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By Martin Harwit (auth.)

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The calculations are based on nuclear reaction rates, computed for different stages of a star's life. They give the following evolutionary history. As a star contracts from an initial dust cIoud, it becomes very bright but radiates at such low temperatures that the star would be off the scale of our diagram, beyond the right edge. At this stage the star radiates only in the infrared ; but the star's life time in this state is so short that such objects have not yet been identified. Theoretical views still differ considerably, and observational data will be needed before we can be sure, but the following evolutionary sequence may be approximately correct.

If these spheres were about 10 m in diameter, cosmic ray particles produced in the sun could have caused spallation, while the hydrogen contained in the postulated ice spheres would have slowed down the neutrons released in spallation (F062). This would have permitted the slowed neutrons to react with Li 6 to form tritium, and with B 10 to form LF, thus leading to an overall enrichment of LF relative to Li 6 . , while on earth the abundance of LF is much greater than Li 6 . In modified versions of this theory, larger spheres containing less H 2 0 ice are found to give similar resuIts, and only a moderate fraction of the earth's material need have initially been bombarded by solar cosmic rays.

Cosmic ray particles mayaIso have their origin in such pulsars. This is as far as the theory of stellar evolution has come: It promises many detailed insights into the nuc\ear history of stars and of the universe, and into the major energy sources available in the universe. lt may eventually also lead to a better understanding of the formation of extremely energetic cosmic ray partic\es, and of the ultimate fate of stellar matter in the uni verse. Studies of stellar evolution then are much more than just a conjecture about the life and death of stars; they may ultimately provide the detailed physical insights necessary for understanding the more important energy generating processes on a cosmic scale.

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