Robert Ferwerda, age 12, of Milwaukee, Wisconsin, for his question:  

What is a supernova?

A supernova is the big brother of a nova and the word "nova" means a new star. The nova was named before astronomers knew that the showy object is far from young. The much flashier supernova happens to be so ancient that the star may be in the last throes of its existence.

In the year 1054 A.D., astronomers in China and Japan spotted what seemed to be a bright new star in the constellation of Taurus, the Bull. For a short time it shone as bright as brilliant Jupiter, brighter than any other star in the sky. Then it faded away. The oriental astronomers named the showy visitor the Guest Star, noted it down in their records and soon forgot it. This was, of course, long before the invention of the telescope.

Now we know that the Guest Star was a supernova a super celestial explosion, the . aftermath of which is still going on. Telescope pictures show its cloudy remains, spreading in a ring of gaseous filaments that look somewhat like a fringe of filmy claws. We call it the Crab Nebula and on a clear winter night you may be able to see it as a hazy blur. It is located in the constellation Taurus, just where the blazing Guest Star was reported more than 900 years ago.

Astronomers suspect that a supernova is the explosion of an entire star. We are not sure what causes the cosmic catastrophe, but the outer and inner shells of starry gases appear to shatter and erupt in all directions at fantastic speeds. The sudden outburst is seen clear across the Galaxy and may be brilliant enough to be seen from other galaxies across the vast reaches of space.

The titanic flash may be 200 million times brighter than our sun and in one second it pours forth more energy than the sun does in 60 years. This phase of the supernova event is short. But the star was a nuclear furnace and the masses of material in its fiery core were projected outward at fantastic speeds. The explosive force continues to toss them onward and outward,~long after the supernova flash has subsided. Astronomers estimate that, after 900 years, gases from the Crab Nebula still are expanding outward at a rate of about 70 million miles a year. It is believed that the entire star was annihilated because the central region of the nebula is now transparently thin.

There are about 100 billion stars in our Galaxy and a supernova explosion is a very rare event. We can expect one of our stars to blow itself to bits in this way about once in 400 years. No supernova has occurred in our Galaxy since the telescope was invented. However, several of these cosmic explosions have been photographed in other galaxies. Only three have been reported in our own Galaxy through thousands of years. All of them were near the densely populated center of the Milky Way.

The Guest Star that exploded 900 years ago was at a distance of 4,100 light years. Its light, traveling at 186,000 miles a second, would take 4,100 years to reach the earth.

The actual explosion, then, occurred some 5,000 earth years ago. Our telescope pictures of the Crab Nebula show the gaseous remains of the explosion as they were some 4,000 years ago. We shall not be able to take pictures of the spreading tentacles as they are today until the year 6067.