JON LOMBERG / SCIENCE PHOTO LIBRARY JON LOMBERG / SCIENCE PHOTO LIBRARY
Spiral galaxy structure, illustration. A spiral galaxy is a gravitationally-bound group of stars that has two distinct regions: a central nucleus & a disk that contains the spiral arms surrounding the nucleus. The nucleus is an elliptical ball of older (population II) stars. It is often yellow in colour because the blue population II stars are no longer shining. In contrast, the spiral arms of the galactic disk contain young (open) star clusters (population I) of hot, blue stars. The individual stars in the galactic disk move in and out of the spiral arms, which are density waves of stars, bunching up as they orbit in the plane of the galactic disc.
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