黑洞的诞生

A new kind of cosmic flash may reveal something never seen before: the birth of a black hole. When a massive star exhausts its fuel, it collapses¹ under its own gravity and produces a black hole, an object so dense² that not even light can escape its gravitational grip. According to a new analysis by an astrophysicist（天体物理学家） at the California Institute of Technology (Caltech), just before the black hole forms, the dying star may generate a distinct burst of light that will allow astronomers³ to witness the birth of a new black hole for the first time.

 

Tony Piro, a postdoctoral scholar at Caltech, describes this signature light burst in a paper published in the May 1 issue of the Astrophysical Journal Letters. While some dying stars that result in black holes explode as gamma-ray bursts, which are among the most energetic phenomena⁴ in the universe, those cases are rare, requiring exotic circumstances, Piro explains. "We don't think most run-of-the-mill black holes are created that way." In most cases, according to one hypothesis, a dying star produces a black hole without a bang or a flash: the star would seemingly vanish from the sky -- an event dubbed⁵ an unnova. "You don't see a burst," he says. "You see a disappearance⁶."

 

But, Piro hypothesizes, that may not be the case. "Maybe they're not as boring as we thought," he says.

 

According to well-established theory, when a massive star dies, its core collapses under its own weight. As it collapses, the protons and electrons that make up the core merge⁷ and produce neutrons⁹. For a few seconds -- before it ultimately collapses into a black hole -- the core becomes an extremely dense object called a neutron⁸ star, which is as dense as the sun would be if squeezed into a sphere with a radius¹⁰ of about 10 kilometers (roughly 6 miles). This collapsing¹¹ process also creates neutrinos, which are particles that zip through almost all matter at nearly the speed of light. As the neutrinos stream out from the core, they carry away a lot of energy -- representing about a tenth of the sun's mass (since energy and mass are equivalent, per E = mc2).

 

According to a little-known paper written in 1980 by Dmitry Nadezhin of the Alikhanov Institute for Theoretical and Experimental Physics in Russia, this rapid loss of mass means that the gravitational strength of the dying star's core would abruptly¹² drop. When that happens, the outer gaseous¹³（气态的） layers -- mainly hydrogen -- still surrounding the core would rush outward, generating a shock wave that would hurtle through the outer layers at about 1,000 kilometers per second (more than 2 million miles per hour).

 

Using computer simulations, two astronomers at UC Santa Cruz, Elizabeth Lovegrove and Stan Woosley, recently found that when the shock wave strikes the outer surface of the gaseous layers, it would heat the gas at the surface, producing a glow that would shine for about a year -- a potentially promising¹⁴ signal of a black-hole birth. Although about a million times brighter than the sun, this glow would be relatively¹⁵ dim compared to other stars. "It would be hard to see, even in galaxies¹⁶ that are relatively close to us," says Piro.

 

But now Piro says he has found a more promising signal. In his new study, he examines in more detail what might happen at the moment when the shock wave hits the star's surface, and he calculates that the impact itself would make a flash 10 to 100 times brighter than the glow predicted by Lovegrove and Woosley. "That flash is going to be very bright, and it gives us the best chance for actually observing that this event occurred," Piro explains. "This is what you really want to look for."

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