from the November 04, 2005 edition -
http://www.csmonitor.com/2005/1104/p02s01-stss.html
A step closer to identifying that monster in the Milky Way
Mystery object is smaller than once thought - and incredibly dense.
Might it be an actual supermassive black hole?
By Peter N. Spotts | Staff writer of The Christian Science Monitor
Astronomers have taken the measure of a monster lurking at the center
of the Milky Way. It's not as big as astronomers once thought. But its
incredible density throws more weight behind the idea that it's a
supermassive black hole, not some oddball collection of other exotic
objects.
The research brings astronomers a step closer to capturing images of
the edge, or "event horizon," of a black hole -- which scientists say
would be the "smoking gun" that proves such entities exist. Once that
horizon is crossed, anything falling in -- including light -- will never
come out.
Supermassive black holes are enormous concentrations of matter
confined to relatively tiny spaces in the centers of most galaxies.
They are the oversized cousins of stellar black holes, which can form
after stars at least 10 times as massive as the sun burn out and
collapse. These smaller black holes tend to be 18 or 19 miles across.
To produce the new results, astronomers aimed a continentwide network
of radio telescopes at a source of radio emissions at the center of
the galaxy, in the constellation Sagittarius. The source is an object
4 million times more massive than the sun.
But the object apparently does not take up much space as previous
measurements had indicated, the new results show. Instead of filling a
patch of space as wide as the solar system, or even as wide as Earth's
full orbit around the sun, the object is smaller than the distance
between the Earth and sun, or 1 astronomical unit. Given estimates of
its mass and its incredible shrinking volume, calculations of its
density are going through the roof.
By closing in on the object's true size, "we're getting tantalizingly
close to being able to see an unmistakable signature ... of a
supermassive black hole," notes Zhi-Qiang Shen of the Shanghai
Astronomical Observatory and the Chinese Academy of Sciences, who led
a team from China, the US, and Taiwan. Its results appear in the
current issue of the journal Nature.
What would that signature look like?
Most likely, it would appear as a black circle, surrounded by a thin,
bright line, astronomers say. This thin line of radiation actually
would come from objects behind the black hole. But a process called
gravitational lensing would focus the radiation around the black
hole. Moreover, the black hole would likely appear off-center inside
its thin halo -- an effect traced to its rotation.
Solving the mystery at the heart of the Milky Way is important,
astronomers say, because supermassive black holes are thought to lie
at the center of most galaxies. Knowing more about the one in the
Milky Way -- if that's really what it is -- will help astronomers
understand the role these objects play in other galactic cores.
Until the smoking gun is spotted, notes University of Maryland
astrophysicist Christopher Reynolds, the object could be something
more bizarre and still fall within the confines of standard
physics. One possibility, although remote, is what he calls a "boson
star," made up of particles associated with the fundamental forces of
nature. Such a star could have the size and mass characteristics of
the object at the heart of the Milky Way.
"There are other families of particles out there that can form massive
compact objects," says Dr. Reynolds. New instruments expected to be
developed over the next decade should help astronomers sift the right
answers from the wrong ones, he says.
Copyright 2005 The Christian Science Monitor.
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