Galaxies

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What is a galaxy?

Galaxies are huge collections of stars, dust and gas. They usually contain several million to over a trillion stars and can range in size from a few thousand to several hundred thousand light years across. There are hundreds of billions of galaxies in the Universe.
What is a spiral galaxy?
Spiral galaxies get their name from the shape of their disks, in which stars, gas and dust are concentrated in spiral arms that extend outward from the central nucleus of the galaxies. They are divided into three main types according to how tightly wound the spiral arms are: Sa, Sb and Sc. Sa galaxies have very tightly wound arms around a larger central nucleus. Sc galaxies have very loosely wound arms around a smaller nucleus. Sb's are between, having moderately wound arms around an average sized nucleus. Our Milky Way galaxy is an example of a spiral galaxy. Spiral galaxies are rich in gas and dust and have a high rate of star formation. Since spirals contain a high fraction of hot, young stars, they are often among the brightest galaxies in the universe. About 20% of all galaxies are spirals.
What is an elliptical galaxy?
Elliptical galaxies are elliptical in shape and are divided into eight subgroups: E0-E7 depending on their elongation. E0 ellipticals are nearly circular, while E7s are highly elongated. Elliptical galaxies contain primarily old stars, and do not have much gas and dust. There is very little new star formation in these galaxies. About 60% of all galaxies are ellipticals.
What is an irregular galaxy?
Irregular galaxies have no particular shape. They are among the smallest galaxies and they contain a vast amount of gas and dust. As a result they have a very high rate of star formation. The Large and Small Magellanic Clouds are examples of irregular galaxies. About 20% of all galaxies are irregulars.
What is the Milky Way?
The Milky Way is the galaxy in which we live. It is a spiral shaped galaxy that contains about 200 billion stars, including our Sun. It is about 100,000 light-years in diameter and about 10,000 light-years thick.
How long would it take a spacecraft to reach the galaxy nearest the Milky Way?
How long it would take a spacecraft to travel to the nearest galaxy depends on how fast it is traveling, however the general answer is a very very long time. The nearest galaxies to us are the two irregular galaxies called the Large Magellanic Cloud and the Small Magellanic Cloud. The nearest large galaxy is the spiral galaxy Andromeda. Here are their distances from us:
 
Large Magellanic Cloud - 179 thousand light-years away
Small Magellanic Cloud - 210 thousand light-years away
Andromeda Galaxy - 2.9 million light-years away
 
One light year is the distance that light travels in one year in a vacuum at a speed of 186,000 miles per second or about 5,880,000,000,000 miles! So, you can see that even the nearest galaxies are incredibly far away from us.
How many stars are in our galaxy?
Our galaxy contains about 200 billion stars.
How many galaxies are in the Universe?
Astronomers think that there are at least several billion galaxies in the Universe. The exact number is not known. This is based on observations of how many galaxies we see in a small part of the sky scaled to give us an estimation of how many galaxies there would be in the whole sky.

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