The corona is a very sparse, but very large, layer of gas that extends from the star’s photosphere and chromosphere out to a distance of about ten million miles away from the Sun. It is much dimmer than the rest of the Sun, and can only be seen when the Sun is blocked from view—either by a scientific instrument called a coronagraph, or naturally during a solar eclipse.

Even though it is less dense than even the best laboratory vacuums on Earth and so far away from the Sun’s core, the corona is very energetic and very hot, with its plasma reaching temperatures of millions of degrees. Astronomers are still trying to figure out how the corona gets so hot. Current research suggests that the strong electrical currents and magnetic fields in and around the Sun transfer tremendous amounts of energy to the corona, either generally or by special “hotspots” that form for short periods of time and then disappear again.