Formation of Galaxies
A galaxy is a huge group of single stars, star systems, star clusters, dust and gas bound together by gravity.Some galaxies even contain planet in them. There are four types of galaxies including Elliptical, Spiral, Barred Spiral, and Irregular. Each galaxy has special characteristics that classify them into each of these categories.
Elliptical
Elliptical look like flat, rounded balls. On the inside, there are billions of starts. Although, there is little dust and gas making it no longer possible to form stars. So, as a result of this, all of the starts currently in these types of galaxies are old.
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Spiral
Spiral galaxies look like bulge in the middle with arms that spread out similar to pinwheels. Inside, there are very bright and young stars. There's a normal amount of gas and dust inside so new stars are able to form in the arms of the galaxy. Very few stars form in the middle of the galaxy.
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Barred Spiral
Barred Spiral galaxies have long arms with a huge bar shaped region of stars and gas that pass through the middle of the galaxy.
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Irregular
Irregular galaxies don't have regular shapes. They can be almost any shape such as the picture below. They are usually smaller than the others and have bright, young stars. There is lots of dust and gas inside these galaxies.
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Galaxies consist of gas, dust, dark matter and hydrogen. Scientists believe that the tiny, hot and dense particle exploded, cooled and expanded. From there, the hydrogen fell into the middle of the galaxy and the dark matter remained around it.