In all sciences we classify things. From animals to elements. It’s just what we like to do and it helps us understand what we are studying better. So what about galaxies. Are they classified? Yes, and that is the topic for today’s blog. There are four main types of galaxies: spiral, barred spiral, elliptical and irregular (some people classify dwarf galaxies as well, but that’s up for debate). But wait, I hear you say. We don’t fully understand what a galaxy is. Well don’t worry, I got you. In the dictionary, it says that a galaxy is “a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction”. That is mostly true. Mostly, because it didn’t mention a little thing called dark matter. Little because it only takes up about 25% of the universe. Oh, wait I forgot, matter that we can see only takes up 5% of the universe. The remaining 70% consists of dark energy. Anyway, the dictionary says that galaxies are “held together by gravitational attraction”, and that is true. There isn’t enough mass that we can view for galaxies to interact and spin etc the way we observe them. So a while back, scientists hypothesised that there was more mass that we just couldn’t see. They named it dark matter. Without it, our universe would be completely different. Also most galaxies have super massive black holes in the centre. Those black holes are mysterious to us because we don’t know how they formed.
OK, now we know what galaxies are, here’s a little quiz. Match the pictures with the words and see how many you get right! Answers are at the bottom of the page.
- Barred Spiral
So now we know what each of them looks like let’s see what distinguishes them from each other. Firstly the spiral.
As you can guess from the name it has spirals. In the centre, it has a round bulge that the spirals come out of. Next up is the barred spiral galaxy.
The barred spiral galaxy is similar to the normal spiral in the sense that it has spirals. But what’s different is that the spirals come out of not that bulge in the middle. No no. But a bar running through the centre. Our galaxy is a barred spiral galaxy. The elliptical galaxy is the penultimate one.
From the name and picture, you probably guessed that they are just a circle or an ellipse. They may look boring but they hold a lot of history to them as they most likely merged with another galaxy and lost a lot of mass. Eventually, the mass all rolled up into an ellipse. On the subject of merging the last galaxy is the irregular.
Irregular galaxies have recently gone through some rough times. Merging with another galaxy is not an easy task. As seen from the photo irregular galaxies are all misshapen and irregular as it states in the name.
So next time you look up in the sky think about all the galaxies and their classifications. To leave you on a nice fact, there are more stars in our observable universe (a circle that has a radius of 13.7 billion light-years around us) than grains of sand on every beach in the world.