They were, in a way, dark days before 1991, when the Compton Gamma Ray Observatory was launched into orbit to study the most violent aspect of the universe. Until then, astronomers were limited in their ability to study the brilliant flashes of light known as gamma-ray bursts. And then for nearly 10 years, the field of gamma-ray astronomy burgeoned. But today, a bright chapter in astronomy is about to be closed -- if only temporarily -- as NASA ends Compton's productive nine-year mission.
It was in September 1991, four months into its operation, when Compton gave astronomers its first major discovery. On a mission to study bursts of gamma rays -- extremely high-energy electromagnetic radiation -- the telescope found quite early that the bursts were scattered nearly uniformly throughout the sky. This showed astronomers that the bursts were not simply from within the galaxy, but also from a variety of deep-space sources. Since then, Compton has detected more than 400 sources of regular gamma-ray bursts.
"The situation before Compton was launched was that there were balloons, and there were small, early versions of instruments being flown on satellites," said James Ryan, a professor of physics at the University of New Hampshire and one of the principal investigators for the mission. "The net result was that we had some fuzzy images that took many years to accumulate."
Gamma rays provide a fascinating window into the universe. One gamma-ray photon can have a billion times the energy of a photon from visible light. Thus, the processes that create these gamma-ray bursts must be extremely powerful events. Finding the sources of these bursts could give astronomers a better understanding of events like the birth of galaxies and the death of a star.
But like many great devices of discovery, the Compton data have raised new questions even as it helped answer old ones. In 1998, NASA announced that it had found the first example in nearly 20 years of a class of gamma-ray emitter called a Soft Gamma Repeater, or SGR. These are neutron stars that pulse gamma rays at irregular intervals. SGR's are believed to exist as only a single, short phase in the life of a highly magnetized neutron star. It is believed that periodically, the magnetic field wrinkles the crust of the star, causing a high-energy starquake that releases gamma rays.
Also, astronomers using Compton found a pulsar that emitted radiation only as gamma rays, meaning that it was invisible to telescopes that cannot detect light in gamma-ray wavelengths. Science still has no perfect explanation for how this pulsar works. "It really poses the question of how these pulsars can be so efficient in accelerating cosmic rays that in turn radiate to produce the emissions that we see," Dr. Ryan said. "They must be incredibly efficient, probably more efficient than any accelerator that we have here on Earth."
In all, Compton opened up gamma-ray astronomy as a veritable field of study. "We have images of the whole sky that we never had before, the sort of science that they've been doing at other wavelengths for years. We finally entered into that," Dr. Ryan said. "Taking us out of a cottage industry of astronomy and putting us into the mainstream -- that's probably the big contribution."
advertisement
