Earth and its star, the sun, are part of a spiral galaxy, the Milky Way, that contains more than 100 billion other stars.

Like a cosmic carousel, the entire Milky Way rotates to the tune of a mysterious barker cloaked in glowing clouds of gas and dust at the galactic core.Who is the barker, and what is his tune? What lies in the heart of the galaxy? Is it really the "Great Annihilator," a monstrous celestial object that slurps up stars by the score? A $617 million astronomy satellite, nudged into orbit last week by the space shuttle Atlantis, may help answer these questions about one of the most intriguing areas of the cosmos.

Called the Gamma Ray Observatory, the 17-ton satellite is one of four Earth-orbiting observatories the National Aeronautics and Space Administration hopes to have working in coming years. The first, the Hubble Space Telescope, was launched in April 1990 and already is at work, despite a design flaw that distorts its images. The other two will be the Advanced X-Ray Astrophysics Facility, scheduled for launch in 1998; and the Space Infrared Telescope Facility, scheduled for launch by the end of 1999.

The Hubble telescope peers into the cosmos with only the human eye's narrow range of visual perception. Visible light is only a narrow band in the spectrum of electromagnetic radiation emitted by stars and other celestial objects.

GRO and its two future companions, however, represent an effort by NASA to open "new windows" on the cosmos. The Earth-orbiting observatories will scoop up invisible forms of electromagnetic phenomena that human beings could see if they had electronic detectors rather than eyes.

These forms range from very short gamma rays to x-rays, ultraviolet rays, infrared rays and radio waves.

Gamma rays have the highest energy of any form of electromagnetic radiation. The photons, or particles, that make up gamma rays have energies millions or billions of times greater than those of visible light.

Since high-energy processes tend to produce high-energy radiation, gamma rays are emitted by some of the most violent processes and exotic objects in the universe. These include exploding stars termed supernovae; super-dense objects termed "black holes" that can gobble up entire stars, and gamma-ray "bursters," mysterious objects that in a few seconds release more gamma energy than the sun could in 500 years.

Gamma rays, in some instances, are the only source of information about this celestial turmoil. Yet since the first detection of gamma rays from space in 1957, they have proven extremely difficult to study.

Gamma rays from celestial objects that reach the Earth's surface are weak and infrequent, obscured in a larger gamma-ray background. The background results from the interaction of cosmic rays with observing instruments and the Earth's atmosphere, which also blocks passage of shorter gamma rays.

Dr. Donald A. Kniffen, of NASA's Goddard Space Flight Center in Greenbelt, Md., describes the GRO as the biggest single advance in gamma ray astronomy.

GRO represents a dramatic improvement in sensitivity, range and resolution over previous gamma ray detectors. Its instruments, for instance, are 10 to 20 times more sensitive than earlier instruments carried aloft on satellites and balloons.

GRO, which weighs almost 18 tons, will orbit 280 miles above the Earth with a life expectancy of four or more years.

GRO is not the usual astronomical observatory because it does not carry telescopes. Gamma rays pack so much energy that they zip right through metal and glass. They thus cannot be focused with the mirrors used in conventional telescopes.

Instead, the new satellite carries four electronic devices that detect gamma rays indirectly, by observing their interaction with matter.

Size is critical for detectors used in gamma ray astronomy. The greater the mass of the detector, the larger the number of gamma rays likely to be detected. Three of GRO's instruments are as large and as heavy as a subcompact car.

Each of the four main detectors onboard GRO is designed for gamma rays that pack different levels of energy. The most energetic gamma rays studied by GRO will have energies one million times greater than the weakest. GRO thus will detect a broad range of gamma ray emissions from celestial objects.

Data from the gamma ray detectors will be stored on magnetic tape, and periodically relayed to ground stations. Scientists will use an array of sophisticated computer technology to analyze the data.

The first 15 months of GRO's mission will be devoted to conducting a full-sky survey of gamma ray sources. It will chart all the gamma ray sources detectable from this part of the galaxy.

Earth is located about 25,000 light-years from the center of the Milky Way. One light-year is roughly 5.9 trillion miles - the distance that light, traveling at about 186,000 miles per second, can cover in one year. The Milky Way itself is about 100,000 light-years in diameter.

In astronomical terms, that puts GRO in prime position for studying mysterious events under way at the heart of the galaxy, which has fascinated mankind for centuries.

Ancient Greeks, for instance, thought a goddess, Hera, lived at the center of the galaxy. They believed that the white band that can be seen stretching across the sky on moonless nights was a stream of mother's milk. Hera supposedly provided it to nourish the Earth. The band actually is the plane of the Milky Way.

Modern scientists have accumulated much evidence challenging that peaceful scenario. It suggests that the galactic core is a killing field for stars and other matter, a place of incredible violence dominated by a black hole.

Black holes are objects so massive that nothing - not even light - can escape their gravitational pull. Scientists believe that a black hole may have been formed at the galactic core by the collapse of a gigantic star.

The black hole, they believe, is constantly increasing in size and mass as its gravitational pull sucks in stars, clouds of dust and gas, and perhaps planets. Some scientists have called the black hole the "Great Annihilator."

Previous studies of gamma ray emissions from the galactic core suggest that vast quantities of matter are being annihilated. In the process, elementary particles of matter called electrons collide with particles of antimatter called positrons. Scientists believe the black hole generates antimatter as it slurps-in stars, spewing out positrons that fuel the annihilation of matter. Each collision between matter and antimatter produces two gamma rays, with a characteristic "signature" - a specific energy level that GRO can detect.

GRO thus could help to determine whether a black hole does exist in the Milky Way's heart, and answer other questions about the galactic core.