Black Hole Caught Red-Handed in a Stellar Homicide

WASHINGTON -- Astronomers have gathered the most direct evidence yet
of a supermassive black hole shredding a star that wandered too
close. NASA's Galaxy Evolution Explorer, a space-based observatory,
and the Pan-STARRS1 telescope on the summit of Haleakala in Hawaii,
were among the first to help identify the stellar remains.

Supermassive black holes, weighing millions to billions times more
than the sun, lurk in the centers of most galaxies. These hefty
monsters lay quietly until an unsuspecting victim, such as a star,
wanders close enough to get ripped apart by their powerful
gravitational clutches.

Astronomers have spotted these stellar homicides before, but this is
the first time they identified the victim. Using several ground- and
space-based telescopes, a team of astronomers led by Suvi Gezari of
the Johns Hopkins University in Baltimore identified the victim as a
star rich in helium gas. The star resides in a galaxy 2.7 billion
light-years away. The team's results will appear in today's online
edition of the journal Nature.

"When the star is ripped apart by the gravitational forces of the
black hole, some part of the star's remains falls into the black hole
while the rest is ejected at high speeds," Gezari said. "We are
seeing the glow from the stellar gas falling into the black hole over
time. We're also witnessing the spectral signature of the ejected
gas, which we find to be mostly helium. It is like we are gathering
evidence from a crime scene. Because there is very little hydrogen
and mostly helium in the gas, we detect from the carnage that the
slaughtered star had to have been the helium-rich core of a stripped star."

This observation yields insights about the harsh environment around
black holes and the types of stars swirling around them. It is not
the first time the unlucky star had a brush with the behemoth black hole.

The team believes the star's hydrogen-filled envelope surrounding the
core was lifted off a long time ago by the same black hole. The star
may have been near the end of its life. After consuming most of its
hydrogen fuel, it had probably ballooned in size, becoming a red
giant. Astronomers think the bloated star was looping around the
black hole in a highly elliptical orbit, similar to a comet's
elongated orbit around the sun. On one of its close approaches, the
star was stripped of its puffed-up atmosphere by the black hole's
powerful gravity. The stellar remains continued its journey around
the center, until it ventured even closer to the black hole to face
its ultimate demise.

Astronomers predict stripped stars circle the central black hole of
our Milky Way galaxy. These close encounters are rare, occurring
roughly every 100,000 years. To find this event, Gezari's team
monitored hundreds of thousands of galaxies in ultraviolet light with
the Galaxy Evolution Explorer, and in visible light with Pan-STARRS1.
Pan-STARRS, short for Panoramic Survey Telescope and Rapid Response
System, scans the entire night sky for all kinds of transient
phenomena, including supernovae.

The team was looking for a bright flare in ultraviolet light from the
nucleus of a galaxy with a previously dormant black hole. Both
telescopes spotted one in June 2010. Astronomers continued to monitor
the flare as it reached peak brightness a month later and slowly
faded during the next 12 months. The brightening event was similar to
the explosive energy unleashed by a supernova, but the rise to the
peak was much slower, taking nearly one and a half months.

"The longer the event lasted, the more excited we got, because we
realized this is either a very unusual supernova or an entirely
different type of event, such as a star being ripped apart by a black
hole," said team member Armin Rest of the Space Telescope Science
Institute in Baltimore.

By measuring the increase in brightness, the astronomers calculated
the black hole's mass to be several million suns, which is comparable
to the size of our Milky Way's black hole.

Spectroscopic observations with the Multiple Meter Telescope
Observatory located on Mount Hopkins in Arizona showed the black hole
was swallowing lots of helium. Spectroscopy divides light into its
rainbow colors, which yields an object's characteristics, such as its
temperature and gaseous makeup.

To completely rule out the possibility of an active nucleus flaring up
in the galaxy, the team used NASA's Chandra X-ray Observatory to
study the hot gas. Chandra showed that the characteristics of the gas
didn't match those from an active galactic nucleus.

For images, video and more information about this study, visit:


For graphics and information about the Galaxy Evolution Explorer, visit:





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