Astronomy Packet: 8
For a very long time scientists thought that the Milky Way Galaxy, the one we live in, was the only galaxy in the Universe. They thought that our galaxy was the entire Universe. Today we know that outside of our own galaxy there are billions and billions of other galaxies just like it.
Because galaxies are so large and the distances between them so unimaginably huge, the galaxies of our Universe do not all orbit around a central object. There is no object large enough to have enough gravity to hold them all. However, gravity still plays an important role in how galaxies interact with each other.
Many galaxies, including our own, have what are called satellite galaxies. These are smaller galaxies which orbit them in the same way the Moon orbits the Earth.
Gravity also causes galaxies to gather close to each other in clumps. Near our own galaxy there are around 20 other galaxies which all are held together by gravity. This is called the Local Group. They do not orbit one another in a circle, but gravity keeps them from drifting apart.
Gravity also causes galaxies and galaxy groups to drift towards one another. Our local group of galaxies is slowly drifting towards a very large group of thousands of galaxies called the Virgo Cluster.
The Structure of the Universe:
If you could see the entire Universe at one time you would notice that all the billions of galaxies are not spread evenly through space. There are areas which have many thousands or millions of galaxies, and there are large areas where there are none.
The areas with a lot of galaxies are arranged like long strings. These strings of galaxies stretch in different directions making what looks like a giant fishing net. In other words, the Universe looks like a giant net. This effect is not caused by gravity. Instead, it was probably caused as the Universe expanded super fast in the first few seconds after the big bang.
Assignment # 2:
Describe in your own words what the Universe would look like if you could see the whole thing at one time.