Medium-Sized Black Holes Now in Stock
PASADENA, CA (JUNE 5, 2001) --
Like T-shirts and baseball caps, black holes appear to
come in three sizes: small, medium, and large. The small and large sizes seem
to be fairly common, but astronomers had to root around the cosmic storeroom
for awhile before they found any mediums. Now, the shelves seem to be well
stocked with them, after astronomers announced the discoveries of about 100
possible medium-sized black holes.
Black holes are objects with such powerful gravity that nothing can escape
from them, including light. Astronomers discover black holes by measuring
their effects on the stars and gas around them. Disks of super-heated gas
surround many black holes, producing copious amounts of X-rays that are
visible to orbiting telescopes.
The "small" black holes are only a few times the mass of our Sun, and
probably form when massive stars collapse. "Large" black holes are a million
to several billion times the mass of the Sun, and inhabit the cores of many
galaxies. These galactic monsters probably formed early in the history of the
universe, perhaps from the collapse of massive gas clouds. "Medium" black
holes range from about 10 to several thousand times the Sun's mass.
Astronomers announced the discovery of the first possible medium black hole
just two years ago. But several teams of astronomers today reported finding
dozens of candidate medium-mass black holes in distant galaxies.
Andrew Ptak of Carnegie Mellon University and Edward Colbert of Johns Hopkins
University found evidence of at least 73 by studying observations of about
750 galaxies by the German ROSAT X-ray satellite. They actually found compact
objects that emit large amounts of X-ray energy. Although it's possible that
the X-rays are produced by some other type of astronomical object, such as
the remnant of an exploded star, the astronomers say it's most likely that
the objects are black holes with masses 10-100 times the Sun's mass.
A team led by Kimberly Weaver of NASA's Goddard Space Flight Center used the
more-powerful Chandra X-Ray Observatory to find evidence of nine medium black
holes in NGC 253, a galaxy about 10 million light-years away. Six of these
objects are within 3,000 light-years of the galaxy's center, in a region of
intense star formation. The putative black holes may someday spiral into the
center of NGC 253 and merge to form a supermassive black hole, Weaver said.
Also using Chandra, other astronomers found more possible medium-sized black
holes in M82 and in the Antenna galaxies -- two spiral galaxies that are
merging to form a single giant galaxy. Like NGC 253, these galaxies are
producing many new stars, leading astronomers to speculate that there may be
an as-yet unidentified connection between medium-sized black holes and these
so-called "starburst" galaxies.
Damond Benningfield
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