Astronomers Probe Space Mystery

GERALD FISHMAN is a happy astronomer. He has gotten his teeth into what he calls "probably one of the most intriguing unsolved mysteries in astronomy."

It's the riddle of the now-you-see-'em, now-you-don't gamma-ray sources that pop up unpredictably all over the sky. They shine brilliantly with gamma-ray "light" for a few seconds or less only to fade and never be seen again. Searches of the locations in the sky where these "bursters" have appeared have so far found nothing unusual.

Astronomers don't know what they are or where they are. They could be close to Earth or billions of light years away. But Dr. Fishman says he thinks that, whatever they turn out to be, "it's going to be something new."

That's the kind of challenge astronomers love to find when they open up a new research field - in this case, the observation of the universe with gamma rays. It also is the major challenge that has emerged from the millions of bits of data taken by the National Aeronautics and Space Administration (NASA) orbiting gamma-ray observatory. Fishman, who works at NASA's Marshall Space Flight Center in Huntsville, Ala., is one of the principal investigators using that facility.

The space shuttle Atlantis deployed the $557-million, 17.5-ton observatory on its 273-by-280 mile-high (440-by-450 kilometer) orbit April 7, 1991. A year ago next Wednesday, NASA gave the satellite its official name - the Arthur Holly Compton Gamma Ray Observatory - in honor of the late Nobel Prize laureate who discovered a fundamental process involved when gamma rays interact with matter. Earlier this month, Fishman joined dozens of other scientists to review the observatory's findings during the World

Space Congress here.

Gamma rays - the "light" by which the observatory sees cosmic phenomena - are an extremely high energy form of electromagnetic radiation. Gamma- ray photons range in energy from about 10,000 times to about 10 billion times the energy of a photon (particle) of visible light.

Gamma rays arise in a variety of ways. Particles accelerating in strong magnetic fields may emit them. Electrons may collide with photons and boost them to gamma-ray energies - the process Compton discovered. Gamma rays may come from fast electrons slowing down in the presence of other matter - so-called braking radiation - or from the collision of atomic nuclei. Matter and antimatter - such as electrons and positrons - may collide and annihilate each other in gamma-ray bursts.

Gamma rays also come from decay of radioactive elements. One of the Compton Observatory's long-term objectives is to search the sky for radioactive material produced in the giant star explosions called supernovae.

These processes all involve high energies. Scientists expect that gamma rays will be a signature of the most violent and spectacular phenomena in the universe, including supernovae. They should flow from the vicinity of black holes. These are objects so compact that not even light can escape their strong gravity.

1 | 2

Next