Alternatively, a plasma can be defined as a collection of charged particles whose dynamics are chiefly governed by electromagnetic forces. But that seems a bit tame to describe what goes on in nuclear fusion reactors, where the temperature hits 100 million degrees in a seething mayhem of particles trapped by invisible magnetic fetters. Nor is it adequate to depict the whirling inferno of plasma that forms around a black hole as it sucks whole stars into oblivion.

Even the most popular descriptions miss the mark. Plasmas are often called the “fourth state of matter,” to distinguish them from the more familiar solids, liquids and gases that fill our world. That's a useful distinction. But it's only meaningful in a few odd spots in the universe where solids, liquids and gases can actually exist.