Black Holes by the Basketful
Other flashes also light up the galactic center: The explosions of the heaviest stars as supernovae. These blasts stir up the gas clouds and magnetic fields that permeate the center of the galaxy. They also leave behind small, dense remnants — either neutron stars or black holes a few times the mass of the Sun.
Streamers of stars and gas orbit the central black hole in this false-color image, which combines views in infrared and radio wavelengths. [NRAO/AUI]
"We think that the net result of this process happening throughout the lifetime of the galaxy is that the one big supermassive black hole is surrounded by an entourage of tens of thousands of small black holes," says Morris. "They are there to stay. The only place they can go is to merge with the central supermassive black hole, which is probably the ultimate fate of most of them. But for the most part, they're doomed to quietly and darkly stay in the immediate vicinity of the one big black hole.
"Ultimately, we'd really like to know whether that's there, because those 10- or 20,000 black holes would wreak havoc with the orbits of stars that we can see. We won't understand the dynamics of the galactic center unless we know whether this elephant in the room is really sitting there or not."
Nor, despite their conclusion that the central object really is a black hole, will they be able to stamp the subject as "case closed" until they actually see the black hole itself — something that could happen within the next few years, Milosavljevic says.
"The good thing about the gas that's flowing into the black hole is that it's transparent, so that you can see it approaching the event horizon," he says. "The bigger the radio telescope that's being used to observe this gas, the closer we get to the actual event horizon. We are on the verge of observing the gas as it plunges across the event horizon. Once that's been done, we will know for sure" that a supermassive black hole inhabits the Milky Way's busy heart.
It's a process that will take years or decades of additional work, which will be enhanced by new telescopes, new observational techniques, and new ideas.
"One of the fascinating things about this study, that's unlike any other that I've done in my lifetime, is that it's a study that lasts a lifetime," says Morris. "Most of the things I do, I work on them for one or two years, get a result, publish it, and then move on. We started this in 1995 and I don't see any end, and I don't see any end of the richness of results that we're getting, either. Each time we get a new capability or a longer time base for studying some of these things, we learn a lot more than we knew before."
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.