Monday, Jan. 03, 1949

Nature's Atom Bombs

Many stars are variable. The "novae" flash into sudden brilliance and then fade back to dimness. Others wax & wane regularly every few days. In a letter to Britain's Nature magazine, D. Stanley-Jones suggests that both types of uneasy stars may be natural versions of the man-made atomic bomb.

An atomic bomb has two "subcritical masses" of plutonium or uranium 235. When the two masses are far away from each other, nothing happens. The neutrons generated spontaneously within each piece escape so rapidly through the surface that a chain reaction cannot get started. But when the two pieces are brought close together, the neutrons stay in the uranium longer. By splitting uranium atoms and thus releasing more neutrons, they start a nuclear chain reaction which does not stop until the explosion has scattered the fissionable material.

Variable stars, says Stanley-Jones, may work the same way. When the star is in ai normal quiescent condition, its heavy fissionable atoms are too far apart for a chain reaction to get started. Being heavy, they sink gradually toward the center of the star. As they sink, they approach one another. When they get close enough, a chain reaction is set off. Its heat and radiation make the star expand until the fissionable materials in it are too far apart to react any more.

In some cases, says Stanley-Jones, the explosion is powerful enough to disrupt the star, blowing a vast halo of luminous material away from its surface. Such stars would be nonrepeating novae. A milder explosion would merely cause a slight expansion and more brightness. After it is over, perhaps the remaining fissionable material falls back toward the star's center and causes, in due course, another moderate explosion. Such stars, exploding gently at regular intervals, Stanley-Jones says, would behave like the "pulsating variables."

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