May 2, 2012
Astronomers Catch Supermassive Black Hole Devouring Distant Star
Lawrence LeBlond for RedOrbit.com
[ Watch the Video ]Astronomers have for the very first time captured a supermassive black hole devouring a wandering star that strayed too close to a phenomena that nothing can escape, not even light.
Although the star most likely met its demise in an untimely fashion, it is an extraordinary event for science and astronomy, as scientists believe it may provide them with greater clues on the inner workings of dark behemoths nestled away in the hearts of many galaxies.
Supermassive black holes are objects millions and perhaps billions of times the mass of our sun. They lay quietly in the darkness awaiting for unlikely victims, such as stars, to happen by and then shred them apart with their extraordinarily powerful gravitational pull.
While scientists first caught a black hole performing stellar homicide last year, this is the first time they have been able to identify the victim. Using a wide selection of ground- and space-based telescopes, astronomers, led by Suvi Gezari of John Hopkins University in Baltimore, have identified the victim as a star rich in helium gas.
“This is the first time we´ve actually been able to pinpoint what kind of star was disrupted,” Gezari told SPACE.com.
Results of the discovery are published in today´s online edition of the journal Nature (PDF document).
The researchers, in June 2010, spotted a bright flare from the previously dormant black hole at the center of a galaxy 2.7 billion light years away.
“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.”
This particular black hole´s mass is estimated to be equivalent to 3 million suns, roughly the size of the black hole sitting at the center of the Milky Way.
“These spectacular events provide a glimpse into otherwise unobservable black holes, telling us about their masses,” Gezari said. “We know that there are strong connections between black holes and the galaxies they reside in, and it turns out that somehow the mass of the black hole and the mass of a galaxy influence each other, so we want to better know what's going on there.”
“Also, people want to understand the physics of black holes and how they affect the geometry of space-time around them,” she added. “We need to know its mass to help pinpoint a lot of those details.”
The observations astronomers made give insights about the harsh environment around black holes and the types of stars swirling around them. 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 most of its hydrogen fuel was consumed, it most likely ballooned in size, becoming a red giant. It may have been circling the black hole in a highly elliptical orbit, similar to the elongated orbit comets have. Then, on one of its close passes, the black hole made its move, stripping the star´s puffed-up atmosphere with its extremely 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 there are stripped stars circling the black hole of our Milky Way as well. They believe these close encounters are rare, only occurring about once every hundred thousand years.
Gezari said her team was looking for a bright flare in ultraviolet light from the nucleus of a galaxy with a previously dormant black hole. Telescopes picked up one of those bright flares in June 2010. Astronomers continued to monitor the flare as it reached peak brightness a month later and then slowly faded during the next 12 months. They compared the brightening event to that of an explosive energy unleashed by a supernova, although the rise to the peak was much slower in this instance, taking nearly 45 days.
“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.
In addition, Gezari and her colleagues analyzed the spectrum of the ejected gas using data from the Multiple Mirror Telescope Observatory on Mount Hopkins in Arizona. Since each element has a unique spectral fingerprint, we were able to determine the spectrum of the gas in this event was mostly helium.
“It is like we are gathering evidence from a crime scene,” Gezari told SPACE.com.
The fact there was mostly helium and very little hydrogen in the gas suggests “the slaughtered star had to have been the helium-rich core of a stripped star,” she explained.
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,” said Gezari. “These observations also give us clues to what evidence to look for in the future to find this type of event.”
The team plans to use the Large Synoptic Survey Telescope, to be completed sometime in 2015, to scan half the sky every night, hoping to detect far more carnage.
“We can measure at what rate stars are being disrupted by black holes as a function of the type of galaxy, measure the masses of the black holes, see what types of stars orbit black holes in the centers of galaxies, and try and better understand the evolution of galaxies over time,” said Gezari.
There´s much to be done, she added.
Image Credit: NASA, S. Gezari (The Johns Hopkins University), and J. Guillochon (University of California, Santa Cruz)