To the astronomers' menagerie of pulsars and quasars, add the noisar.

A noisar is a newly discovered rare star that emits X-rays, but not in a steady stream or in evenly spaced pulses like other X-ray sources in the sky. Instead, it sputters irregularly, spewing out X-rays in no particular pattern.

Scientists often call any patternless signal "noise." Noisar is a contraction of "noisy star."

The first noisar was discovered by astronomers John Middleditch and William Priedhorsky of the Los Alamos, N.M., National Laboratory, who plan to publish their finding in Astrophysical Journal.

Their X-ray source, called Sco X-1, is 10 quadrillion miles from Earth in the constellation Scorpius. It has been known for more than 20 years. Not until they accidentally "listened" at one particular radio frequency, however, did they hear its noise.

Ordinary X-ray sources are so rare that only a few hundred are thought to exist in a galaxy of 100 billion stars. Noisars must be even rarer.

Ordinary X-rays are believed to result when two stars orbit each other, the more massive partner drawing gas from the other through its gravitational pull. As the gas hits the heavier star, the impact releases energy, some in the form of X-rays.

In the case of Sco X-1, the energy is the equivalent of 1,000 hydrogen bombs exploding over every square inch of the recipient's surface.

In most X-ray objects, the energy radiated to Earth is continuous, or is pulsed because the recipient star is spinning. The noisy emission by Sco X-1, however, suggests that some unknown process is at work. The astronomers say that their discovery is "shocking" and that they cannot explain it.