MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Image showing the stages in the life and death of a star system, and its cyclical nature. At right we see a star exploding at the end of its life, a supernova. As the debris cloud expands it forms a diffuse nebula and joins the other gases in the interstellar medium. Later, due to turbulence and random motions in the interstellar medium, dense knots appear -- regions that are denser than others (bottom left). Regions such as these undergo gravitational contraction and lead to the creation of protoplantetary discs and then planetary systems (top). When the star expires, the whole cycle starts all over again.
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