Most of the planetary nebulae scattered across the sky are great shells of luminous gas. At the core is a star about 100 times as big as the sun. Last week Dr. Fred Lawrence Whipple of Harvard Observatory reported that he had calculated the average lifetime of planetary nebulae at 30,000 years—in astronomical time, a split second.
Reason why planetaries are so shortlived: they are constantly and rapidly expanding, constantly dissipating. Some, thousands of light-years distant, grow and die so fast that before earthly astronomers see them, they have been dead and gone for...
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