By Scott Dance, The Baltimore Sun
8:47 PM EDT, May 2, 2012
It's rare to witness the entirety of a murder. But that's how some local scientists investigated exactly what happened during a fatal attack in 2010.
The victim? A star — a massive red giant — 2.7 million light-years away that had lost its outer layers in previous brushes with its attacker. The perpetrator was a massive black hole that swallowed the star and spewed its guts out into space over the course of a year.
A team led by a Johns Hopkins University researcher conducted the probe, and astronomers say its findings could lead to discoveries that shed new light on the central role black holes may play in the growth of galaxies.
"It will tell you if a black hole is somehow influencing how the galaxy evolves," said Suvi Gezari, the associate research scientist at Hopkins who led the research effort.
The research appeared online Wednesday in Nature, a weekly scientific journal, and is considered the most direct evidence of a long-held theory — that black holes feed on stars that get too close.
One astronomer called the research "a hallmark of a maturing field," leapfrogging to bring observation up to speed with theory and moving right past by adding new questions to be answered.
"We've known for years that black holes grow by gobbling up gas around them," said Joshua Bloom, an astronomy professor at the University of California, Berkeley. "Adding stars as one of their feeding sources also gives us some insight of how black holes grow in the universe."
The team, which also included scientists at the Space Telescope Science Institute, had been scanning the skies for years to detect such an event, watching for a flare of light from the center of a galaxy with a dormant black hole.
They finally spotted one in June 2010. It took nearly a month and a half for the burst of light to peak, and a year for it to fade as gases were absorbed into the black hole and spilled into space.
The explosion initially looked like a supernova, but the longer it took to brighten and then fade, the more the researchers' interest was piqued, they said.
Using spectroscopy, which can show the temperature or gaseous makeup of an object, they observed a large amount of helium being devoured. Helium is typically only found in such quantity in the core of a star. The rarity of such conditions led researchers to rule out phenomena like a supernova or an active galactic nucleus. It could only be a black hole devouring a star.
While some helium gas was being absorbed into the black hole, some of it was also spilling out at speeds as high as 20 million miles per hour, as fast as it might have in a supernova but giving off ultraviolet light for far longer than such an explosion would have.
Gezari scrutinized ultraviolet images of the gas explosion using NASA's GALEX satellite telescope, and Armin Rest, an assistant astronomer at the Space Telescope Science Institute, monitored it using a telescope atop Mount Haleakala, Hawaii. Along with other researchers across the country, they parsed what they saw to determine the size and type of the star and the harshness of conditions surrounding the black hole.
Their research concluded that the star must have had earlier brushes with the black hole, which stripped its outer layers of hydrogen away so that little was left of the star save its helium core. Astronomers had already predicted that such stripped stars surround the black hole at the center of the Milky Way galaxy. Without hydrogen fuel, the star likely expanded to become what is known as a red giant.
Previous measurements made using the Hubble Space Telescope have shown there is a connection between the sizes of black holes and the galaxies that surround them, Gezari said. This incident shows activity around a black hole in a galaxy that is relatively young, she said, and the research can be duplicated to learn more about black hole behavior.
A more active black hole "could quench star formation and reduce the size of the galaxy," she said.
This is not the first time astronomers had seen signs of a black hole destroying a star. But the extent of this event enabled researchers to determine the black hole's mass for the first time, Gezari said. The black hole observed in 2010 is the size of several million suns, researchers said, about the same size as the black hole at the core of the Milky Way. Many of the stars around it are about 2.5 billion years old, compared with many 13.6-billion-year-old stars in the Milky Way, Gezari said.
The observations could have applications for studying black holes, she said. Black holes, thought to be formed when stars die and collapse into themselves, are hard to observe in space because their high gravity does not allow light to escape.
If more of them can be spotted while they are destroying wandering stars, more can be understood about the relationship between black holes and their galaxies, Gezari said. Such incidents only happen once in about 100,000 years in a given galaxy.
One new question, for example, is why the star did not simply disappear into the darkness of the black hole, Bloom said. What forces sent some of the star's gases spewing into space haven't been explained.
"With a more powerful survey, we could find enough of these events to actually answer those questions," Gezari said.
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