The corona is the outermost layer of the Sun's atmosphere, a vast region of extremely hot, glowing gas that extends millions of miles into space. Even though the corona is much hotter than the Sun's surface (millions of degrees compared to thousands), it's also much less dense, so you can't normally see it. The bright light from the Sun's surface overwhelms it.

You can only see the corona during a total solar eclipse, when the Moon blocks the Sun's bright face. Then the corona appears as a shimmering white halo around the dark disk of the Moon, with delicate streamers and loops of glowing gas extending outward.

The corona is important because it's where the solar wind begins: streams of charged particles that flow out from the Sun and travel through the entire solar system. When these particles reach Earth, they can create the beautiful auroras near the poles and occasionally disrupt satellites and power grids.

Scientists still study the corona to understand why it's so much hotter than the Sun's surface, which seems to defy common sense. It's like the air around a campfire being hotter than the burning wood.