Vernon Coleman, age 10, of Chattanooga, Tenn., for his question:
WHAT IS A NOVA?
A nova is a star that suddenly explodes and blasts part of its matter into space. Astronomers do not know what causes novae. The word nova comes from the Latin word for new since people at one time believed that novae were newly created stars.
An exploding nova quickly becomes much brighter than before. It remains bright for a time and then fades slowly. All novae observed so far have been distant stars too faint to be seen without a telescope before they exploded.
In a typical nova explosion, the star loses only about a hundred thousandth part of its matter. The matter it throws off is a shell of glowing gases that expands outward into space at a tremendous speed.
Some novae develop so rapidly that they reach their greatest brightness in a few hours or a day. They start to fade gradually almost at once and return to their original brightness in several months or a year.
Slower novae may take a month to reach maximum brightness. The fading process of these novae may last many years.
Several novae, called recurrent novae, have flared up more than once since they were first observed. Astronomers believe that all novae may be recurrent, but that most flare up so rarely that recurrence has not been noticed.
Astronomers observe about two novae a year in the Milky Way galaxy but they believe that between 25 and 40 probably occur. Most novae go unnoticed because they are so far from the earth. Novae in one of the Milky Way's neighboring galaxies occur at about the same rate as novae in the Milky Way.
A supernova is a star that explodes much more violently and shines much more brightly than a nova. A supernova throws off as much as 10 percent of its matter when it explodes.
Supernovae and novae differ so much in the percentage of matter thrown off that scientists belive the two probably develop differently.
A supernova may increase in brightness as much as a billion times in a few days. Astronomers believe that about 14 supernova explosions have taken place in the Milky Way during the past 2,000 years.
The Crab Nebula, a huge cloud of dust and gas in the Milky Way, is the remains of a supernova seen in AD 1054. Supernovae are also rare in other galaxies.
The sun is a huge, glowing ball of gases at the center of the solar system. The sun is only one of billions of stars in the universe.
Will our sun become a nova someday? Perhaps, say the scientists. But it won't be for a long, long time. Our sun, they estimate, is now about 4.6 billion years old. It will continue to shine for at least another 5 billion years.