Stars like our Sun can burn brightly for billions of years, generating heat in their cores by fusing hydrogen into helium. When a medium-size star—up to eight solar masses—finally runs out of fuel, nuclear burning no longer drives an outward pressure gradient, and the star starts to collapse due to its own gravity. The contracting star generates a final burst of heat by fusing helium into carbon and oxygen, which causes the outer layers to expand outward.

The star turns into a red giant. In doing so, it engulfs any nearby planets but cannot sustain heat production. Within a billion years, all that remains of the star is the dense (109 g/cm3) stellar core composed of carbon and oxygen. The hot, Earth-sized ember, now called a white dwarf, glows faintly as it cools over tens of billions of years. (See Physics Today, March 2019, page 14...

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