Cradled by Destruction
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, 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
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
"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.