Black holes may have the ultimate appetite, but their size is limited. [Credit: Dana Berry/STScI/NASA]
“I am so much in awe that I always spell ‘Universe’ with an uppercase ‘U,’” says Yale astrophysicist Priyamvada Natarajan. The capital letter pops out when Natarajan speaks, especially when she talks about her research involving the universe’s most exotic and massive marvel—the black hole.
Black holes are “perhaps the universe’s biggest prima donnas,” says Natarajan. According to the prevailing theory, black holes are bottomless pits growing in size and strength as matter falls into them until there is nothing left to consume. However, Natarajan and her colleague astrophysicist Ezequiel Treister, of the European Southern Observatory in Chile, have determined that this picture of the boundless black hole seems to be exaggerated.
Their theoretical research reveals black holes have a size limit, or a maximum possible mass. This turns out to be around 10 billion times the mass of the Sun—similar to the difference in mass between the Hoover Dam and a standard brick. This size limit implies that despite its almost unimaginable size and power, a central black hole will never devour its host galaxy.
A black hole is a celestial body so massive that its gravity is strong enough to trap even light, the fastest moving thing in the universe. Imagine a bowling ball creating a depression on a large trampoline. Marbles rolling slowly along the surface of the trampoline near the bowling ball will eventually end up in the funnel-like indentation. As slow moving marbles gather at the center, the depression becomes incrementally deeper, making it possible to capture faster marbles. Taken to the extreme, the depression becomes so deep that even the fastest marbles cannot roll past it. This is a black hole.
By this logic, as long as a black hole has something to absorb—like particles of gas and dust in the cosmos—it will continue to grow.
As matter nears a black hole it becomes caught in a disc of swirling gas, much as a leaf drifting down a river gets trapped in a whirlpool. The captured matter spirals around this stellar eddy, rotating faster and faster, being compressed and warmed up by gravity. Like the hot, shining Sun, this heated matter releases energy in the form of light, X-rays and other radiation that escapes the black hole. But once the matter crosses a boundary close to the black hole called the event horizon, the light it emits also gets trapped, and the material “disappears.”
What Natarajan and Treister have shown in their paper, published in the October 15 edition of the journal Monthly Notices of the Royal Astronomical Society, is that it is possible this release of energy will actually disrupt the flow of matter into a black hole.
The energy spouted by a black hole is like a spray of foam from the center of a powerful whirlpool. At low levels, this spray does not have much effect, but at higher energies the spray will prevent the leaf from ever reaching the whirlpool’s center. If the spray is sufficiently strong, it will even push the leaf back into the river, with no chance of the whirlpool capturing it again.
This finding has important implications for the formation of galaxies as the universe cooled after the Big Bang, says Karl Gebhardt, an astrophysicist at the University of Texas in Austin who was not involved in the research. Researchers think black holes reside at the center of almost every large galaxy, including our own, the Milky Way. And as it turns out, the size of the central black hole is proportional to the size of the galaxy itself.
“What we are really learning here is how large a galaxy can be,” says Gebhardt. By knowing the maximum size of galaxies and measuring their distribution, scientists can learn some fundamental properties about the early universe, like the distribution of matter and the size of the density fluctuations that evolved into stars and galaxies.
“The universe is a story,” Natarajan says. “What scientists are trying to do is get at the individual threads and weave out the story line.”
Like supporting actors in a play, black holes work with the rest of the cast and are integral in telling the story of the universe. Maybe black holes are not prima donnas after all.
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