On Monday, millions of people will see something they’ve never seen before: the Sun’s corona — its hot outer atmosphere. It extends hundreds of thousands of miles from the surface of the Sun. But it’s so faint that it’s overpowered by the Sun’s brilliant disk. The only time we can see it is during a total solar eclipse.

The corona is made of super-heated gas known as plasma. In fact, the corona is heated to a couple of million degrees Fahrenheit, and sometimes hotter. By comparison, the Sun’s surface is only about 10,000 degrees.

Scientists still aren’t sure just why the corona is so hot. One idea that’s been gaining steam, so to speak, is that the Sun produces millions of “microflares” — small eruptions from its surface. They blast plasma into space.

The plasma thins out as it moves away from the Sun. Eventually, it forms the solar wind — a flow of charged particles that races through the solar system at a million miles per hour or faster.

During a solar eclipse, the corona looks like a silvery halo around the intervening Moon. It may be especially intense during this eclipse because the Sun is nearing the peak of its 11-year magnetic cycle.

One note of caution: It’s safe to look at the Sun when it’s fully eclipsed. At all other times, though, it’s too bright to view without proper eye protection. So stay safe as you view the corona — part of the grandeur of a solar eclipse.

More about the eclipse tomorrow.