An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time

THE SUN

The Sun

The Sun. Our star, the Sun, is a powerhouse of nuclear energy, shining via a process called nuclear fusion. Deep in its core, smaller atomic nuclei are smashed together into larger ones, releasing energy each step of the way. This image, taken in 1973, from Skylab, the early US space station, shows an enormous eruption (flare) on the Sun's surface. (Courtesy of NASA)

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