One of the greatest mysteries spotted by the Hubble Telescope could be called the ultimate UFO. Except it wasn't "flying;" it was just located in deep space, beyond detectable galaxies. Two years ago, the object began to brighten, peaking at 21st magnitude in two colors near infrared. After brightening for about 100 days, it faded out and became unviewable (you can see it in its invisible and explodey states above). This object didn't behave like a supernova, and researchers are still trying to figure out why something flared up and died down in what amounts to the middle of nowhere, universe-wise. Now some researchers are working on an answer. Sky and Telescope reports on a new paper (PDF) published by some UC Berkeley researchers on the strange object:

The mystery object did not behave like any known kind of supernova. It is not even in any detectable galaxy. "The shape of the light curve is inconsistent with microlensing," say the researchers. They recorded three spectra of it - and its spectrum, they write, "in addition to being inconsistent with all known supernova types, is not matched to any spectrum in the Sloan Digital Sky Survey database" of vast numbers of objects. "We suggest that the transient may be one of a new class."

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So basically the object emitted light in spectra unknown for stellar explosions. Could it have been an artificial structure out there in the black? Hubble Finds a Mystery Object [via Sky and Telescope]