In the past few months, you might recall warnings of incoming pieces of the sun. Called coronal mass ejections (CMEs), these energetic solar-storm particles can trigger amazing displays of auroras. But they can also wreak havoc on spacecraft, navigation systems and communications. In principle, a big enough CME could do so much damage to electrical infrastructure that power grids could be offline for months.

Scientists try to predict the storms by looking at sunspot activity. Still, a CME can reach Earth in just days. What gives these storms their explosive energy? One clue lies in the fact that the sun’s corona (atmosphere) is actually much hotter than the sun’s surface, as explained in this clip from PBS’s NOVA program, “Secrets of the Sun,” airing Wednesday, April 25, at 9 PM.

Watch Understanding Solar Activity on PBS. See more from NOVA.