When we look up at the night sky, we notice that there are many stars in our sky. Stars must like to live together in star cities - galaxies. Our city of stars is called the Milky Way , and it is home to 100 billion stars, including the Sun. The stars stay together because of the force of gravity.
Galaxies come in different shapes and sizes.
Some galaxies don't seem to fit into these groups, so they are called irregulars . During the early 1900's, which is not very long ago, astronomers were unaware that there were other galaxies outside our own Milky Way Galaxy . When they saw a small fuzzy patch in the sky through their telescopes, they called it a nebula . When examined closely, some of the nebulae had a spiral shape. These nebulae were all believed to be part of our Galaxy, our community of stars.