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Given a scientific passage and an answer, generate a question for the given answer.

Passage: Stars shine because of nuclear fusion. Fusion reactions in the Sun's core keep our nearest star burning. Stars are made mostly of hydrogen and helium. Both are very lightweight gases. A star contains so much hydrogen and helium that the weight of these gases is enormous. The pressure at the center of a star is great enough to heat the gases. This causes nuclear fusion reactions. Answer: nuclear fusion
Output:
What process, which results because of great pressure at the center of a star, causes stars to shine?