A galaxy is a large group of stars, gas, and dust bound together by gravity. Our solar system resides in the Milky Way galaxy, a spiral galaxy that is part of a group of galaxies called the Local Group. There are billions of galaxies in the Universe, but only three outside our Milky Way Galaxy can be seen without a telescope - the Large and Small Magellanic Clouds and the Andromeda galaxy. The Large and Small Magellanic Clouds are about 160,000 light years away and are satellites of the Milky Way. They can be seen from the southern hemisphere. The Andromeda Galaxy is a larger galaxy that is about 2.5 million light years away and can be seen from the northern hemisphere with good eyesight and a very dark sky. The other galaxies are even further away from us and can only be seen through telescopes.
The smallest galaxies may contain only a few hundred thousand stars and be several thousand light years across, while the largest galaxies have trillions of stars and may be hundreds of thousands of light years across. Galaxies can be found by themselves, in small groups and in large clusters.
Galaxies sometimes collide with each other.These collisions can trigger bursts of star-formation in addition to changing the shapes of the galaxies that collide. However, when galaxy collisions occur, individual stars do not collide, due to the vast distances between them.
Galaxies are classified by shape. There are three general types: elliptical, spiral, and irregular.
Beautiful galaxies by Hubble
Credit: NASA, ESA
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