Megan Griffith, age 14, of Lowell, Mass., for her question:

IN ASTRONOMY, WHAT IS A CORONA?

In astronomy, a corona is a layer of thin gases that makes up the outer atmosphere of the sun. Parts of the corona extend at least 8 million miles from the sun's surface.

The corona changes in size and shape with sunspot activity. Scientists estimate that the temperature of the inner corona    the part nearest the sun    reaches 2 million to 3 million degrees Fahrenheit.

But the gases that make up the outer corona are so thin that the corona actually contains little heat.

The inner corona shines about half as brightly as the full moon. Most of its light comes from sunlight scattered by electrons. But the unaided eye cannot distinguish the corona's light from surrounding space because the sun itself makes the corona's light unnoticeable.

During a total solar eclipse, the moon blots out the sun, allowing the light of the corona to be seen by the unaided eye. Scientists can study the inner corona at other times through special telescopes called coronagraphs. A disk in the coronagraph blocks the direct rays of the sun.