BERKELEY - Two decades ago, when University of California,
Berkeley physicist Charles Townes and his colleagues, including
post-doc Reinhard Genzel, claimed to have evidence for a massive
black hole at the center of the Milky Way, few believed them.
Not that astronomers hadn't long suspected a black hole at
the center of most galaxies, including our own, that had formed
from the collapse of many stars into a mass so compact that
not even light could escape its gravitational tug. But the evidence,
Genzel now says, wasn't convincing enough for most of their
Now, it turns out, Townes and Genzel were right.
Last week, Genzel and colleagues at the Max Planck Institute
for Extraterrestrial Physics in Germany reported in Nature nearly
incontrovertible evidence that our galaxy indeed has a massive
compact object at its core — a black hole.
The evidence "makes it very difficult for anyone to say
it's not a black hole, and the doubters will now have to give
up their doubts," Townes told the San Francisco Chronicle
"The case for a black hole at the center of our galaxy
was already very strong, mostly through Reinhard's previous
work," added black-hole modeler Eliot Quataert, assistant
professor of astronomy at UC Berkeley, in a recent interview
with UC Berkeley Media Relations. "This just makes it closer
to being airtight. There's no reasonable alternative interpretation
to what's going on in the galactic center."
Genzel, now a director of the Max-Planck Institute for Extraterrestrial
Physics near Munich, Germany, who spends part of his time as
a physics professor at UC Berkeley, has tracked for the past
10 1/2 years a star that orbits in a highly eccentric, comet-like
path around the center of the galaxy, which is located in the
constellation Sagittarius. His hope has been that, by charting
the exact path of this star, called S2, he could deduce the
mass of the object it orbits.
Today, he and his colleagues have observed about 2/3 of the
star's entire 15.2-year orbit, and calculate that the galactic
core has the mass of some 3 million stars like our sun, all
tightly packed into a region less than the size of our solar
Stars buzz "like a swarm of flies" around the center
of the galaxy, Genzel said in a phone interview from Germany,
but few have short enough periods to track in a few years' time.
And only recently, with the advent of adaptive optics, have
astronomers been able to pick out individual stars in this tight
cluster. Adaptive optics is a technique to remove the twinkle
from stars, by flexing segmented mirrors fast enough to stabilize
and focus the bouncing image created by turbulent air in the
Using new adaptive optics on the European Southern Observatory's
Very Large Telescope (VLT), located in the high-elevation Atacama
Desert of Chile, Genzel and his team were able to get the resolution
needed to follow S2.
The key to their recent success, however, was a chance observation
this spring of S2 as it swung through the part of its orbit
closest to the galactic center. Genzel's team measured a velocity
of 5,000 kilometers per second — 170 times faster than
the Earth orbits the sun. With a newly installed wide-angle
near-infrared camera on the VLT, they captured images of S2
with other radio sources nearby, allowing the researchers to
pinpoint the exact location of S2 relative to the radio source,
Sagittarius A*, long suspected as the black hole at the center
of the galaxy. Sagittarius A*, or SgrA*, is a bright spot of
radio waves and X-rays that astronomers see when they look
the galactic core.
"We could actually nail down the position of the suspected
culprit, the radio source SgrA*, on the infrared images. So
we can say, during the time the star was moving fast, it was
also very, very close — three times the size of Pluto's
orbit in our solar system — to that radio source,"
Genzel said. "That's why we now know that not only is
there a mass there, that mass actually is identical with that
source, as expected."
With S2 orbiting so close to a mass so large, the central object
could only be a black hole, Genzel and his team concluded. Other
proposed objects at the center, such as a cluster of neutron
stars or smaller black holes, would not fit within such a confined
Theoretically, a black hole is a singularity in spacetime,
a mathematically precise point into which all the mass has been
squeezed. This singularity can never be seen, however, because
it is surrounded by a spherical region into which all infalling
matter disappears and from which nothing, not even light, escapes.
This "horizon" is located at the so-called Schwarzschild
radius, which for a 3 million solar mass black hole sitting
where our sun is, would extend about one tenth the distance
to the Earth, Genzel said. The star S2 approached within 2,000
Schwarzschild radii of the galaxy's central black hole.
Genzel said that some 20-30 other stars orbit closer to the
galactic core than S2, some of which no doubt pass even closer
to the Schwarzschild radius. With techniques like stellar infrared
interferometry, which should become available within five years,
it would be possible to follow the orbits of these stars and
perhaps see weird gravitational effects from the nearby black
"If we could see a star get as close as10-15 Schwarzschild
radii, that would be in the region of strong gravity, so then
you could test some extra things that are predicted by general
relativity," Genzel said. "These stars would have
velocities not 5-8,000 km/sec, but 30,000 km/sec."
The star S2, though continually bombarded by X-rays emitted
by gas streaming into the black hole, is sufficiently far away
that it probably doesn't even know it's orbiting a black hole,
Townes, Genzel and their colleagues were the first to estimate
the mass of the black hole at the galactic core. Based on infrared
tracking of the motion of ionized hydrogen gas clouds around
the galactic core, they calculated that the black hole contained
about 4 million solar masses, not far from Genzel's new estimate
of 3 million solar masses.
But many objected, saying that the amount of radiation emitted
by the radio source Sagittarius A* was 10,000 to 100,000 times
less than expected from a black hole at the center of the Milky
Since then, Quataert said, astrophysicists have come to realize
that gas flowing toward a black hole actually has a hard time
hitting it. Rather than streaming through the horizon and growing
the hole, much of it is swirled around and thrown out in the
form of jets. His analogy is water draining out of a bathtub,
but instead of swirling down the drain, it shoots out of the
tub. Quataert models black hole dynamics, including the inflow
of gas, to better understand radiation from black holes.
Now that the identity of the object at the center of the galaxy
is nearly certain, Genzel is eager to explore the other data
he has on the star cluster at the core.
"This a real fantastic laboratory at the galactic center
to understand how a nuclear star cluster comes about, how new
stars are being formed in an environment where there shouldn't
be any new stars," he said.
In these studies, he acknowledges a friendly rivalry with UCLA
astronomers, who have unpublished data supporting Genzel's new
With evident pride in his former student, Townes said, "Genzel's
group did it first and best. There's no doubt about that."