Zeroing in on a Dark Heart
The central question that Ghez, Genzel, and others hoped to answer was whether a supermassive black hole inhabits the center of the Milky Way.
As early as 1980, radio observations revealed that gas was swirling around a dark, massive object at the center of the galaxy. That object, which glowed steadily at radio wavelengths, was designated Sagittarius A* ("A-star") for its location in the constellation Sagittarius. And by the early '90s, observations of the motions of stars and gas clouds suggested the dark object was more than two million times as massive as the Sun.
But that region of the galaxy is about 27,000 light-years away, so it looks tiny in even the largest telescopes, and it is hidden behind thick clouds of dust in the galaxy's disk. No telescope had been able to probe deeply enough into the heart to determine just how tightly the mass was packed, leaving a lot of wiggle room for interpretation. The large mass could have been concentrated in a single black hole, but it just as easily could have been spread out in a cluster of smaller black holes, neutron stars, or other dense, faint objects.
Narrowing the list of possibilities required sharper pictures. With enough detail, astronomers could see many individual stars orbiting the concentrated mass at the galaxy's center. Plotting the orbits of the stars, then applying the laws of orbital motion, would allow astronomers to calculate the mass of the central object and narrow the estimates of its size. Those measurements, in turn, would whittle the list of possible explanations.
Genzel began obtaining those measurements with a large telescope in Chile, while Ghez soon followed with the 10-meter (33-foot) Keck telescope in Hawaii. Both telescopes are above most of the water vapor in Earth's atmosphere, allowing them to view the universe at infrared wavelengths, which penetrate the galaxy's dust clouds.
In addition, they could apply a new technique, called speckle imaging, that compensates for Earth's atmosphere by taking thousands of short-exposure images and stacking them together. Each frame "freezes" the atmosphere in place, eliminating the blurring caused when starlight passes through atmospheric layers of different density. Instead of a fuzzy blob, a star looks like a crisp pinpoint — a necessity for plotting precise orbits of stars in the crowded "downtown" of the Milky Way's center.
"I was looking for a problem that would really benefit by having that tool, that hammer so to speak," Ghez says. "I was looking for the right nail. At that point, it was clear that the question of whether there is a supermassive black hole was the perfect question to address, because the technology had just gotten to the point that you could observe the center of the galaxy."
So the two teams began probing about two dozen stars that are within a few light-weeks of Sagittarius A*. (The orbit of Neptune, the most distant planet from the Sun, spans about two-thirds of a light-day, which is the distance that light travels in one day.) The stars are much bigger and brighter than the Sun, so they are relatively easy to pick out.
Over the following decade, the teams plotted the positions of their target stars several times a year, primarily in summer in the northern hemisphere, when the center of the galaxy is in best view.
As the orbits began taking shape, another technological advancement sharpened the view even more.
Called adaptive optics, it was developed by the military to track orbiting satellites and space junk. Both the Keck system and the one used by the Very Large Telescope in Chile, a group of four eight-meter telescopes which Genzel has used over the last decade, beam a laser into the upper atmosphere, where it strikes sodium atoms, causing them to glow. The telescope keeps this pinpoint of light in sharp focus by altering the shape of a small secondary mirror to compensate for the blurring motions of the atmosphere. If this laser-generated "guide star" is in good focus, so are the stars that appear near it in the sky.
The combination of techniques allowed the teams to plot detailed orbits for each of the stars in their studies. One of the stars, a bright blue sizzler designated S2 or SO-2, has already made a full turn around Sagittarius A*, passing less than one light-day from it, but most of the others still haven't completed the circuit. Even so, the astronomers have plotted the orbits with enough accuracy to greatly refine the measurements of the size and mass of the object they are orbiting.
This animation shows stars orbiting the supermassive black hole. [Keck/UCLA Galactic Center Group]
In addition, continued observations with radio telescopes show that the central object remains fixed in place relative to the stars around it like the hub of a merry-go-round. "This source is motionless, which is perfectly consistent with it being very massive because the more massive you are the less you move," says Milos Milosavljevic, an astronomer at The University of Texas at Austin who studies Sagittarius A* and other supermassive black holes. "We see the stars but do not see anything terribly interesting at the location of the common center, the place where a large amount of mass has to be located to explain the motion of the stars."
The conclusion, says Ghez, was as inescapable as a black hole's gravity.
"[We went] from knowing there's four million times the mass of the Sun confined in a region where you see lots of other stuff that could easily explain the mass that you're seeing, down to a region that's the size of our solar system," she says. "And at that point, there are no other alternatives to explaining this mass concentration. You really are left with a supermassive black hole."
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.