What is a Galaxy?

Earth is our home planet. It orbits the Sun together with the other eight planets, comets, asteroids and all of the other stuff that makes up the Solar System. But where is the Solar System located? It turns out that the Solar System is part of a much larger structure, called a galaxy. The Sun is a star, just like the others you see when you look up on a clear night. They are very far away from us and therefore look dim compared to the Sun. All of these Stars are all part of the same galaxy. That galaxy is called the Milky Way. A galaxy is a very large group of stars, dust, gas and dark mater that is gravitationaly bound together. A galaxy can range in size from a few hundred million stars up to large galaxyies containing 100 trillion stars.

Andromeda galaxy, also called M31. Credit: NASA

Galaxies fit into 3 main categories; spiral, elliptical and irregular. A spiral galaxy is shaped like a giant pinwheel and is what we normally think of when we think of a galaxy. Our own galaxy, the Milky Way and the Andromeda galaxy, which is pictured above, are both examples of spiral galaxies. It is thought that the spiral arms are caused by a density wave within the galaxy. All of the stars orbit around the center of the galaxy and the speed with which they rotate will change as the move through the density wave. The best way to imagine this is traffic on a busy road. The effect of this density wave and the change in speed is the spiral arms. The spiral arms can make beautiful structures. There are plenty of variety in spiral galaxies and no two are alike. Some of them have very tight spirals, while some have much more open spirals. Spiral galaxies can also have different numbers of arms. A large number of spiral galaxies have a feature at the centre called a bar and are therefore called barred spiral galaxies. It is thought that these bars are temporary features. In the image below of Galaxy NCG 1300 you can see the straight bar in the center surrounded by two spiral arms.

NCG 1300 – A barred spiral galaxy. Credit: NASA

The second type of galaxy is called an elliptical galaxy. These look more like giant clouds of stars and are not as structured as spiral galaxies. Elliptical galaxies are some of the biggest galaxies in the universe. They come in a variety of shapes from spherical like a ball to elongated like a rugby ball. You can see an example of one below;

NGC 3610- an example of an elliptical galaxy. Credit: NASA

It is thought that elliptical galaxies are formed by the interactions and mergers of other galaxies. As galaxies move through space they often collide with each other. This will eventually happen with the Milky Way and the Andromeda galaxy. After this collision, it is likely the result will be that the two galaxies will merge into one large elliptical galaxy. All we have to do it come up with a name for it, maybe Milkdromeda? We have plenty! In any case this is about 4.5 billion years away. In the next image you can see two galaxies in the process of merging.

Two galaxies which are in the process of colliding. Credit: NASA

The third main type of galaxy is called an irregular galaxy. These are galaxies that don’t fit into the other two categories. In general they are small and don’t have any central bulge or spiral arms. It is thought they are formed through interactions with other galaxies, collisions or may be young galaxies that have not reached symmetry yet.

Irregular galaxy named IC 3583. Credit: NASA

It’s hard to grasp how large galaxies really are. Estimates place the number of stars in the milky way at 400 billion and the number of galaxies in the observable universe at 2 trillion. At the centre of most galaxies is a supermassive black hole. The back hole at the center of the Milky Way galaxy is called Sagittarius A*. It has a mass, 4 million times greater than the Sun. Interestingly, up until relatively recently astronomers called galaxies nebulae. From ancient times, people were aware of the Milky Way. Some astronomers speculated that the Milky Way could be a large cloud of stars all orbiting a center of mass but it wasn’t until the 1900s that astronomers realised that some of the nebulae they saw, such as the Andromeda nebula, were actually large islands of stars and these islands of stars were located outside the Milky Way. In the 1840s, William Parsons built a 72 inch telescope at Birr Castle in County Offaly, Ireland. It was the biggest in the world at the time and in 1845, he was the first to note the spiral pattern in the galaxy now known as M51 or the Whirlpool Galaxy, which you can see in the image below;

M51 – The Whirlpool galaxy. Credit: NASA

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