A dying star has been caught in the act of resurrecting itself by eating its neighbor.

Together, the stars represent a previously unseen stage in the lifecycle of millisecond pulsars, the fastest-spinning objects in the universe.

"It’s really a missing link in the chain from young pulsar to old pulsar," said Anne Archibald, a McGill University graduate student and lead author of the study published in Science Thursday.

Pulsars are a special class of neutron stars, the corpses of massive stars that exploded as supernovae. They’re born spinning quickly, up to tens of times per second, and sweep the sky with a beam of radio energy as they rotate. Eventually, they slow down to the point where they can no longer emit radio waves and die a second death.

But until now, scientists couldn't explain how some old, dead pulsars become millisecond pulsars, which rotate hundreds of times a second. The new discovery of an intermediate step between the two appears to be the missing link.

Astronomers have long theorized that these superfast stars share their orbit with a companion star from which they leech extra material. The material settles around the pulsar's middle in a so-called accretion disk. As material from the disk falls onto the surface of the pulsar, it imparts enough angular momentum to spin back up into what scientists call a "recycled pulsar."

"We mean it in the same sense as recycling your plastics," Archibald said. "These pulsars have died and become invisible and useless to us, but they get brought back to life by getting fed from a companion."

Astronomers suspect that another type of pulsar-star pair is a way station between normal pulsars and millisecond pulsars. Called a low-mass X-ray binary, such a system is also made up of a neutron star and an accretion disk, but it doesn't emit radio waves. While the neutron star feeds off its neighbor, the gas flowing between them blocks low-energy radio waves from escaping the system. But when the accretion disk runs out, the radio waves come back, and astronomers can recognize it as a pulsar.

Until recently, this was only a theory. But Archibald and an international team using radio telescopes on four continents, stumbled on just such a pulsar right in the middle of this metamorphosis.

The system, called J1023, was discovered in 2007 when the Green Bank Telescope in West Virginia was down for repairs. The radio telescope couldn't be steered to different points in the sky, but resourceful astronomers took data anyway, observing whatever happened to pass overhead. That survey uncovered a millisecond pulsar about 4,000 light years away, spinning 592 times per second.

But this wasn't the first time this system caught astronomers' attention. Another survey in 1999 missed the pulsar but identified its companion as a sun-like star. When they looked again with radio telescopes in 2000, they saw evidence for an accretion disk around a neutron star. By 2002, the accretion disk had disappeared, but the neutron star was still not emitting radio waves as would be expected of a pulsar.

It was only the 2007 observations that pinned it down as a millisecond pulsar. Astronomers had managed to catch the system changing over the course of 10 years, an eye blink on astronomical timescales.

"This is a completely new thing, seeing it go from one state to another," said co-author Maura McLaughlin of West Virginia University. "We’ve never seen that before, ever."

Aside from proving that recycled pulsars form through cannibalism, J1023 provides a living laboratory for studying how these systems evolve.

"Studying this system will teach us an awful lot about the recycling process," McLaughlin said. "We may see the radio pulsations disappear and come back again a year from now. There are lots of neat things we can do."

"It’s a bit of a landmark in tying together what we think happens in these dead stars," said Don Backer, an astronomer at the University of California, Berkeley and one of the discoverers of the first millisecond pulsar. "You can see the transitions and understand more about how both of these stars work. That’s a great deal of fun."

Image: Anne Archibald