by Playfuls Staff |
6th December 2006
Black holes have been foreseen in the 18th century, by geologist John Michell, and their existence further proven by French physicist Pierre de Simon Laplace.[more]
General relativity (as well as most other metric theories of gravity) not only says that black holes can exist, but in fact predicts that they will be formed in nature whenever a sufficient amount of mass gets packed in a given region of space, through a process called gravitational collapse; as the mass inside the given region of space increases, its gravity becomes stronger and (in the language of relativity) increasingly deforms the space around it, ultimately until nothing (not even light) can escape the gravity; at this point an event horizon is formed, and matter and energy must inevitably collapse to a density beyond the limits of known physics.
For example, if the Sun was compressed to a radius of roughly three kilometers (about 1/232,000 its present size), the resulting gravitational field would create an event horizon around it, and thus a black hole.
Supermassive black holes are believed to exist in the center of most galaxies, including our own Milky Way. This type of black hole contains millions to billions of solar masses, and there are several models of how they might have been formed. The first is via gravitational collapse of a dense cluster of stars. A second is by large amounts of mass accreting onto a "seed" black hole of stellar mass. A third is by repeated fusion of smaller black holes. Effects of such supermassive black holes on spacetime may be observed in regions as the Virgo cluster of galaxies, for example, the location of M87 and its neighbors.
Now it appears that NASA has discovered a black hole in the center of a galaxy situated at more than 4 billion light years away from us, which is viciously devouring a nearby star.
For the past two years, scientists monitored the drama as the star, residing in a galaxy in the Bootes constellation, was ripped apart by the black hole.
Lead author of a new paper appearing in the recent issue of Astrophysical Journal Letters, Dr. Suvi Gezari of the California Institute of Technology, Pasadena, California said such type of events were very rare.
"This type of event is very rare, so we are lucky to study the entire process from beginning to end," said Dr. Gezari.
The black hole, tens of billions of times as massive as the sun, is 4 billion light-years away in the Bootes constellation. By studying black holes, scientists say they hope to gain an understanding of the evolution of the universe. The phenomena are patches of concentrated matter in the universe whose gravitational pull is so strong that not even light can escape.
“This will help us greatly in weighing black holes in the universe, and in understanding how they feed and grow in their host galaxies as the universe evolves,” Christopher Martin, a Caltech academic and principal investigator for the telescope, said in the statement.
"Part of the shredded star swirled around the black hole, then began to plunge into it, triggering a bright ultraviolet flare that the Galaxy Evolution Explorer was able to detect. Today, the space-based telescope continues to periodically watch this ultraviolet light fade as the black hole finishes the remaining bits of its stellar meal," said Dr. Gezari.
"The observations will ultimately provide a better understanding of how black holes evolve with their host galaxies," she said.
In a typical galaxy, a star strays into a black hole about once every 10,000 years, as was the case with the Bootes constellation event, the agency said.
“For perhaps thousands of years, the black hole rested quietly deep inside an unnamed elliptical galaxy,” NASA said. “Then a star ventured a little too close to the sleeping black hole and was torn to shreds by the force of its gravity. Part of the shredded star swirled around the black hole, then began to plunge into it, triggering a bright ultraviolet flare that the Galaxy Evolution Explorer was able to detect.”
Scientists continue to use the telescope to observe the ultraviolet light as it fades while the black hole snacks on the final table scraps from the devoured star.
"We looked at the galaxy in 2003 and there was no ultraviolet light coming from the galaxy at all," Gezari said. "And then in 2004, we suddenly saw this very bright source."
"The only way to explain such a luminous ultraviolet flare is if the black hole swallowed a star," Gezari said.
