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February 12, 2012

The twins of Gemini stand high in the east as darkness falls on February nights — the stars Pollux and Castor. Pollux is the brighter of the two, and has a slight orange tint. The twins soar high overhead in late evening.

Gemini also harbors a much more exotic object: a dead star that’s one of the closest neutron stars to Earth, born in a spectacular explosion hundreds of thousands of years ago.

The object was first detected in the 1970s, as a source of gamma rays. But it proved so mystifying that astronomers gave it a name with a double meaning: Geminga. It not only signifies that it’s in the constellation Gemini, but in a Milanese dialect it means “it’s not there,” because the object proved hard to see at wavelengths other than gamma rays.

Finally, in the early 1990s, an orbiting satellite detected pulses of X-rays from Geminga. These indicated that the object was a pulsar — a rapidly spinning neutron star, which is the crushed corpse of a once-mighty star that exploded as a supernova.

Today, Geminga spins four times a second. As it ages, though, it slows down. By measuring this decrease, astronomers have deduced that Geminga is about 340,000 years old.

Geminga is only about 800 light-years from Earth, which makes it one of the closest neutron stars yet discovered. So anyone who looked skyward about 340,000 years ago likely would have seen a great explosion — one that, for a while, outshined every other star in the night sky.


Script by Ken Croswell, Copyright 2011


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