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
can identify the victim. Using a slew of ground- and space-based
telescopes, a team of astronomers led by Suvi Gezari of The Johns
Hopkins University in Baltimore, Md., has identified the victim as a
star rich in helium gas. The star resides in a galaxy 2.7 billion
light-years away.
Her team's results recently appeared in an 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. 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,
we know from the carnage that the slaughtered star had to have been the
helium-rich core of a stripped star," Gezari explained.
This observation yields insights about the harsh environment around black holes and the types of stars swirling around them.
This
is not the first time the unlucky star had a brush with the behemoth
black hole. Gezari and her team think the star's hydrogen-filled
envelope surrounding its core was lifted off a long time ago by the same
black hole. In their scenario, 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. The 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. Only its core remained intact. The
stellar remnant continued its journey around the black hole, until it
ventured even closer to the behemoth monster and faced its ultimate
demise.
Astronomers have predicted that stripped stars circle the
central black hole of our Milky Way galaxy, Gezari pointed out. These
close encounters, however, are rare, occurring roughly every 100,000
years. To find this one event, Gezari's team monitored hundreds of
thousands of galaxies in ultraviolet light with NASA's Galaxy Evolution
Explorer (GALEX), a space-based observatory, and in visible light with
the Pan-STARRS1 telescope on the summit of Haleakala in Hawaii.
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. They found one in June 2010, which was spotted with
both telescopes. Both telescopes continued to monitor the flare as it
reached peak brightness a month later and then slowly began to fade over
the next 12 months. The brightening event was similar to that of 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, since we realized that 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, Md.
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 MMT
(Multiple Mirror Telescope) Observatory on Mount Hopkins in Arizona
showed that 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.
"The
glowing helium was a tracer for an extraordinarily hot accretion
event," Gezari said. "So that set off an alarm for us. And, the fact
that no hydrogen was found set off a big alarm that this was not typical
gas. You can't find gas like that lying around near the center of a
galaxy. It's processed gas that has to have come from a stellar core.
There's nothing about this event that could be easily explained by any
other phenomenon."
The observed speed of the gas also linked the
material to a black hole's gravitational pull. MMT measurements revealed
that the gas was moving at more than 20 million miles an hour (over 32
million kilometers an hour). However, measurements of the speed of gas
in the interstellar medium reveal velocities of only about 224,000 miles
an hour (360,000 kilometers an hour).
"The place we also see
these kinds of velocities are in supernova explosions," Rest said. "But
the fact that it is still shining in ultraviolet light is incompatible
with any supernova we know."
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.
"This is the first time where we have so many
pieces of evidence, and now we can put them all together to weigh the
perpetrator (the black hole) and determine the identity of the unlucky
star that fell victim to it," Gezari said. "These observations also give
us clues to what evidence to look for in the future to find this type
of event."
This article was first published at www.newswise.com