Alvin David Williamson, age 12, of Gary, Indiana, for his question:
How fast did the largest supernova expand?
Through the ages astronomers observed sudden beacons among the stars. For a few weeks or months they outshone the brightest stars, then faded away. These spectacles were called novas, or new stars. Later, astronomers discovered that a few were outstanding. They are called supernovas and these stupendous events are very, very rare.
Telescopes keep an eye on thousands of galaxies far beyond our Milky Way. And every year they photograph catastrophic explosions in a few of these remote star systems. An event of this sort is a super¬nova, a star that explodes with enough blazing radiance to outshine the billions of stars in an entire galaxy.
It has been estimated that the average galaxy can expect one supernova explosion in about four centuries. During the past thousand years, three have been observed in our own Milky Way galaxy. And without a doubt, the biggest one occurred in the year A.D. 1054. It was recorded by Chinese astronomers as a super brilliant new star that suddenly appeared in the constellation Taurus, the Bull. They called it the Guest Star, for it faded and soon was forgotten.
In the 1700s, astronomers understood that novas and supernovas are ordinary stars that explode, shooting out shells of blazing gases at fantastic speeds. In exploring ancient records, they found the old report of the Guest Star and searched for its remains. They found a most remarkable nebula, a pale patch of glowing, hazy light.
Its filmy gases, still expanding, spread outward somewhat like crab legs. And it is agreed that this famous Crab Nebula in Taurus resulted from the Guest Star explosion of 1054 A.D. The width of the stupendous nebular is some 400 million million miles. Those exploding gases have spread that far during the past nine centuries.
Astronomers can estimate the rate at which a celestial object is approaching or receding from the earth. Filaments of the Crab Nebula have been checked and rechecked. Those approaching the earth still are expanding at faster than 600 miles a second. However, we have no need to worry about being engulfed for our safe distance from the Crab Nebula is estimated to be around 4,500 light years. Even if its gases spread at the speed of light, they could not reach us for at least 4,000 years.
We might assume that, nine centuries ago, the gases of the Crab expanded outward at even higher speeds. Observations of supernovas in other galaxies suggest that this is possible. In some cases, they expand at 1,000 miles per second. Maybe this is at or near the top speed of expanding supernovas.
Most likely a star explodes because the internal forces that hold it together are unstable. An ordinary nova shoots off its outer shell, with a small fraction of its gases. A supernova shoots off a tenth of its total gaseous material. A super supernova, with the brilliance of 10 billion suns, may lose nine tenths of its material.