Gamma-Ray Bursts by Andrew Fraknoi
"The gamma-ray burst traveled through intergalactic space at the speed of light for eleven billion years, during which time the Sun and the planets were born." — Timothy Ferris, in the film version of Seeing in the Dark.
The Sun produces enough energy in one second to supply all the energy needs of Earth's civilization for nearly a million years, but astronomers have recently discovered violent events in the universe that make the Sun look dim by comparison. These brief outpourings of energy are called gamma ray bursts. They are short in duration, lasting from a fraction of a second to a few minutes, and they emit most of their energy as gamma rays.
Gamma rays belong to the same family as light, x-rays, and the radio waves that travel between a radio station and your car radio. Called electromagnetic radiation, such waves travel at the speed of light and bring us almost all the information we have about the universe. Gamma rays are much shorter in wavelength than visible light, and carry a lot more energy—so much energy, that here on Earth we typically detect strong gamma-ray bursts only from the detonation of nuclear bombs. The earth's atmosphere blocks most gamma rays from space, so gamma-ray astronomy was not possible until astronomers put gamma-ray detecting telescopes into space.
The first gamma-ray bursts were discovered by accident in the 1960's, by spy satellites looking for gamma-rays from secret nuclear bomb tests. Once scientific satellites began looking for them, gamma-ray bursts were found to occur at random, all over the sky. This randomness provided a key clue to explaining them. Events that are truly distributed all over the sky are very rare. The planets of our solar system, for example, are not randomly distributed—they circle the Sun in a flat disk. The island of stars that includes our Sun (the Milky Way Galaxy) is a flat disk, so the stars we see through telescopes are not distributed randomly: Most are found in the disk of the Milky Way. The fact that gamma-ray bursts come from all directions therefore indicated that they originate, not in the solar system or the Milky Way, but out in the wider universe.
Astronomers were at first reluctant to accept this conclusion, because the farther away the bursts were, the greater the energy they had to produce to become noticeable to us. You can construct a lighthouse that is visible from ten miles away, or a larger one that can be seen from a hundred miles away, but if you can see a lighthouse a billion miles away, you know it must be much more powerful than any you've seen before. Similarly, for gamma-ray bursts to come from billions of light years away—as it now seems certain that they do—they must have more power than anything previously known to astronomers. Gamma-ray bursts typically emit as much energy in a few seconds as an entire galaxy of stars will emit in years. Think about this: a single sudden explosion, which becomes as bright as the combined light of millions of billions of stars and then quickly fades away. What could be the source of these mind-boggling bursts?
Astronomers don't yet have all the answers—in part because there are evidently two or more ways to produce a gamma-ray burst—but it now seems clear that many bursts are associated with the violent death of the largest stars in the universe. As discussed on the Lives of the Stars page, the most massive stars end their careers in titanic explosions called supernovae. One supernova blossomed in the skies of Earth in the year 1054, becoming so bright that for a few weeks it was visible in broad daylight. (Today, almost a thousand years later, the tattered remnant of that supernova is visible through a small telescope as the Crab Nebula, in the constellation Taurus, the bull. You can find it with our star chart. The Crab is one of the best studied and most interesting objects in the sky.) Astronomers have observed thousands of supernovae—most of them in other galaxies, since that's where most of the stars are—and even use them to chart the dimensions of the universe.
But it now appears that certain dying stars—if they are really huge, spinning rapidly, and are surrounded (as often happens) by a thick shell of gas and dust ejected in their death throes—can explode as even more violent hypernovae. In these cases, the core of the star collapses to be a super-compressed, spinning black hole, which stirs and energize the wreckage around it to briefly emit a tremendous burst of gamma rays.
According to the hypernova model, the explosion and the spinning black hole will continue to produce great jets of energy that energize the leftover material surrounding the black hole. Thus the burst area continues to glow with electro-magnetic waves—not gamma rays any more, but less energetic x-rays—and then the even less energetic wavelengths of visible light. This visible-light afterglow can be photographed with medium to large-sized telescopes on Earth for hours, days, or even weeks following the initial gamma-ray burst.
The afterglow starts to fade right away, so the earlier you catch the explosion, the more likely you are to be able to photograph the afterglow. Astronomers have sent x-ray telescopes into orbit to scan the sky, to catch the first x-ray afterglow of a burst as soon as possible. The latest of these satellites, called Swift, has been making a slew of discoveries since its launch in November 2004. True to its name, the Swift satellite instantly dispatches e-mail alerts to amateur and professional astronomers around the world.
Many observers alerted by Swift will be on the wrong side of the world or under cloudy skies, but a few have a chance to catch the visible afterglow. As there are many more amateur than professional astronomers, and many have the kinds of telescopes and CCD cameras needed to measure the fading brilliance of a gamma-ray burst, the odds are that the discovery will be made by an amateur.
As described in our film, the amateur astronomer Michael Koppelman captured just such an image, and it turned out to be one of the most distant gamma-ray bursts ever observed—its distance a staggering 11 billion light years. One day when the different kinds of gamma-ray bursts are better understood, it is likely that the amateur astronomy community will have played a significant part in exposing the secrets of these violent events.
The distribution of 2704 gamma-ray bursts on the sky:
NASA Painting of a Gamma-ray Burst:
NASA Painting: Hypernova:
Crab Nebula from the Hubble:
Swift Spacecraft Illustration: