.
That energy travels back out into the galaxy in radiation and high-speed winds. This process, called AGN feedback, can heat up or blow away the gas that would otherwise have collapsed to form new stars. Many astronomers now think AGN feedback is what shuts down star formation in big galaxies, turning them from blue, actively star-forming spirals into red, dead ellipticals over billions of years.
One striking observation is the M-sigma relation: a tight mathematical link between the mass of a supermassive black hole and the speed at which stars in the galactic bulge orbit. A galaxy with stars moving fast in its bulge has a heavy central black hole; a galaxy with slower stars has a lighter one. This was discovered around 2000 and strongly suggests that galaxies and their central black holes grow together, not independently.
The 2017 first direct image of M87*, and the 2022 image of Sagittarius A*, came from the Event Horizon Telescope, a worldwide network of radio dishes that used a technique called very-long-baseline interferometry to act as a single Earth-sized telescope. The images show a bright ring of glowing gas with a dark central shadow, exactly as Einstein's equations predicted. The shape and size of the shadow match general relativity to approx. 10% precision, the best test of strong-gravity behaviour ever made.
The black hole at the very centre of our own galaxy is Sagittarius A*. For its smaller cousins, see stellar black holes. To learn what happens at the edge of a black hole, read about the event horizon.