For more than 50 years, astrophysicists have speculated that inside a superdense neutron star, nuclear matter might flow without any resistance whatsoever—much like electricity does in earthy materials known as superconductors. Now, two teams say they have direct evidence of such bizarre "superfluidity" in a neutron star, and other researchers seem convinced. "I think it's a defensible claim," says theorist Krishna Rajagopal of the Massachusetts Institute of Technology in Cambridge. "The only explanation [of the observation] that I'm aware of is the one presented in papers."
Ordinary superconductivity  is weird to begin with. When some metals are cooled nearly to absolute zero, the electrons in them form hard-to-break "Cooper pairs" that flow without resistance. In 1959, just 2 years after physicists worked out that explanation for superconductivity, some of them proposed that similar pairing may happen inside incredibly hot, hugely pressurized neutron stars. The core of a massive star that has died in a supernova explosion, a neutron star consists of neutrons seasoned with a few protons and electrons, and it packs as much mass as one or two suns into a globe less than 20 kilometers across.
Scientists have accumulated indirect evidence for such pairing and superfluidity. For example, spinning neutron stars called pulsars emit clocklike streams of electromagnetic pulses. Usually incredibly steady, that pulsing sometimes speeds up abruptly. Such "pulsar glitches" likely result from brief interactions between the neutron star's solid crust and superfluid interior.
Now, two teams of physicists say they have more direct evidence for superfluidity in the heart of a neutron star. The data come from NASA's orbiting Chandra X-ray Observatory, which between 2000 and 2009 took occasional observations of Cassiopeia A (Cas A), a neutron star 11,000 light-years from Earth. The youngest known neutron star in the Milky Way, Cas A was born in a supernova explosion that astronomers may have spotted 330 years ago. The observations show that its temperature is falling at an alarming rate: from 2.12 million K to 2.04 million K, or 4%, in 10 years.
That huge cooling rate shows that Cooper pairs are forming, says Dany Page, a theoretical astrophysicist at the National Autonomous University of Mexico in Mexico City. In 2009, Page and colleagues predicted that a neutron star, which cools by emitting particles called neutrinos, should undergo a sudden increase in cooling when neutrons start to pair. That's because when two neutrons form a pair, they release energy that can go into producing more neutrinos. The researchers had hoped to demonstrate that brief period of intense cooling by measuring the temperatures of many neutron stars of different ages.
Then they got lucky. Last year, Wynn Ho of the University of Southampton in the United Kingdom and Craig Heinke of the University of Alberta in Edmonton, Canada, found that Chandra data showed Cas A was cooling so fast that they could measure the change. So instead of looking at many neutron stars, researchers needed to look at only this one. "This is definitely the first time that we've been able to see an appreciable temperature change in a neutron star," says Andrew Steiner, a nuclear astrophysicist at Michigan State University in East Lansing and member of Page's team.
The researchers find that they can explain Cas A's massive cooling rate if the neutrons in the star (and, separately, a few protons) are pairing, as they report  this week in Physical Review Letters. The fierce cooling must be a recent spurt, Page says, because otherwise the star would have to have started out "unbelievably hot." Likewise, it can't go on forever because Cas A would get far colder than neutron stars generally do. Ho, Heinke, and colleagues report a similar analysis in a paper in press  at the Monthly Notices of the Royal Astronomical Society.
The real gold is in the observation itself, Rajagopal says. "To see a neutron star cooling before your eyes is a tremendous discovery," he says. Page says that if researchers keep watching Cas A for a few decades, they should see the cooling slow down as all the neutrons pair up and extra neutrino emissions fade.