Tuesday, 15 May 2012

Black hole caught destroying star

"Black holes, like sharks, suffer from a popular misconception that they are perpetual killing machines," says researcher Ryan Chornock from the Harvard-Smithsonian Centre for Astrophysics. Occasionally a star wanders too close, and that's when a feeding frenzy begins. "Actually, they're quiet for most of their lives. Supermassive black holes are giant gravity wells that usually lurk dormant and undetected at the centre of galaxies, but can occasionally be tracked by the scraps left over from their stellar fests. Scientists have witnessed the rare spectacle of a supermassive black hole devouring a star that had ventured too close - an event that occurs about once in 10,000 years. "

If a star passes too close, the black hole's gravitational pull can rip it apart before sucking in its gases, which are heated by the friction and start to glow - giving away the silent killer's hiding place.

Smoking gun

She says this was the first such space feast observed from beginning to end, and "that is very exciting because that time scale is how we determine how big the black hole is" Observations over several months allowed the team to conclude that the black hole was at the centre of a galaxy 2. 7 billion light years away, and about three million times the mass of our Sun, making it about the same size as the Milky Way's central black hole. Its victim was probably a star in its late, red giant phase which had tempted fate by wandering to within about 150 million kilometres of the black hole, about the same distance as the Earth is from the Sun. "This is the first time where we actually have enough detailed information that we can actually determine what kind of star was torn apart by a black hole and how big the black hole was that did it," says Gezari. "Initially we didn't know exactly what this flare was because it was so bright that when we looked at the galaxy we couldn't see the stars to determine how far away the galaxy was," study co-leader Suvi Gezari of John Hopkins University in Baltimore. Their report in the journal Nature describes the flare brightened to a peak that July, before fading away over the course of a year. Chornock and colleagues observed such a glow in May 2010 through a telescope mounted on Mount Haleakala in Hawaii, as well as a NASA satellite.

Eaten alive

The scientists concluded that the star had lost its hydrogen outer shell in a previous pass by the black hole, leaving just its helium core to be consumed in round two.

"It was really spectacular to have so much info and have all the pieces of evidence come together to form a consistent picture of what happened," says Gezari.

Black holes are very dense regions in space-time with a gravitational force so strong that even light cannot escape. Scientists who study them hope to learn more about the evolution of galaxies.

Stars in our own Milky Way galaxy, including the Sun, are too far away to be at risk of being consumed, says Gezari.

"We would have to wait at least 10,000 years before we would be able to see a star being gobbled by our own black hole," she says.

"So the best way to find these events is not to wait around for our own Milky Way galaxy to gobble a star, we actually have to look at hundreds of thousands of galaxies in the sky to catch one in the act of shredding up a star."

Dr David Floyd from the Monash Centre for Astrophysics in Melbourne says scientists normally notice a flare in a distant galaxy and then turn their telescopes towards the object after the process has already begun.

"What makes this so special was the fact that they actually caught the black hole as it was ripping the stellar core apart," says Floyd.

"Now increasingly with new telescopes such as Pan-STARRS, which is the one that was used here, and the LSST or Large Synoptic Survey Telescope, which will be built in Chile, we will be able to scan the skies continuously letting us catch these events as they happen."

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