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 Richard Sullins, age 12, of Spruca Pine, North Carolina, for his question:   

 Could the Crab Nebula occur again?

The stupendous Crab Nebula was created by a star and so far as we know that same star can never perform an encore. However, somewhere in the Milky Way some other star is preparing to dazzle billions of ordinary everyday stars with another such supernova explosion. Astronomers estimate that this happens to only. one star in 100    and it happens only once in the lifetime of such a star.

Cosmology is the branch of astronomy that deals with the infinite scope of the universe, with all the dynamic factors that govern its history. At present, most of it is shrouded in mysteries, though recently several theories and new observations have come together. These nec 7 concepts have shaken some older ideas and stretched our minds to make room for larger ones. For example, Jupiter is not the sort of planet we thought it was. Bigger surprises came from outside the solar system with quassars, pulsars and strange pockets called blnclc holes.

This could be the Decade of Cosmology. If it is, the stupendous Crab Nebulla will have helped to make it possible. Actually the story began about 8,000 years ago, when a barely visible star exploded and flashed a dazzling display of expanding gases across the galaxy. Its light took 5,000 years to reach the earth. In 1054, it was recorded by Chinese astronomers as a brilliant new star in the constellation Taurus. It soon faded to what looks like a very dim star. Centuries later, telescopes revealed it to be a cloud of nebulous gases with expanding filaments like tentacles. This Crab Nebula now spans six light years, though we must allow for the fact that its image takes 5,000 years to reach us.

A few years ago radio telescopes traced the strange signals coming from pulsars. These dynamic little neutron stars had been predicted, they have intense fields of gravity called black holes. The logical place to look for a pulsar was in the heart of an old supernova. Sure enough, one was found in the Crab Nova    in the midst of an intense gravitational field. These discoveries told part of the life history of a supernova star.

Astronomers are not sure what triggers the stellar explosion. But most agree that the structure of such a star is unstable. Its mass may be 1.5 times that of our sun. Our average star has consumed almost half its hydrogen fuel in about five billion years. In the far future it is expected to die down gradually. But apparently a massive unstable star cannot grow old gradually in the average starry fashion. It reaches a critical stage and blows up. The exploding gases spread through the galaxy with energies approaching the speed of light. The pulsar in the Crab Nebula rotates at about 30 times per second, though it is gradually slowing down. Apparently the remaining core of the flashy old star is dying.

Some astronomers suggest that the hot dense core and the gravity that holds the structure together become unbalanced as the fuel is consumed. When the inner and outer forces reach a critical stage, gravity collapses the star. The core is compressed, then gaseous shells explode out into space. The Crab Nebula star lost too much energy for another such explosion. But about every 100 years, some other massive, unstable star in our galaxy blows up with a supernova explosion.

 

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