Share this article

 Email

 Print

 

Republish this article

The text of this article by Futurity is licensed under a Creative Commons Attribution-No Derivatives License.

More
Top Stories

Lemur personalities vary from shy to ‘mean as sin’

Gastric bypass results without the surgery?

Grooves in Mars dunes likely from dry ice, not water

Nasty toxin helps E. coli survive in water

Could cocoa tame obesity’s inflammation

Humans polluted with lead 8,000 years ago

Eyeball discovery may mean safer surgery

How corals build their skeletons

Sea birds ‘do math’ to divvy up turf

Supermassive black holes, weighing millions to billions times more than our 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 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 appear in the May 3 online edition of the journal Nature.

Crime scene

“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,” says Gezari, an associate research scientist in physics and astronomy at Johns Hopkins. “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,” she said. “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.

Second encounter

This apparently was 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 then have been relatively 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 star continued its journey around the center, until what remained ventured even closer to the black hole to face its ultimate demise and was completely disrupted.

Astronomers believe that stripped stars circle the central black hole of our Milky Way galaxy, Gezari pointed out. These close encounters, however, would be rare, occurring roughly every 100,000 years.

Strange supernova?

To find this one event, Gezari’s team monitored hundreds of thousands of galaxies in ultraviolet light with the Galaxy Evolution Explorer, a space-based observatory, and in visible light with the Pan-STARRS1 telescope on Mount Haleakala, Hawaii.

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 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,” says 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 information on an object’s characteristics, such as its temperature and gaseous makeup.

“The glowing helium was a tracer for an extraordinarily hot accretion event,” Gezari says. “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).

“The place we also see these kinds of velocities are in supernova explosions,” Rest says. “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 says.

“These observations also give us clues on what evidence to look for in the future to find this type of event.”

More news from Johns Hopkins University: http://releases.jhu.edu