Copyright © 2007 Dorling Kindersley
A galaxy is a vast collection of stars, gas, and dust spinning in space and held together by gravity. All the stars in the sky belong to our own galaxy, the MILKY WAY.
The Universe contains a hundred billion galaxies, and there are four main types. Spiral galaxies have a central bulge of stars, with other stars in a pattern of curved arms. Barred spirals have arms coming from a bar through their center. Ellipticals are round or oval, with no spiral arms. An irregular is a galaxy with no special shape. Galaxies in a GALAXY CLUSTER are mostly spirals and ellipticals.
A few galaxies, called active galaxies, create huge amounts of energy. At their center, they have a massive black hole that generates a trillion times more power than our Sun and spits out jets of electrically charged particles. Quasars and radio galaxies are both types of active galaxies.
Our home in the Universe is the Milky Way Galaxy. It is a spiral galaxy that contains our Sun and 200 billion other stars, among vast clouds of dust and gas. The Milky Way measures about 100,000 light-years across.
Earth sits out near the end of one of the Milky Way’s spiral arms, so we have an excellent view of the rest of our galaxy. From Earth, the Milky Way appears as a pale band of light across the night sky. It is a flat spiral, and we see it from the side, so it seems long and thin to us. The dark rifts in the Milky Way are huge dust clouds that hide the stars behind them.
A galaxy cluster is a large number of galaxies that are grouped together in space. The Virgo Cluster, for example, contains at least 2,000 galaxies.