Chandra Observatory reveals new star generation

Chandra Observatory reveals new star generation
http://www.physorg.com/news7225.html


Black holes are often regarded as galactic bullies. With an infamous
appetite for stars and mayhem, they're nothing but menacing destroyers.
Or are they? For the first time, scientists using NASA's Chandra X-ray
Observatory have tested and proven how a black hole at the center of
the Milky Way is protecting and nurturing a flock of young stars.

"Many scientists are going to be very surprised by these results," said
Chandra scientist Rashid Sunyaev.

Called Sagittarius A* ("A-star"), the black hole hosts a ring of stars
sitting one light-year from its center, a relatively slim distance in a
galactic sense. Until now, why the stars are orbiting so dangerously
close to the hole's lethal center was an ongoing debate among
astronomers.

Stars are thought to emerge out of clouds of cold, dark gas floating in
space. The idea is that at some point, a cloud begins to collapse under
its own pull of gravity. As the cloud shrinks, the competition for
space inside causes atoms to grow hot enough to start fusing together.
The continuing process releases light and heat, and provides enough
explosive pressure to halt the cloud's collapse and stabilize its
formation into a star.

Black holes, with all of their brooding turmoil, hardly seem a safe
place for nurturing stars. "Massive black holes are usually known for
violence and destruction," said project lead scientist Sergei
Nayakshin. In fact, black holes arise from the death of stars, and have
a vicious reputation for light-warping gravity or devouring anything
that comes near them. In such an environment, the gas clouds that form
stars should be ripped apart by the black hole's tidal forces, which
stretch, flatten and accelerate matter at great strength. Given the
object's treacherous demeanor, it's a real surprise Sagittarius A* has
a maternal side.

"In one of the most inhospitable places in our galaxy, stars have
prevailed," said Nayakshin. "It appears that star formation is much
more tenacious than we previously believed."

Navakshin and Sunyaev tested a pair of explanations for the black
hole's ability to offer safe harbor. In one possibility, known as "the
disk model," the gravity of the dense disk of gas that swirls around
Sagittarius A* is strong enough to offset the black hole's distorting
tidal forces. With the two forces in balance, gas clouds can naturally
settle in and form stars. The second, "migration" option suggests the
stars formed in a cluster far somewhere in space and were drawn into
the black hole. The migration scenario predicts about one million
low-mass stars in Sagittarius A*. The disk model, on the other hand,
suggests the number of stars to be well below one million.

To sort out which explanation is likeliest, the two scientists counted
the stars around the black hole. They estimated the number of stars in
Sagittarius A* by comparing the amount of X-ray light seen around the
black hole to the amount emitted from the Orion Nebula, known to have a
few thousand stars. The two researchers determined the black hole holds
about 10,000 low-mass stars. The relatively low number of stars ruled
out the one-million-star migration model and offered solid evidence to
support the protective disk concept.

"We can now say that the stars around Sagittarius A* were not deposited
there by some passing star cluster. Rather, they were born there," said
Sunyaev.

What's more, growing up in the tough neighborhood of a black hole
appears to change the stars themselves. Despite their difficult
surroundings, stars born in the disks around black holes tend to grow
larger or more "massive" than free-floating stars.

Despite the malicious reputation of black holes, NASA's Chandra X-ray
Observatory has shown there's a bright side to these dark wonders. Not
only is Sagittarius A* proving a stable, nurturing environment for
stars to grow up safe and sound, but it's also a place where they
develop to be big and strong as well.