Surviving in a Bad Neighborhood
Young stars crowd near central black hole
Like a cartoon guru sitting atop a mountain, the supermassive black hole at the center of the Milky Way is surrounded by young followers — a tightly packed cluster of stars. Most of the stars are small, faint, and old. But orbiting within half a light-year of Sagittarius A*, the teams found almost 200 stars that are only about six million years old.
"That introduces some tremendous challenges," says Mark Morris, a professor of physics and astronomy at UCLA. "We expected that the tidal forces from the black hole would prevent star formation from taking place near the black hole, and so it was a growing surprise as we realized that there is a young cluster of stars there."
A view of the star cluster and gas clouds at the center of the Milky Way. [ESO/S. Gillessen et al.]
"You don't expect to see young stars near a black hole because a black hole is not a nice neighbor to a stellar nursery," says Andrea Ghez. "Black holes tend to shred things apart. To get stars to form you need a gentle environment that allows big, fragile clouds of gas and dust to collapse under their own self gravity. If you have a black hole right next to you and it shreds the cloud apart, it's very hard to imagine how to get stars to form."
Yet the same measurements that revealed the mass of the black hole also revealed the masses of the stars that orbit it. A star's mass provides an upper limit to its age because heavier stars "burn" the nuclear fuel in their cores much more quickly than modest stars like the Sun.
The observations, along with the temperatures and other characteristics of the stars, show that the heaviest stars are a few dozen times as massive as the Sun, indicating a maximum age of six million to seven million years. These stars are gravitationally bound to a massive star cluster that surrounds the black hole. Since the stars in a cluster are all born together, the entire cluster is the same age as its heaviest members.
What's more, two other clusters, known as the Arches and the Quintuplet, lurk near the black hole, each with its own population of young stars. Each of the three clusters contains several times as many stars as the young clusters in Earth's portion of the Milky Way, which is far out in the galaxy's disk.
"Stars are forming out of those clouds [around the black hole], but not in the same way they do out in the disk or in our suburban neighborhood where we live," says Morris, who has been studying the center of the galaxy since the 1970s. "The stars that form in the galactic center, at first sight, seem to form in these incredibly rich clusters that form all at once, in one fell swoop, 10,000 stars — bang! — in very compact, massive clusters."
"It's an interesting problem — the paradox of youth," says Ghez. "How in the world do you get these young stars in a region where you just don't expect them? There's been a huge industry of theoretical ideas trying to overcome this conundrum."
One idea says the stars aren't really young at all, but instead have been altered by mergers or other events that make them look young.
A second idea says the cluster's stars are young, but they formed farther from the black hole and were quickly funneled inward, perhaps by gravitational encounters with medium-sized black holes. Yet astronomers know of no way for that many stars to be pushed such a great distance in so short a time, and there's no evidence yet of another good-sized black hole in the vicinity of Sagittarius A*.
A third idea says the stars actually formed where we see them today, in a disk encircling the black hole — an idea supported by the orbits of many of the young stars, which follow a similar orbital plane. (In fact, there may be two groups of the young stars, each following its own orbital path.)
Yet like the other two, that scenario presents some serious questions, Ghez says. "If you think star formation happened at the center of the galaxy, where did all that gas come from? How do you funnel it in? Is it a cloud that just happened to pass by in the right kind of orbit that got captured in this region? Is it the collision of clouds that allows the material to stream into the center? Or is it a recent merger? We don't know. We have no idea. This is one of the $64,000 questions." — DB
Is there a supermassive black hole at the center of the Milky Way galaxy?
Results to Date
There is a supermassive black hole, roughly 4.1 million times the mass of the Sun, at the center of the galaxy.
The Black Hole at the Center of Our Galaxy, by Fulvio Melia, 2003
Sagittarius A*, the black hole at the center of the Milky Way, is about 16 million miles (25 million km) in diameter, or about half the distance from the Sun to Mercury, the innermost planet.