One of the brightest stars in the sky could get even brighter - as bright as a full moon - and last for as long as a year.

The star is Betelgeuse, which marks the left shoulder of the constellation Orion. It is visible in the winter sky over most of the world as a bright, reddish star. That star could explode as a supernova within the next 100,000 years -- relatively soon for a star.

If it goes nova, it could offer astronomers a close look at how supernovae evolve and the physics that governs how they work.We'd learn a huge amount, said Phil Plait, an astronomer who has worked on the Hubble Space Telescope, on his blog.

The problem is that is not clear when that will happen. While stories have been circulating that the star could explode in 2012, the odds of that are actually quite small. Betelgeuse may explode tomorrow night, or it may not go kerblooie until the year 100,000 A.D. We don't know. But given that huge range, the odds of it blowing up next year are pretty slim, Plait wrote.

The explosion won't do the Earth any harm, as a star has to be relatively close -- on the order of 25 light years -- to do that. Betelgeuse is about 600 light years distant.

A supernova is what happens to a massive star towards the end of its life (usually a star more than two to three times the sun's mass). Stars shine via nuclear fusion, turning hydrogen into helium, and then helium into carbon and heavier elements. Eventually, the star starts making iron. When that happens, the amount of energy from fusing into anything heavier is less than what it takes to fuse the atoms. So the iron falls as ash to the center of the star.

When the iron core reaches a certain size, the star can't generate enough energy to support the outer layers (which are made up of hydrogen, helium and other heavier elements). The star collapses in a few seconds, and the mass of material bounces off the core, with so much energy that the star van shine more brightly than the galaxy it is in. What's left is a neutron star, a ball of neutrons a few miles across and so dense that a cubic centimeter weighs a metric ton.

The sun won't do that because it isn't heavy enough; instead it will expand into a red giant and then slough off its outer layers, ending its life quietly as a white dwarf star. But Betelgeuse is one of the most luminous stars known. It is far larger than the sun, and about 20 times as massive. Were it placed at the center of the solar system it would extend all the way to the asteroid belt, beyond mars.

Stars that massive don't last long, however. Betelgeuse is thought to be only 10 million years old, as the more massive a star is the shorter its lifespan, which is why astronomers think it has an outside chance of exploding relatively soon.

The last time a supernova visible to the naked eye went off in the neighborhood was in 1987. Supernova 1987A was in the Magellanic Cloud, and was about 168,000 light years distant, and it was a heavily studied phenomenon. Its peak brightness was third magnitude, which is not very bright (and not visible in much of the northern hemisphere).

Before that, there was a naked-eye supernova in 1604. That one was close, as these things go, about 20,000 light years away. It was bright enough to rival Venus in the night sky, and was called Kepler's Star, as Johannes Kepler was among the first to observe it and record where it was - in the constellation Ophiuchus.

Arab and Chinese astronomers also recorded one that is now the Crab Nebula, in 1054. The remnant of the star can be seen by amateur astronomers in the constellation Taurus. It was so bright it could be seen in daylight for three weeks and was visible to the naked eye for two years. The nebula is about 6,000 light years away.