Thanks to two orbiting X-ray observatories, astronomers
have the first strong evidence of a supermassive black hole
ripping apart a star and consuming a portion of it.
The event, captured by NASA’s Chandra and ESA’s XMM-Newton X-
ray Observatories, had long been predicted by theory, but never
confirmed.
Astronomers believe a doomed star came too close to a giant
black hole after being thrown off course by a close encounter
with another star. As it neared the enormous gravity of the
black hole, the star was stretched by tidal forces until it was
torn apart. This discovery provides crucial information about
how these black holes grow and affect surrounding stars and
gas.
“Stars can survive being stretched a small amount, as they are
in binary star systems, but this star was stretched beyond its
breaking point,” said Stefanie Komossa of the Max Planck
Institute for Extraterrestrial Physics (MPE) in Germany, leader
of the international team of researchers. “This unlucky star
just wandered into the wrong neighborhood.”
While other observations have hinted stars are destroyed by
black holes (events known as “stellar tidal disruptions”),
these new results are the first strong evidence. Evidence
already exists for supermassive black holes in many galaxies,
but looking for tidal disruptions represents a completely
independent way to search for black holes. Observations like
these are urgently needed to determine how quickly black holes
can grow by swallowing neighboring stars.
Observations with Chandra and XMM-Newton, combined with earlier
images from the German Roentgen satellite, detected a powerful
X-ray outburst from the center of the galaxy RXJ1242-11. This
outburst, one of the most extreme ever detected in a galaxy,
was caused by gas, heated to millions of degrees Celsius, from
the destroyed star being swallowed by the black hole. The
energy liberated in the process was equivalent to a supernova.
“Now, with all the data in hand, we have the smoking gun proof
that this spectacular event has occurred,” said coauthor
Guenther Hasinger, also of MPE.
The black hole in the center of RXJ1242-11 is estimated to have
a mass of about 100 million times Earth’s sun. By contrast, the
destroyed star probably had a mass about equal to the sun,
making it a lopsided battle of gravity. “This is the ultimate
David versus Goliath battle, but here David loses,” said
Hasinger.
The astronomers estimated about one percent of the star’s mass
was ultimately consumed, or accreted, by the black hole. This
small amount is consistent with predictions the momentum and
energy of the accretion process will cause most of the
destroyed star’s gas to be flung away from the black hole.
The force that disrupted the star in RXJ1242-11 is an extreme
example of the tidal force caused by differences in gravity
acting on the front and back of an object. The tidal force from
the moon causes tides in Earth’s oceans. A tidal force from
Jupiter pulled Comet Shoemaker-Levy apart, before it plunged
into the giant planet.
The odds stellar tidal disruption will happen in a typical
galaxy are low, about one in 10,000 annually. If it happened at
the center of the Milky Way Galaxy, 25,000 light-years from
Earth, the resulting X-ray outburst would be about 50,000 times
brighter than the brightest X-ray source in our galaxy, beside
the sun, but it would not pose a threat to Earth.
Other dramatic flares have been seen from galaxies, but this is
the first studied with the high-spatial resolution of Chandra
and the high-spectral resolution of XMM-Newton. Both
instruments made a critical advance. Chandra showed the
RXJ1242-11 event occurred in the center of a galaxy, where the
black hole lurks. The XMM-Newton spectrum revealed the
fingerprints expected for the surroundings of a black hole,
ruling out other possible astronomical explanations.
Information and images about the event are available on the
Internet at:
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