LYNETTE COOK / SCIENCE PHOTO LIBRARY LYNETTE COOK / SCIENCE PHOTO LIBRARY
Spiral galaxies. Artist's impression of a barred spiral galaxy (at centre) and a nearby regular spiral galaxy. Galaxies are massive collections of stars, gas and dust. Spiral galaxies are flattened, disk-like systems which are composed of a central core surrounded by spiral arms rotate around the nucleus. In a regular spiral galaxy, the arms emerge directly from the side of the core whilst in a barred spiral they begin at the ends of a bright, central bar which passes through the core. 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.
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