May 15, 2008 -- Radio astronomers have accidentally discovered the rapidly expanding remnants of a 140-year-old supernova explosion in our own galaxy -- making it the youngest supernova in the Milky Way.
The discovery of the cloud dust-veiled explosion from the 1870s begins to fill in a missing supernovae population in the Milky Way. Astronomers figure there ought to be three such stellar self-destructions per century in this galaxy, based on what they see in others.
The next-youngest supernova remnant in the Milky Way is Cassiopeia A, which is thought to be about 330 years old.
The only other young supernova close enough to study is 1987a, seen exploding in the Large Magellanic Cloud (LMC) in 1987. The LMC is a nearby dwarf companion galaxy and not part of the Milky Way, however.
The newest Milky Way supernova remnant, called G1.9, was identified by astronomers who were following up on radio wave observations of the same object made by University of Cambridge astronomer David Green in 1985. They were looking with the space-based Chandra X-Ray Observatory and wanted to publish the 23-year-old radio image in their research paper.
At the time, they thought they were looking at a supernova that happened as long as 1,000 years ago.
"It was a nice radio image that we wanted to use," said astronomer Stephen Reynolds of North Carolina State University in Raleigh. When they took a closer look, however, something didn't fit. The arc-like, shockwave shape of the supernova explosion was too small.
"We couldn't see any reason that the radio image was smaller than the X-ray image," said Reynolds. That's when they called Green back and asked him to look at G1.9 again using the same powerful Very Large Array radio telescope in New Mexico that he used in 1985.
New radio images revealed that the X-rays were not mistaken -- the shock wave from the supernova was 16 percent larger than it had been in 1985. That could mean only one thing: The remnant was expanding very quickly and, therefore, is much younger than they thought.
The discovery of G1.9 underlines the need to return to objects years later to see how they change, explained Roger Chevalier of the University of Virginia.
"Hopefully this will spur people to go back and look at other small radio (wave-emitting) objects," Chevalier told Discovery News.
Another clue to the youth of G1.9 is the kind of X-rays it was giving off. Normally, supernova remnants emit X-rays when the shockwave of the explosion slams into interstellar gas, which then heats up to X-ray-emitting temperatures. But G1.9 was giving off far more energetic X-rays -- more like what's expected from a particle accelerator. That requires particle speeds that would only be available soon after a star explodes, Reynolds explained to Discovery News.
Only four supernovae have been found giving off those sorts of extreme X-rays, said Reynolds. "This is the most extreme of the four," he added.
For all those powerful X-rays, however, G1.9 is also one of the most obscured supernovae in the galaxy, Reynolds said. That has led the researchers to believe that it's located somewhere in the crowded neighborhood of the galactic center. Gas and dust block visible light, but are transparent to radio waves, infrared light and some S-rays.
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