By Robert Roy Britt, Senior Science Writer. posted: 15 November 2004
Astronomers think they have found a rare if not unique black hole very near
the center of the Milky Way. That would make two of the beasts in that part
of the galaxy.
The discovery also adds weight to the idea that black holes come in three sizes,
essentially small, medium and large.
Stellar black holes -- the remains of collapsed stars, are common. They typically
harbor as much mass as a few suns. And for years, scientists have known there
are supermassive black holes in many galaxies; one with the mass of more than
three million suns anchors the Milky Way.
The newly detected object appears to be an intermediate mass black hole, packing
about 1,300 solar masses.
Intermediate mass black holes ought to exist, some theorists say, because they
should have been the building blocks of supermassive black holes. A few should
be left scattered around any respectable galaxy. But attempts to discover them
-- data suggest two others exist in our galaxy -- have so far proved inconclusive.
Black holes can't be seen, because everything that falls into them, including
light, is trapped. But the swift motions of gas and stars near an otherwise
invisible object allows astronomers to calculate that it's a black hole and
even to estimate its mass.
If the newfound object, catalogued as GCIRS 13E, is indeed a middleweight black
hole, it is likely a rare variety, perhaps one of kind, that formed farther
out and has been lured to the galactic center. It is now less than 1.5 light-years
from the fringes of the known supermassive black hole. That's much closer than
our Sun is to the next nearest star.
Orbiting the presumed middleweight are seven stars, each of which in its prime
was more than 40 times the mass of the Sun. Even as corpses they contain five
to 10 solar masses. The whole setup is racing around the galactic center at
626,300 mph (280 kilometers per second).
Theory holds that these stars could not have formed in their present location,
because the gravity of the nearby supermassive black hole wouldn't have allowed
a gas cloud to contract into a star, says study leader Jean-Pierre Maillard
of the Institute of Astrophysics in Paris.
On the other hand, Maillard told SPACE.com, the stars could not have formed
too far from their present location. Why? Because there wasn't time. Massive
stars die young. The seven examined in the study can't be more than 10 million
years old, or they would have exploded already. So the seven stars, along with
the middleweight black hole, all had to migrate inward within the past 10 million
years -- an eyeblink in the 13 billion years of the galaxy's lifetime.
All this means the cluster probably formed about 60 light-years out beyond
its current orbit, the calculations show.
Maillard said the seven stars are the remains of what likely was once a cluster
of many stars. In such a globular cluster, as astronomers call it, a middleweight
black hole could develop through runaway star collisions, other research has
found.
" It might be unique," Maillard said of the black hole candidate. Other
middleweights might exist in the galaxy, he said, but probably none so close
to the center.
The study relied on data from several telescopes, including the Gemini Observatory
in Hawaii and the European Southern Observatory in Chile.
There are other clues as to what's there. The location of the apparent middleweight
black hole coincides with a source of X-rays noted by the Chandra X-ray Observatory.
Black holes are known to create intense X-ray emissions as gas swirls inward
and is superheated.
Maillard cautions, however, that more observations are needed to pin down with
certainty the existence and identity of the object.
The discovery, announced last week, is detailed in the journal Astronomy and
Astrophysics.
This article is part of SPACE.com's weekly Mystery Monday series.
Blackhole encart affiché en grand
The orbit of a particle near a black hole depends on the curvature of space
around the black hole, which also depends on how fast the black hole is spinning.
A spinning black hole drags space around with it and allows atoms to orbit
nearer to the black hole than is possible for a non-spinning black hole, as
seen in the right-hand artist's rendering of a stellar black hole. In the left
one, no evidence for spin was found.
Blackhole encart affiché en petit (sur le coté)
The orbit of a particle near a black hole depends on the curvature of space
around the black hole, which also depends on how fast the black hole is spinning.
A spinning black hole drags space around with it and allows atoms to orbit
nearer to the black hole than is possible for a non-spinning black hole, as
seen in the right-hand artist's rendering of a stellar black hole. In the left
one, no evidence for spin was found.
|
