DAVID A. HARDY / SCIENCE PHOTO LIBRARY DAVID A. HARDY / SCIENCE PHOTO LIBRARY
Magnetic spiral arm theory. Artwork of the theory of spiral galaxy formation which states that the arms are held in place by loops of a magnetic field (purple) from the nucleus. Matter is passed away from the nucleus by the field, as shown by the arrows. The nucleus of a spiral galaxy (upper left) typically consists of old stars, appearing yellower than the arms, which mainly contain young blue stars. The idea that the arms are held by magnetic fields has now largely been displaced by the "rotating disc" theory, in which stars are falling into towards the heavier nucleus, but revolving around it at the same time.
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