Kris Berglund, age 13, of Rockford, Ill., for her question:

What is the sun's corona?

The rosy gold face of the sun is a surface layer of gases called the chromosphere. Above the chromosphere is a filmy halo of gases called the corona, reaching no one knows how far out into space. The ghostly corona is outdazzled by the brilliant face of the sun, and in the past it could be studied only when the vivid surface of the sun was blotted out during a total eclipse. Now science has an instrument with which the solar corona can be studied at all times.

The sun's corona was studied extensively during the IGY when several surprises were discovered. The temperature of the pale halo of yellow and gray is between one and two million degrees. No one knows why it should be so much hotter than the surface of the sun. No one knows, either, where the corona ends or how far its gases extend out into space. It is made of the gaseous plasma that fills the spaces of the solar system. Getting thinner as it goes, the corona may well extend beyond the orbit of the Earth.