For the first time astronomers are studying a rapidly whirling pulsar whose companion star is big enough to temporarily block out the pulsar's characteristic radio signals.

"This is the first time that this has happened," Roger Blandford, an astrophysicist at the California Institute of Technology, said Wednesday. Pulsars are believed to be a type of neutron star, dense clumps of condensed cosmic matter that are the remnant cores of supernovas, rare stars that suddenly increase in brightness by 10 million to 100 million times.The core of the remnant neutron star is compressed with such force that the spinning ball left behind contains more mass than is found in the sun, which is thousands of times larger.

Princeton University astronomer J.H. Taylor, who along with a team of scientists, pinpointed the unusual star by using a radiotelescope in Puerto Rico, said it is the second fastest pulsar known.

He estimated that the 10-mile-wide pulsar, spinning at mind-boggling speed, makes a complete revolution every 1.6 milliseconds. A millisecond is one one-thousandth of a second.

Every 9.17 hours, the pulsar orbits a mysterious companion star that eclipses the pulsar for about 50 minutes in every orbital cycle.