J.D. Harrington
Headquarters, Washington
202-358-0321
j.d.harrington@nasa.gov
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 http://hubblesite.org/news/2012/18.
For graphics and information about the Galaxy
Evolution Explorer, visit http://www.nasa.gov/galex and http://www.galex.caltech.edu.
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