MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Illustration of a red giant star shedding its outer layers. Red giants are very evolved stars, that have depleted their stores of hydrogen. This leads to the core collapsing under gravity, which makes it hotter. This fresh heat pushes on the star's outermost layers and gradually the star loses mass as its atmosphere escapes, bit by bit, into space. This is how planetary nebulae are formed. In this picture, a red giant has already expelled much of its atmosphere in a series of shells.
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