I learned via Phil Plait’s Bad Astronomy blog that the nearest candidate for a supernova, the binary star IK Pegasi, is located just 150 light years away. Wikipedia’s description of this star system is succinct.
The primary (IK Pegasi A) is a main sequence, A-class star that displays minor pulsations in luminosity. It is categorized as a Delta Scuti variable star and it has a periodic cycle of luminosity variation that repeats itself about 22.9 times per day. Its companion (IK Pegasi B) is a massive white dwarf—a star that has evolved past the main sequence and is no longer generating energy through nuclear fusion. They orbit each other every 21.7 days with an average separation of about 31 million kilometres, or 19 million miles, or 0.21 astronomical units (AU). This is smaller than the orbit of Mercury around the Sun.
What makes it a supernova candidate? Phil Plait explains.
IK Peg A is aging. It’s still fusing hydrogen into helium in its core like the Sun does. But remember, those pulsations are telling us it’s nearing the end of its life too. At some point in the future, probably in a few dozen or hundred million years, it too will swell into a red giant.
When it does, [. . . m]aterial from IK Peg A will flow onto the white dwarf. Separated by a mere 30 million kilometers or so (closer than Mercury is to the Sun), this transfer of mass will flow steadily. As the matter piles up on the surface of the white dwarf it gets fiercely compressed and hot. At some point the temperature gets high enough to flash fuse it into helium. There will be an explosion — big, but not big enough to destroy the star — called a nova. Some of the hydrogen will remain, as will the helium. When things calm down, the material from the red giant will start to pile up again.
Lather, rinse, repeat.
But every time it does this, not all the added material blows away. The mass of the dwarf increases. It’s also possible that the matter from the red giant will accumulate slowly enough that it will pile up without a nova explosion. Either way, the mass of the white dwarf increases. And remember, IK Peg B is already pretty massive. It can only gain so much more mass before something very bad happens…
One day, something very bad happens. When the dwarf reaches a mass of about 1.4 times the Sun, the physical forces inside the star can no longer support its own mass. The white dwarf starts to collapse, and the core temperature rockets skyward. A fusion chain reaction is ignited in the dwarf, and the conditions inside it cannot stop it. Within seconds, the chain reaction runs out of control, consuming the bulk of the star, and it explodes.
Right now, IK Pegasi isn’t close enough to be a threat to Earth–IK Pegasi would have to be within 75 light years of Earth to have an impact. More, IK Pegasi is moving relative to Earth, the longer the time that IK Pegasi B refrains from going supernova translating to a still greater distance. Commenter Eric Mamajek calculates that none of the known nearby likely candidates for supernova status are likely to enter within the lethal range in the time they have left.