Junior Astronomer - Galaxies
A galaxy is a collection of stars, gas and dust. Most galaxies contain billions of (over 1,000,000,000) stars similar our Sun! Some of the stars are much smaller than our Sun, and we call then red dwarfs because they are a red colour. There are also much bigger stars than our Sun, which we call giant stars. These are very bright and often blue coloured! Our galaxy is one of over 170 billion (170,000,000,000) galaxies in the Universe, each containing billions of stars! There are lots of different types of galaxy. We label galaxies according to shape; our galaxy is disc shaped (a bit like a pancake) and it has spiral arms, so we call it a spiral galaxy. There are also galaxies that are circular, like a football or rugby ball shape, which we call elliptical galaxies. Some galaxies are unique and can't be classified normally - we call these irregular galaxies. Astronomers aren’t very good at thinking of imaginative names! The galaxy below on the left is called the Andromeda Galaxy! It is a nearby spiral galaxy that we think looks like our galaxy. If Andromeda wasn’t so dim it would appear much larger than the Moon in our night sky.
The Andromeda galaxy is over 2 million light-years away from us. That means that the light from a star in the Andromeda galaxy takes 2 million years to get to us! Andromeda is moving towards us at 250,000 mph and in around 2 billion years our galaxy will collide with it! However as the stars are so far apart it is unlikely that any two stars will actually collide. Most likely both galaxies will combine and form one massive elliptical galaxy. The galaxy below on the right is the Whirlpool Galaxy and consists of the remnant of two galaxies than have collided!
When we compared how fast the stars in a galaxy move to how heavy we think galaxies are we realised we had a big problem! Galaxies need to be a lot heavier than just all the matter that is in the stars, gas and dust. We call this unknown matter dark matter because we can't see it using light. Scientists have some theories as to what dark matter is, but it is still one of the biggest questions in astronomy!
Most galaxies are grouped into large groups or clusters which can often contain hundreds of galaxies in a relativily small space. A cluster is a dense region of galaxies, with lots of collisions and interactions between galaxies. A group is less dense, but stil contains a lot of galaxies. Groups normally contain a few large spiral galaxies, and lots of small irregular. Our galaxy, the Milky Way, is in a galaxy group unimaginatively called The Local Group.
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