Black Hole Binge?
But it's an anemic black hole.
The supermassive black holes in the centers of many other galaxies are surrounded by large, turbulent disks of infalling material, known as accretion disks. As the gas, dust, and demolished stars spiral toward the black hole's "surface," the event horizon, they are heated to millions or billions of degrees. That produces enormous amounts of energy, particularly X-rays.
An X-ray view of the center of the galaxy. The black hole is at the center, but is too small to see. [NASA/CXC/MIT/F.K.Baganoff et al.]
Magnetic fields in the disk funnel some of the material from the innermost region of the accretion disk into thin jets that shoot into space at near the speed of light. The jets produce lots of radio energy, which radio telescopes on Earth detect as active galactic nuclei (AGNs). In fact, it was the discovery of AGNs that motivated the earliest searches for a supermassive black hole at the center of the Milky Way.
Yet the Milky Way's black hole is an inactive galactic nucleus. It does have a small accretion disk, which produces a faint glow in visible, infrared, and other wavelengths. But the total amount of material in the disk probably equals the mass of a pulverized asteroid or two, not a star.
"We have these young stars in this region losing mass," says Ghez. "The mass should fall into the black hole, and this is one of the reasons why it's odd that the black hole today is so quiet. ... That's an interesting problem that people are currently scratching their heads about."
The black hole probably hasn't always been so quiet, though. A century ago or longer, it seems to have gorged itself on something — either a star or two or a vast amount of gas. Space-based X-ray telescopes have seen evidence of the feast as "light echoes" — the sudden brightening of large gas clouds near the galactic center as they are illuminated by an older source of energy, which could be superheated gas as it spiraled into the black hole.
"We're seeing evidence that there was a tremendous flash of X-rays, one or a few hundred years ago, at or near the center of the galaxy," Morris says. "We can't really pinpoint exactly where, but it could easily have been the central supermassive black hole eating a star. [It] is the kind of flash that you would get if a black hole ate a star, and that happens once in a while."
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.