Astronomers have long held that pulsars -- quickly rotating neutron stars that emit a radio wave beam as they spin -- stop radiating when they become too slow.

But that theory can't support the discovery of Matthew Young, a Ph.D. student at the University of Western Australia, whose findings were published in this week's edition of Nature.

Young has found a pulsar with a rotation period of 8.51 seconds, the slowest period ever found since the first pulsar was discovered more than thirty years ago. At that rate, astronomers say, it should stop emitting light completely.

"This pulsar is on the slab, so to speak, but its heart is still beating," Young said in a statement. "By rights it should be a corpse."

Pulsars are special kinds of neutron stars, which are formed along with supernovae during the death of a massive star.

Neutron stars with strong magnetic fields can radiate as they spin -- like a lighthouse -- sending off what appear to stationary observers as pulses of light.

But as the pulsar's energy is spent, the object slows and eventually stops pulsing, becoming invisible to astronomers.

Young had been using data from the CSIRO, an Australian research center, to look for missing beats from a previously-discovered pulsar -- named J2144-3933 -- when he found that there were no hidden pulses at all.

That fixed the period of rotation at 8.51 seconds, more than three seconds longer than any pulsar discovered before, and too slow, according to current theories, to continue to radiate.

The discovery suggests that there are perhaps tens of thousands of slowly-spinning pulsars yet undiscovered by astronomers.

The finding is pushing astronomers to rethink the theories on which they base their research.

"It challenges our basic ideas of how pulsars turn off when they get to have these very long periods," says Josh Grindlay, a professor of astronomy at Harvard University.

Some astronomers have suggested that the pulsing process might not be directly related to the spinning speed, as had been assumed.

Or, Grindlay suggested, there could be a greater number of electrons in pulsars than previously believed.

Pulsars interest scientist in part because of their extremely condensed state. They weight about as much as the sun but have a radius of only 10 kilometers (6.2 miles) in radius.

Their pulses are also used by scientists as extremely accurate clocks.