Black hole caught red-handed in stellar homicideMay 03, 2012
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 has identified the victim as a star rich in helium gas. The star resided in a galaxy 2.7 billion light-years away.
Her team's results will appear in the May 3 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," said Gezari, an associate research scientist in the Henry A. Rowland Department of Physics and Astronomy at Johns Hopkins' Krieger School of Arts and Sciences. "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, we know 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.
This is not the first time the unlucky star had a brush with the behemoth black hole. Gezari and her team think the hydrogen-filled envelope surrounding the star's 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. 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. The stellar remains continued its journey around the center, until it ventured even closer to the black hole to face its ultimate demise and was completely disrupted.
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 the Galaxy Evolution Explorer (GALEX), a space-based observatory, and in visible light with the Pan-STARRS1 telescope on Mount Haleakala, 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.
In June 2010, they spotted one with both telescopes. Both 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.
By measuring the increase in brightness, the astronomers calculated the black hole's mass at roughly 3 million suns, which equals the weight of our Milky Way's black hole.
Spectroscopic observations with the MMT 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 on what evidence to look for in the future to find this type of event."
Johns Hopkins University
Related Black Hole Current Events and Black Hole News Articles
Big surprises can come in small packages
Astronomers using the NASA/ESA Hubble Space Telescope have found a monster lurking in a very unlikely place.
Mysterious quasar sequence explained
Quasars are supermassive black holes that live at the center of distant massive galaxies.
NASA's RXTE Satellite Decodes the Rhythm of an Unusual Black Hole
Astronomers have uncovered rhythmic pulsations from a rare type of black hole 12 million light-years away by sifting through archival data from NASA's Rossi X-ray Timing Explorer (RXTE) satellite.
Fascinating rhythm: light pulses illuminate a rare black hole
The universe has so many black holes that it's impossible to count them all. There may be 100 million of these intriguing astral objects in our galaxy alone.
Astrophysicists Detect Destruction of Three Stars by Black Holes
Researchers from MIPT and the Space Research Institute of the Russian Academy of Sciences have reported registering three possible occasions of thetidal destruction of stars by supermassive black holes at the centers of galaxies.
All-You-Can-Eat at the End of the Universe
At the ends of the Universe there are black holes with masses equaling billions of our sun. These giant bodies - quasars - feed on interstellar gas, swallowing large quantities of it non-stop.
White dwarfs crashing into neutron stars explain loneliest supernovae
A research team led by astronomers and astrophysicists at the University of Warwick have found that some of the Universe's loneliest supernovae are likely created by the collisions of white dwarf stars into neutron stars.
Out of An Hours-long Explosion, A Stand-In For The First Stars
Astronomers analyzing a long-lasting blast of high-energy light observed in 2013 report finding features strikingly similar to those expected from an explosion from the universe's earliest stars.
Supermassive black hole blows molecular gas out of galaxy at 1 million kilometers per hour
New research by academics at the University of Sheffield has solved a long-standing mystery surrounding the evolution of galaxies, deepening our understanding of the future of the Milky Way.
A hotspot for powerful cosmic rays
An observatory run by the University of Utah found a "hotspot" beneath the Big Dipper emitting a disproportionate number of the highest-energy cosmic rays. The discovery moves physics another step toward identifying the mysterious sources of the most energetic particles in the universe.
More Black Hole Current Events and Black Hole News Articles