In the previous chapter we discussed our home in the Universe: the Milky Way Galaxy. But ours is not the only galaxy. Here we discuss all the others. Some are like our own: others are completely different.

The number of galaxies defies the imagination. Immense, faintly glowing, slowly rotating, they drift about, influenced by their mutual gravitational attraction. Occasionally they actually encounter one another. You might think of them as resembling snowflakes in a blizzard.

Our own Galaxy is disk-shaped, with a central bar and winding spiral arms. Other galaxies lack the central bar; yet others are more nearly elliptical and also lack the spiral arms. Yet still others are completely irregular in shape. Some have lots of interstellar gas; some hardly any.

And some are the seat of violent, hugely powerful explosions. These objects emit vast quantities of energy as optical light, radio waves and other forms of radiation. These emissions probably arise from matter falling into giant black holes lying at their cores.

Galaxies lie all about us: indeed, some are so close they can be seen with the naked eye. But for years nobody knew what they were. We begin with the story of how astronomers came to discover their true nature.