The corona is the hot extended atmosphere of a star. In the case of the Sun, it extends millions of miles (km) into space, and is heated to more than a million degrees, compared to about 10,000 degrees Fahrenheit (5,800 Centigrade) for the surface. It is heated by magnetic activity at or near the Sun's surface, although astronomers are still trying to determine the exact mechanism. Although it is hot, the corona is so thin and faint that it's visible from Earth's surface only during a total lunar eclipse. Astronomers study it with special instruments that block out the disk of the Sun, or with space telescopes. Other stars generate their own coronas.