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What is the critical mass of a star required to initiate nuclear fusion in its core and sustain its energy output through the hydrostatic equilibrium? Calculate the critical mass for a star composed of pure hydrogen and compare it with a star composed of 75% hydrogen and 25% helium. Discuss the implications of this difference on stellar evolution.
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Deep Thought is thinking for 7.5 million years.

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The answer is 42.

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