X-ray Image of Sagittarius A*

By discovering that stars can form in disks around massive black holes, astronomers have shown that these galactic monsters can nurture stars as well as destroy them. Here, Sterl Phinney, an expert on the physics of black holes, explains the astrophysical significance of these results.

I'd like to start by commending NASA for its bravery: most NASA press releases say "scientists have discovered something", but this one says "NASA scientists have failed to discover something". This is a bit like Sherlock Holmes' famous "Silver Blaze" mystery about the theft of a race horse, where Sherlock Holmes describes "the curious incident of the dog in the nighttime." When Inspector Gregory says, "the dog did nothing in the nighttime", Holmes replies, "that was the curious incident". This is an important principle of science, as well as detective work: when expected things do NOT happen, it can be just as important a clue as discovering something that did happen.

X-ray Image of Orion Nebula

All through our galaxy and others, we see stars forming. Always before, when we've seen heavy stars form, they have been accompanied by a retinue of lighter stars. As the newly formed young stars are settling down, their bubbling makes some X-ray emission, as for example in the spectacular Chandra X-ray image of the thousands of young light stars in the Orion nebula surrounding the 4 massive stars of the Trapezium. But here in the depths of the galactic center, where the densities of the gas are a billion times greater than the other places stars form (that's a million times greater than the density difference between air and mud), we see massive stars, but Chandra doesn't see the expected X-rays from light stars. So, either the light stars are not there at all, or in that weird environment, stars form so differently that they don't emit X-rays while settling down.

Why does all this matter? Although the black hole in the center of our Milky Way is now lurking, dim and incognito, there is good evidence that it was once active like a quasar. Quasars are brilliant beacons of light that astronomers can see to the edges of the universe. They consist of giant black holes swallowing huge amounts of hot gas. They are rare, incredibly bright, but short-lived. Forty years ago Donald Lynden-Bell pointed out that although quasars are rare, they are so short-lived that most galaxies must have been through a quasar phase, and so most galaxies must have a giant, but "quiet" (no longer swallowing gas) black hole in them. Lynden-Bell's prediction has since been spectacularly verified by the discovery of giant black holes in the center of our Milky Way and all the other galaxies near it.

Animation of Stars Forming Around Black Hole

But for 40 years, there have been nagging puzzles about quasars. The theoretical models of the gas disks that feed quasar black holes, pioneered by Rashid Sunyaev and others, predicted that beyond a certain radius, the disks should have been unstable: gravitational attraction of the gas should have made it clump up and collapse into stars, as shown in this animation. So the question was: how could the gas get through this radius intact, to feed the quasar, and were stars actually formed or did something prevent it?

A related puzzle: the gas around even the most distant quasars, only one tenth as old as the Milky Way, is as full of elements made in exploding stars as is the Milky Way. How did so many stars form quickly so near the black hole?

We had no way of seeing the answers to these puzzles until very recently.

Illustration of Disk Around Black Hole

Now the pieces are coming together. In the last few years we have seen the following results: first, X-ray and infrared observations have confirmed the presence of dense disks of gas around a light year from most accreting black holes.

Second, infrared observations with the Keck and VLT telescopes revealed the disk of massive young stars around the Milky Way's black hole. These Chandra observations seem to rule out the idea that the stars formed elsewhere and were dragged in, so they probably formed from the gas around the black hole.

Third, in 1971 the final flight of the Stratoscope II balloon telescope (the NASA test predecessor of Hubble Space Telescope) discovered, and ground-based telescopes and HST confirmed that in the center of the Milky Way's nearby sister galaxy, Andromeda (M31), there is a disk of young blue stars only a light year across, inside a ring of older stars.

So if stars are forming so close to the big black holes in the two galaxies nearest to us, stars are probably forming in the gas disks around all big black holes.

NASA and the European Space Agency are building a really exciting thing called the Laser Interferometer Space Antenna (LISA), to detect gravitational waves from these giant black holes and stars falling into them. The bad news is that if there had been enough very low mass stars, LISA might have seen some of them orbiting in our own Milky Way. This now looks unlikely. The good news is that lots of massive stars around all big black holes will create lots of small black holes to fall into the supermassive black holes. So, LISA will see hundreds of these, to great distances, on their way in, providing fantastically precise tests of black hole physics and relativity.

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Revised: September 13, 2012