Knock 'em dead: how does one extinguish life on Earth? Let me count the ways

Natural History, May, 2005 by Neil deGrasse Tyson

But the ultraviolet blasted in every direction by a supernova is just a mosquito bite compared to the gamma rays let loose from a hypernova.

At least once a day, a brief burst of gamma rays--the highest of high-energy radiation--unleashes the energy of a thousand supernovas somewhere in the cosmos. Gamma-ray bursts were accidentally discovered in the 1960s by U.S. Air Force satellites, launched to detect radiation from any clandestine nuclear-weapons tests the Soviet Union might have conducted in violation of the 1963 Limited Test Ban Treaty. What the satellites found instead were signals from the universe itself.

At first nobody knew what the bursts were or how far away they took place. Instead of clustering along the plane of the Milky Way's main disk of stars and gas, they came from every direction on the sky--in other words, from the entire cosmos. Yet surely they had to be happening nearby, at least within a galactic diameter or so from us. Otherwise, how was it possible to account for all the energy they registered here on Earth?

In 1997 an observation made by an orbiting Italian X-ray telescope settled the argument: gamma-ray bursts are extremely distant extragalactic events, perhaps signaling the explosion of a single supermassive star and the attendant birth of a black hole. The telescope had picked up the X-ray "afterglow" of a now-famous burst, GRB 970228. But the X rays were "redshifted." Turns out, this handy feature of light and the expanding universe enables astrophysicists to make a Fairly accurate determination of distance. The afterglow of GRB 970228, which reached Earth on February 28, 1997, was clearly coming from halfway across the universe, billions of light-years away. The following year Bohdan Paczynski, a Princeton astrophysicist, coined the term "hypernova" to describe the source of such bursts. Personally, I would have voted for "super-duper supernova."

A hypernova is the one supernova in 100,000 that produces a gamma-ray burst, generating in a matter of moments the same amount of energy as our Sun would emit if it shone at its present output for a trillion years. Barring the influence of some undiscovered law of physics, the only way to achieve the measured energy is to beam the total output of the explosion in a narrow raw--much the way all the light from a flashlight bulb gets channeled by the flashlight's parabolic mirror into one strong, narrow beam. Pump a supernova's power through a narrow beam, and anything in the beam's path will get the full brunt of the explosive energy. Meanwhile, whoever does not fall in the beam's path remains oblivious. The narrower the beam, the more intense the flux of its energy, and the fewer cosmic occupants will see it.

What gives rise to these laserlike beams of gamma rays? Consider the original supermassive star. Not long before its death from fuel starvation, the star jettisons its outer layers. It becomes cloaked in a vast, cloudy shell, possibly augmented by pockets of gas left over from the cloud that originally spawned the star. When the star finally collapses and explodes, it releases stupendous quantities of matter and prodigious quantities of energy. The first assault of matter and energy punches through weak points in the shell of gas, enabling the succeeding matter and energy, to funnel through that same point. Computer models of this complicated scenario suggest that the weak points are typically just above the north and south poles of the original star. When seen from beyond the shell, two powerful beams travel in opposite directions, headed toward all gamma-ray detectors (test-ban-treaty detectors or otherwise) that happen to lie in their path.