The Milky Way's Supermassive Black Hole Is... Tiny?

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At the center of the Milky Way exists a supermassive black hole four million times more massive than our sun. We haven't actually seen it, but we know it's there through numerous pieces of evidence. One, there's an extremely bright and compact radio signal coming from that location. Two, astronomers have observed dozens of stars dancing around it at high speeds. And three, in 2018, scientists detected clumps of gas nearby moving at about 30% the speed of light, consistent with hot gas orbiting a black hole.

So this black hole is there and it's big – supermassive, in fact! But as dense as the Milky Way's central black hole is, it's positively puny compared to other galactic supermassive black holes. Our neighboring galaxy Andromeda's supermassive black hole is 230 million solar masses! Making that ~58x mass difference even more remarkable is the fact that Andromeda only has between 2.5 and ten times more stars than the Milky Way, so its black hole much bigger in proportion. Farther away, Quasar 3C 273, the brightest quasar in Earth's sky, is 886 million solar masses! (A quasar is a supermassive black hole at a galactic center surrounded by a gaseous accretion disk.)

NASA, A. Martel (JHU), H. Ford (JHU), M. Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA

"If you placed quasar 3C 273 in our Milky Way, it's center would be forty billion times brighter than what we [are] measuring there now," Radboud University astrophysicist Heino Falcke wrote in his recent book Light in the Darkness.

Falcke knows a lot about supermassive black holes. He's been studying the Milky Way's galactic black hole for almost his entire career, and recently held a leading role with the Event Horizon Telescope project, which in 2019 produced the first direct image of a black hole.

In his book, Falcke described how small the Milky Way's black hole is compared to a quasar. More than 750,000 of these supermassive black holes are known to exist.

"Quasars gobble up approximately one sun per year..." he wrote. Our black hole consumes ten million times less mass. "Our Galactic Center would thus be a black hole on a starvation diet–though fasting would be a strange analogy, since ten million times less mass would still be three moons per year that it was devouring."

The largest black hole spotted to date is called TON 618. At 66 billion solar masses, it is 15,300 times more massive than our black hole.

Some physicists think that a few black holes could tip the scales at over a trillion solar masses, large enough to distort the very shape of their host galaxies!

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