Anne Jacobson, age 13, of Waterford, Wis., for her question:
WHAT IS A NOVA?
In what can be described as a typical nova explosion, the star itself loses only about a hundred thousandth part of its matter. What is thrown off is a shell of glowing gases that expand outward into space at a tremendous speed.
People once believed that a nova was a newly created star. This isn't the case. A nova is a star that suddenly explodes and throws part of its own matter out into space. As the nova explodes, there would be a growing brightness followed by a fading and a return to its original brightness.
All novae that's the plural of a nova occur on distant stars and can only be observed through powerful telescopes. The word nova comes from the Latin word for new.
Some novae develop so quickly that they reach their greatest brightness within a few hours or a day. Fading generally starts almost immediately and the original brightness is back within several months. Some, however, may take a month to reach their full brightness and several years to fade.
About two novae each year are observed by astronomers in our Milky Way galaxy, but it is more likely that between 25 and 40 occur. Most go unnoticed since they happen on stars so far away from earth. Novae in one of the Milky Way's neighboring galaxies, astronomers believe, occur at about the same rate as novae in the Milky Way.
A star that explodes much more violently than a nova and shines more brightly for a longer period of time is called a supernova. As much as 10 percent of a star's matter can be thrown off during a supernova explosion. Brightness may increase as much as a billion times within a few days. Astronomers believe that perhaps 14 supernova explosions have happened in the Milky Way during the past 2,000 years. The Crab nebula is the remains of a supernova seen in 1054 A.D.
Some novae have flared up more than once since they were first observed by astronomers. These are called recurrent novae and are believed to explode regularly. Most flare up so rarely, however, that recurrence has not been noticed.
Astronomers do not know what causes novae. This isn't anything for any of us to worry about, but some astronomers believe that our own sun may some day be involved in a nova explosion. But this won't happen for at least 5 billion years from now. If it does, the outer regions of the sun may expand about 40 million miles to about the distance of Mercury. The earth's temperature will then become too high for life to exist, but probably by that time the entire population of the earth will have moved by rocket ships to a distant, safe planet.