October 13, 2005

Big stars are born near Milky Way’s black hole

By Deborah Zabarenko

WASHINGTON (Reuters) - Dozens of massive stars, destined
for a short but brilliant life, were born less than a
light-year away from the Milky Way's central black hole, one of
the most hostile environments in our galaxy, astronomers
reported on Thursday.

On Earth, this might be a bit like setting up a maternity
ward on the side of an active volcano. But researchers using
the Chandra X-ray Observatory and other instruments believe
there is a safe zone around black holes, a big dust ring where
stars can form.

Black holes, including the one at the center of our galaxy,
are monstrous matter-sucking drains in space, with
gravitational pull so strong that nothing, not even light, can
escape once it comes within the hole's grasp.

These young stars, however, are just far enough away to be
held in orbit around the hole much as planets are kept in orbit
around the sun, according to Sergei Nayakshin of the University
of Leicester, United Kingdom.

At less than a light-year's distance, the 50 or 100 massive
young stars are quite close to the black hole, but not close
enough to be drawn in, Nayakshin said in a telephone interview.

A light-year is about 6 trillion miles, the distance light
travels in a year. By comparison, Earth is about 26,000
light-years from the galactic center where the black hole lies.


The dusty zone in which these big stars thrive makes them
impossible to see with optical telescopes, but the orbiting
Chandra detected them by the X-rays they emit. Some smaller
stars were also detected.

The massive stars buried in the cosmic murk are each
between 30 and 50 times the mass of the sun, Nayakshin said.

The more massive the star, the brighter it shines, so that
a star with 50 solar masses would be five orders of magnitude
brighter than the sun; it would shine with the brilliance of
100,000 suns, Nayakshin said.

Over the course of perhaps 5 million years or so -- a mere
blink of an eye in astronomical time -- these high mass stars
would likely lose 80 percent of their mass and explode as
supernovae, transforming into smaller black holes around the
supermassive black hole in the center of our galaxy.

Unlike the sun, which burns its fuel slowly, these massive
stars live fast and die young.

"These stars live a short life because they're so luminous,
they just use up all of their energy too quickly," Nayakshin
said. He is co-author of a study to be published in an upcoming
issue of the Monthly Notices of the Royal Astronomical Society.

These findings also contradict theorists who believe
massive stars form elsewhere in the galaxy and migrate toward
the black hole, he said. And this research may shed some light
on how such big, rare stars are created.