The first we usually know about a supernova is when a seemingly innocuous star flares up without warning to become as bright as a whole galaxy. Now, astronomers have found a star that appears to be on the brink of such a cataclysmic explosion. They spotted the star, known as M31N 2008-12a, in the nearby Andromeda galaxy in 2008 when it underwent a smaller explosion called a nova. This happens when a white dwarf—the burnt-out remnant of a star like our sun—is in a binary pair with a normal star and the small, dense white dwarf steals material—hydrogen and helium—from its companion that builds up in a thick outer layer (artist's conception, pictured above). If the layer gets thick enough, it can ignite a fusion burn, blowing off some of the material in a flash as bright as hundreds of thousands of suns—a nova. Novae are rare, but even rarer are ones that repeatedly blast off their outer layer; only a handful are known. M31N 2008-12a appeared to be one of the few, going nova five times between 2009 and 2014—a much higher frequency than other recurrent novae. A team of astronomers reported today at the National Astronomy Meeting in Llandudno in the United Kingdom, that, following observations with the Liverpool Telescope and NASA’s Swift x-ray observatory, they think the white dwarf must be on the brink of reaching a critical mass—1.4 times that of our sun—beyond which a much more powerful fusion reaction involving carbon deep in the heart of the star will be sparked. When this happens, the white dwarf will be blown apart in a matter of seconds in a flash brighter than many billions of suns. So get ready for the fireworks; it could happen any time in the next few hundred thousand years.