How scientists lost a 5,000-year-old star for three years

Astronomy can be pretty tricky business. For one thing, all the stuff you're looking at is actually spread very far apart, and apparently sometimes a star will just flat-out disappear on you. That's what happened to a team operating NASA's Fermi Gamma-ray Telescope, who discovered a 5,000-year-old pulsar in 2009 only to lose track of it for the next three years.

To understand how this happened, you first have to understand the nature of the star in question. A pulsar is made up of what's left over when a larger star is destroyed in a supernova. Those remnants are then compressed into a much smaller volume, and that compression leads to a very fast rotation. They're called "pulsars" because they seem to pulse on and off to our eyes (or our telescopes) when the beams of light emitted from their poles spin toward and away from us. When it was first spotted, this particular pulsar (designated J1838-0537) was spinning about seven times per second.

"The pulsar is, at 5,000 years of age, very young," said Holger Pletsch, a scientist at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Germany. "It rotates about its own axis roughly seven times per second, and its position in the sky is towards the Scutum constellation."

Now, when we say the Fermi team "lost" this star, we don't mean that they saw it in the sky, then didn't see it again for three years. The Fermi telescope doesn't necessarily work that way. It scans the sky for any sign of particles (or photos) of high-frequency gamma radiation, then translates whatever it finds into a kind of sky map. Scientists are then able to study the results to find both constant sources of gamma radiation, like stars, and infrequent bursts of it, like, say, solar flares. According to Pletsch, this particular pulsar was visible in the Fermi data until September 2009, at which point "it seemed to suddenly disappear."

But how? Stars don't just go away, and if they do there's a definite explosion in space that would be easy for Fermi to spot. So what gives?

After carefully studying the data, the Fermi team determined that the star sort of hiccuped at some point in its rotation, causing it to speed up very, very slightly, just enough that its rotational pattern was no longer detected by the telescope.

"If the sudden frequency change is neglected, then after only eight hours, a complete rotation of the pulsar is lost in our counting, and we can no longer determine at which rotational phase the gamma-ray photons reach the detector aboard Fermi," Pletsch said.

Using algorithms on the ATLAS supercomputer, the team was able to pin down the pulsar yet again this year.

The sudden change in rotation of a star is a freaky occurrence even to astronomers, and there's still no clear explanation of what causes it. Some theorize that "star quakes" are to blame, while others think it has to do with the way fluids inside the star interact with harder outer crusts. Either way, it definitely makes watching the sky more interesting.

A paper outlining the discovery will be published in an upcoming issue of Astrophysical Journal Letters.

(Via Huffington Post)

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