Like ducklings hatched in an alligator swamp, baby stars can form on the edge of a monstrous black hole. Observations by NASA's Chandra X-ray Observatory suggest that young stars in the neighborhood of our galaxy's giant black hole were actually born there, despite the hostile conditions.
In the past decade, astronomers have detected dozens of massive, luminous stars within one light-year of the supermassive black hole in the Milky Way's center. The young stars--just 4 million years old--puzzled astronomers, who thought the black hole's gravity would tear apart nearby gas clouds before they could condense into stars. Instead, astronomers suggested that the stars had been born in a massive cluster at a distance of a hundred light-years or so and then migrated inward.
But according to Sergei Nayakshin of the University of Leicester, United Kingdom, and Rashid Sunyaev of the Max Planck Institute for Astrophysics in Garching, Germany, Chandra's x-ray measurements rule out the migration scenario. To survive the hundred-light-year trek to the Milky Way's core, a star cluster has to be very massive--around a million solar masses--and would contain hundreds of thousands of low-mass stars, which also should end up close to the black hole. Such low-mass baby stars are known to produce x-rays through magnetic flares. But Chandra doesn't detect such x-rays, Nayakshin and Sunyaev report today in an online paper in Monthly Notices of the Royal Astronomical Society. Thus, they conclude, the close-in stars must be local.
Naykshin and Sunyaev think that the stars formed in the dense disk of gas and dust that spins around the black hole. If this accretion disk is massive enough, contracting gas clouds within it can withstand the black hole's disruptive forces and spawn new stars. "It's remarkable that a black hole can help create new stars, not just destroy them," says Nayakshin.
"It's an exciting paper," says theoretical astrophysicist Simon Portegies Zwart of the University of Amsterdam, the Netherlands, who carried out supercomputer simulations of migrating star clusters. However, he thinks Nayakshin and Sunyaev are jumping to conclusions. Less-massive star clusters spiraling inward from a smaller distance--say, 30 or 40 light-years--would deliver a much smaller number of low-mass stars, so you would expect less x-ray emission, Portegies Zwart says. "I'm convinced that the migration scenario still has a role to play."