A flare is an explosion in the chromosphere and corona of a star which results in the ejection into space of a burst of charged particles and radiation, especially X-rays. Solar flares, which are mostly quite minor affairs as stellar flares go, occur regularly on the Sun and give rise to auroras when they reach the Earth. Once or twice a decade, much larger solar flares known as coronal mass ejections release enough energy to disrupt electricity supplies and communications satellites. One such outburst in 1989 brought down the power grid in northern Quebec. However, even solar activity of this magnitude is mild by normal stellar standards.

From studying records of other lone stars in the Galaxy of comparable size, brightness, age, and composition to the Sun, astronomers have found that most Sun-like stars are prone to eject "superflares," with about 10,000 times more energy than the event that caused the Canadian blackout, about once a century. A solar superflare would be enough to melt large flood plains on the icy satellites of Jupiter and Saturn and instantly destroy about half of Earth's ozone layer. Although it would not wipe out life on Earth, it might have serious, short-term consequences for the food chain at high latitudes (where the ozone depletion would be greatest) and result in elevated levels of ionizing radiation over the whole planet with mutagenic and possibly long-term evolutionary effects. If the Sun is indeed unusual in its stability and hospitability toward life, this could have important implications for the frequency with which life and intelligence emerge on other worlds.