A galaxy is a collection of stars that is bound together by gravity. Galaxies are categorized based on their shape into three main groups: elliptical, spiral, or irregular.
The term galaxy is derived from the Greek word for milky, galaxias, in reference to our own galaxy, the Milky Way. In the observable universe, there could be as many as 100 billion galaxies. The closest galaxy to the Milky Way is the Andromeda Galaxy, which is 2.5 million light-years away.
Galaxies are categorized based on their shape into three main groups: elliptical, spiral, or irregular. This classification scheme was invented and developed by the astrophysicist Edwin Hubble. Elliptical galaxies are galaxies that are elliptical in shape. Most galaxies are elongated, but some are more circular shapes. The largest galaxies ever observed are elliptical galaxies. Spiral galaxies are characterized by a central bulge surrounded by spiral arms. In a spiral galaxy, the older stars are towards the center of the galaxy, whereas the newer stars are in the spiral arms. An example of a spiral galaxy is our own, the Milky Way. Two thirds of observed galaxies are spiral shaped. Irregular galaxies don’t fit into the elliptical and spiral galaxy categories. This irregular shape can occur due to the gravitational effects of other nearby galaxies.
The realization that our we exist in a galaxy among many, many others, came over time. Many philosophers and scientists had put forward an idea that the band of light we see in the night sky is caused by lots of other stars. Proof of this came from Galileo Galilei. Using a telescope, he could see that the Milky Way band composed of many faint stars. Our solar system is only one small part of a much larger galaxy.
The first documented description of galaxies outside the Milky Way was the observation of clouds which were categorized as nebulae. Astronomers thought that these blurry objects were part of the Milky Way galaxy. While working at the Mount Wilson Observatory in California, Edwin Hubble made observations on the distance of stars and other celestial bodies. He calculated that the objects were too far away to be part of the Milky Way. Until this realization, it was commonly accepted that the universe only consisted of the Milky Way.
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Use these encyclopedias as a springboard for individual and class-wide projects!