tures through them. These galaxies are classified as SB (fig. 3).

Globular Clusters

These are not really galaxies in their own right, however, they are associated with galaxies. Look at fig 4. If you think that this globular cluster looks like a very round elliptical galaxy, you would be right! Elliptical galaxies and globular clusters have a lot in common. There is no gas or dust in a globular cluster, and the stars are old. In fact, globular clusters may be the some of the oldest objects in the universe.
The big difference is size! Globular clusters contain hundreds of thousands or millions of stars. The stars at the centre of a globular cluster are very densely packed. A planet orbiting one of the central stars would be bathed in very high radiation levels and probably would not experience night since there would be so many stars in the sky. Elliptical galaxies can contain hundreds of billions of stars! And while elliptical galaxies are sometimes round, globular clusters are never elliptical.
Globular clusters are found in 'halos' around galaxies. The Milky Way, for example, has over one hundred globular clusters throughout it's halo (fig. 5).

The Earth's place in the Universe

Our Sun lies at the centre of our Solar System. Its gravity holds the planets in place and provides the heat and light essential for life on Earth. The Solar System lies in an unremarkably sized galaxy which we call either 'The Galaxy' or 'The Milky Way'. It is located about 1/3rd of the way in from the edge of The Galaxy, in one of the spiral arms. Each spiral arm is given the name of a prominent constellation within the arm or line of sight of the arm (fig. 6). This is part of a local cluster of galaxies of different types (which includes the Magellanic clouds and M31). The galaxies in the local cluster are separated by distances much larger than the distances between the stars but these distances are miniscule compared with the distances between adjacent clusters of galaxies. Our cluster is just one of billions that make up the universe as a whole.

Fig. 4: M13, one of the most spectacular globular clusters in the halo of our own Galaxy.

Fig. 5: A diagram showing the position of the Sun and the halo of globular clusters round our Galaxy.

Fig. 6: A diagram showing our Galaxy from above.

Graphics and Text (modified by KW) courtesy of  the "windows on the universe" website.