Kaler, James B. Department of Astronomy, College of Liberal Arts and Sciences, University of Illinois, Urbana-Champaign, Illinois.
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An intermediate state in the evolution of a star in which it swells to enormous proportions before its death. During the longest and most stable phase of a star's life, the star, like the Sun, derives its energy from the thermonuclear fusion of hydrogen into helium deep in its dense, hot (107 K and up) core. It is then said to be on the main sequence. When the hydrogen fuel is gone and the central energy source is thereby exhausted, the core contracts and heats under the action of gravity, fresh hydrogen is ignited in a shell that surrounds the spent core, and the star becomes much more luminous, larger, and cooler at its surface. The lower surface temperature produces a redder color, hence the common term red giant. Stars like the Sun brighten by a factor of 1000 and grow in radius by a factor of over 100 to more than half the size of Earth's orbit (7.5 × 107 km or 4.7 × 107 mi).
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