Well, this is awesome. 

Scientists have been keeping an eye on an unusual star called V838 Monocerotis since 2002, when it had an explosion that made it, for a few weeks, 600,000 times brighter than our sun.

Ever since then, light from that outburst has been rippling across the universe and illuminating space dust in what the European Space Agency calls "the most spectacular 'light echo' in the history of astronomy." The echo alone, according to NASA, spans six light years of outer space. And, now, thanks to a time-lapse look from Hubble photos of the light echo, you can watch the way the echo has transformed—about 20,000 light-years away from Earth at the outer edge of our Milky Way galaxy—over a four-year period: 

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(It's also worth checking out the light echo in GIF form that Jesus Diaz posted over at Gizmodo.)

So what caused this stellar explosion in the first place? It was clear from the get-go that this thing was unlike the novas and supernovas that skywatchers know well. Novas are mammoth nuclear explosions that happen after a normal star "dumps hydrogen onto a compact white-dwarf companion star," according to NASA. Supernovas—which are even more dazzling than novas and temporarily burn brighter than everything else in the galaxy around them—can be caused by nuclear fusion or by the collapse of a star's core. 

But none of that happened this time. Whereas a typical nova blasts a star's outer layers into space and a supernova blows apart a star entirely, V838 Monocerotis didn't even lose its outer layers. From NASA

Instead, it grew enormously in size. Its surface temperature dropped to temperatures that were not much hotter than a light bulb. This behavior of ballooning to an immense size, but not losing its outer layers, is very unusual and completely unlike an ordinary nova explosion.

This sudden and unpredictable explosion, then, may be evidence of a stage in star evolution that is "rarely seen," NASA says. One theory: A pair of stars got so close to each other that their atmospheres merged. A slightly less romantic way of describing this event: stellar cannibalism, in which one star swallows the one next to it.  

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