Scientists used to think that all the stars in the universe were part of one giant grouping of stars. Then, in 1917, Thomas Wright suggested that there might be lots of different large groups of stars. A few years later this was proven by other astronomers and the idea of the galaxy became real.
What is a Galaxy?
A galaxy is a group of stars and other space stuff. The stars tend to spin around a center of high gravity, sort of like the planets spin around the Sun in the Solar System. Galaxies are huge and can have trillions (way bigger than billions!) of stars.
As big as galaxies are, they are generally separated by large areas of empty space. There are even clusters of galaxies that are separated by even larger areas of space. Scientists think there are over 100 billion galaxies. Wow, the universe is huge!
Milky Way
We live in the galaxy called the Milky Way. The Milky Way is part of cluster of around 3,000 galaxies called the Local Group. The Milky Way is a spiral shaped galaxy and is estimated to be made up of around 300 billion stars.
Drawing of the Milky Way galaxy. Source: NASA
Types of Galaxies
There are four main types of galaxies depending on their shape:
Spiral - The spiral galaxy has a number of long arms that are spiraling around the center. In the center of the spiral galaxy are older stars while the arms are generally made of new stars.
Barred spiral - This type of galaxy is similar to the spiral but has a long bar in the middle with spirals coming off the ends.
Elliptical - A mass of stars clumped together in the shape of an elliptical disc.
Irregular - Any other shaped galaxy is generally lumped into the category of irregular. It is thought that most irregular galaxies are formed by two of the other three types of galaxies crashing into each other.
Barred spiral galaxy NGC 1300. Source: NASA, ESA, and The Hubble Heritage Team
Fun Facts about Galaxies
The word galaxy comes from the Greek word for "milky".
Some scientists think that most of the mass of a galaxy is made up of a mysterious substance called dark matter.
It is thought that there is a massive black hole in the center of galaxies.
The closest galaxy to the Milky Way is Andromeda, which is around 2.6 million light years away from us.
Many galaxies are more than 100,000 light years across in distance.
It takes over two hundred million years for the sun to orbit the center of the galaxy. This is called a galactic year.
