LYNETTE COOK / SCIENCE PHOTO LIBRARY LYNETTE COOK / SCIENCE PHOTO LIBRARY
Spiral galaxy. Artist's impression of a large, spiral galaxy with small, spherical groups of stars (globular clusters) nearby. Galax- ies are massive collections of stars, gas and dust Spiral galaxies are flattened, disk-like systems composed of a central core surrounded by spiral arms which circularly orbit around the nucleus. The core generally has a much higher density of stars than the arms; the core's stars also tend to be older than those in the arms. Globular clusters follow highly elliptical orbits around the centre of their galaxy. These orbits often do not lie in the plane of the galaxy.
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