Adam Ogden, age 16, of Billings, Mont., for his question:

HOW LARGE IS THE LARGEST STAR?

There are about 200 billion billion stars in the known universe. That's the number 2 followed by 20 zeros. Most stars are between 1 million and 10 billion years old, with some of them 20 billion years old. The largest stars have a diameter of about 1 billion miles, which is about 1,000 times that of our sun.

The largest known stars, called supergiants, include Antares and Betelgeuse. Antares has a diameter 330 times that of the sun while Betelgeuse actually expands and shrinks. Its diameter varies from 350 to 450 times that of the sun.

Astronomers divide stars into four groups by size, but the sizes in each group vary greatly. Supergiants, the largest, are from 100 to 1,000 times that of the sun. Giant stars are from 10 to 100 times larger.

Most of the stars are about the same size as the sun. They may be thought of as being medium size. A few called dwarfs, the smallest stars, are smaller than the earth.