But, since it can't be harnessed, it's just as well that this all happened 20,000 light-years away.
"This was quite a find," said Strohmayer. "We had suspected that such an explosion could exist. But because they are so rare, we didn't know if we could actually observe one.
Dead but not done
Neutron stars are the walking corpses of the cosmos. Here's how they form:
Big stars end their lives in a giant explosion called a
into an area 7 to 12 miles (11 to 20 kilometers) across.What caused the flare?
Daily helium explosions on a neutron star create carbon ash. Strohmayer suspects that this ash had built up for about a year and was packed so tight below the surface that it finally fused and exploded.
The three-hour event was far larger than most neutron-star flares, which last about 10 seconds. In fact, an initial 10-second flare was observed, and may have triggered the longer flare, which ignited a few seconds after the first one died down.
Both explosions were observed with the Rossi X-ray Timing Explorer.
"Such a long burst -- with a rich assortment of X-ray data -- will provide new insights into the physics of neutron stars and thermonuclear explosions," Strohmayer said.
Stealing energy
The neutron star is part of a binary system called 4U 1820-30. It orbits in tandem with a low-mass dwarf star. Neutron stars in this configuration, which is typical, steal helium gas from their companion.
The gas slams into the surface of the neutron star, builds up, and finally jump-starts helium fusion that creates flares on a regular basis.
"Over the course of a year or two, more and more helium rains down upon the neutron star," Strohmayer said. "This helium ignites and produces carbon. The carbon ash builds up under layers of new helium and other gaseous metals. When enough carbon builds up -- and the pressure raises the temperature to many times that of our Sun's core -- carbon will begin to fuse."
He estimates that it would take about a billion trillion pounds of carbon and a temperature of a billion degrees to create a three-hour flare like the one he saw.
The explosion gives researchers data to chew on, and may improve understanding thermonuclear reactions and how neutron stars become so dense.
"With the Rossi Explorer, we are now not only seeing the brilliant activity on the surface of a neutron star, we are also seeing what goes on underneath," said NASA's Jean Swank, project scientist for the satellite. "According to this recent observation, the physical forces are even more terrific than we expected."
The results were presented November 8 at a meeting of the High-Energy Astrophysics Division of the American Astronomical Society in Honolulu.
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