Inside a pulsar the neutrons are thought to decay into protons and electrons, which are then accelerated by the pulsar's powerful magnetic field. Since protons and electrons are charged particles, they release intense synchrotron radiation at radio wavelengths from the magnetic poles of the pulsar. Previously astronomers believed that once a pulsar slowed beyond a certain point, the radio emissions would stop. However PSR J2144-3933 - which is now believed to be 280 million years old - defies this result. "By rights it should be a corpse," says Young.

The team discovered the pulsar while using the Parkes radio telescope in Australia to look for something else. "I was looking for missing pulses, points at which the pulsar skips a beat," says Young. "At first glance it seemed that this pulsar was missing two beats out of three. Then it became clear that it was really only beating a third as fast as we'd thought."

"This is a bit of a problem for the theories," says Manchester. "Perhaps the matter and anti-matter process can happen at lower spin rates than we thought. Or perhaps the pulses are powered by something else. Whatever the case, the theory needs a rethink."

The discovery also highlights the dangers of relying on fully automated software to locate interesting astrophysical objects. When PSR J2144-3933 was discovered in 1993, a software package determined its period to be 2.84 seconds. Almost half of the pulsars discovered in the past few years were detected by automated software at the Parkes telescope.