So far we have seen that everything in the Universe is structured and organized, from the moons which orbit the planets, to the Solar System itself where everything orbits the Sun. The force responsible for this organization is gravity.
The power of gravity causes matter to attract, pulling together into stars, worlds, and planetary systems. Once we leave the Solar System, does this complex organization end? No, it does not. This same gravity that makes the Solar System’s worlds orbit the Sun, causes billions of stars to clump together orbiting in giant circles around a common center. These collections of stars and gas are called galaxies.
Think of a galaxy as a giant Solar System. Instead of planets circling a sun, there are stars and dust clouds circling the center. In the Solar System the Sun’s strong gravity holds onto the planets. It can do this because it is much bigger than the planets are. What do you think might be in the center of a galaxy that could be big enough to hold onto all the stars?
The answer is a very complicated one. At the center of most galaxies we find two important things that hold onto the galaxies’ stars. Firstly, there are many, many stars packed in tightly. If you look at a picture of a galaxy, often the center glows very brightly. This is caused by the glow of so many stars so close together. Secondly, most galaxies also have what is called a supermassive black hole in their centers. These black holes can contain as much matter as a billion stars, but all of that matter is squished up into a space smaller than a pinhead.
Assignment # 1:
In your own words, describe to me what you think a galaxy is.
Check your answer
A galaxy is a group of stars, gas, and dust, held together by gravity.