It's not a supernova. Nor is it a galaxy, or a black hole.
In fact, astronomers have no idea what the mysterious object that in February 2006 suddenly flared up in an otherwise barren patch of sky might be, or even what it's made of.
Researchers working on something called the Supernova Cosmology Project had pointed the Hubble Space Telescope at a very distant star cluster, 8.2 billion light-years away or more than halfway across the universe.
But they noticed something else — a point of light where there hadn't been one before.
Over the next three months, the object got brighter and brighter until it was 120 times its initial luminosity.
Then it slowly got dimmer again, at about the same rate, until by the end of the year it was gone.
Astronomers led by U.C. Berkeley astrophysics grad student Kyle Barbary put the light coming from it through a mass spectrometer to see what it was made of — but couldn't get signatures for any known elements.
"Because we can't see anything we recognize in the spectrum, we can't tell if it's even in [our] galaxy or in another galaxy," Barbary told New Scientist magazine in an article posted Tuesday morning.
It's not a supernova, which would have flared up much more quickly, then died out even more quickly.
All the scientists know is that it's no closer to Earth than 130 light-years away — and no further than 11 billion light-years away.
As Sky and Telescope magazine noted last week, "that leaves a lot of leeway."
"We are hoping someone else might have seen something similar," Barbary told New Scientist, "or might be able to shed some light on it."