Like
ballerinas pirouetting around an invisible leader, a collection of stars orbits
our galaxy's gravity sink, or black hole. New infrared images of the cosmic
dance confirm that this supermassive black hole weighs as much as 4 million
suns.
Supermassive
black holes can weigh as much as a billion suns or more and are thought to reside
at the centers of most, if not all, galaxies. They can't be seen, because
their gravity is so powerful it traps even light, but astronomers infer their
presence by watching the motions of stars and gas around them.
Over a
period of 16 years, beginning in 1992, researchers monitored 28 stars orbiting
the Milky Way's central region, where the supermassive black hole called Sagittarius
A* is thought to lurk.
By watching
how the central stars orbited Sagittarius A*, to which they are gravitationally
bound, the researchers inferred properties of the black hole itself, such as
mass and distance. They found that at least 95 percent of the mass affecting
the stars must be within the black hole. Results gave a precise distance of
27,000 light-years from Earth to the presumed black hole. One light-year is the
distance light will travel in a year, or about 6 trillion miles (10 trillion km).
"Undoubtedly
the most spectacular aspect of our long-term study is that it has delivered
what is now considered to be the best empirical evidence that supermassive
black holes do really exist," said team leader Reinhard Genzel of the
Max-Planck-Institute for Extraterrestrial Physics in Germany.
The new
images also reveal common properties among the orbiting stars. For instance,
the team found the innermost stars trek around the black hole in random orbits,
while six of the 28 stars, which reside farther out, orbit the black hole in
the same plane, just as our planets mostly do around the sun.
One
particular star, known as S2, orbits the Milky Way's center so fast that it
completed one full revolution within the study's 16-year period.
The
researchers hope to continue to study the dancing stars to solve a long-held
riddle as to how such stars ended up in their orbits about Sagittarius A*. They
are too young to have migrated far, and scientists think it's improbable the
stars formed in their current orbits where they'd be exposed to the extreme
tidal forces of the black hole.
One
explanation put forth recently is that the stars formed out of material
that survived after a gas cloud plunged in toward the central black hole.
This scenario was based on computer simulations. The researchers suggest that
the six stars orbiting in a disk formed about 6 million years ago in this
gas-cloud scenario.
The
innermost stars could have once been in pairs, said lead researcher Stefan
Gillessen, also of the Max-Planck-Institute. And so when the binary stars got
too close to the supermassive black hole, the gravitational energy may have
been shuffled around between the stars. In that way, one member of the pair
would've been kicked out while the other remained. These innermost stars — each
missing a partner — are estimated to be about 50 million years old.
The recent
finding involved observations in 1992 with the SHARP camera aboard the European
Southern Observatory's New Technology Telescope at the La Silla observatory in
Chile; and more recent observations using instruments aboard ESO's Very Large
Telescope.
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