Understanding galaxies

If you look up to the sky on a very dark, clear night, you might see a bright band of light. In the past, people believed it was a flock of mythical birds crossing the sky, embers thrown up by a girl, or a river of milk spilled by a goddess. They came up with wonderful stories, but they couldn’t have even imagined what we know now! We call this band of light the MILKY WAY. The Milky Way is actually an extremely large collection of stars, planets, cosmic dust, gases and matter that are pulled together by gravitational forces - it’s a GALAXY.

The Sun and the Earth are part of the Milky Way galaxy too! For a long time, scientists thought that the Milky Way was the only galaxy in the universe - that it was the universe! But they couldn’t have been more wrong! The universe is made of an unimaginable number of galaxies, which come in all shapes and sizes! All galaxies, including ours, are formed in a similar way.

When stars are born out of clouds of gas and cosmic dust, they start to attract each other and rotate around a point in space - their common centre of mass. More and more stars are pulled closer together and create gigantic clusters. The gravitational forces in the centre become so strong that they create a black hole. The black hole’s gravitational force holds the galaxy together. If we could look at this galaxy from outside, it would resemble a thin pinwheel, with long spiral-shaped arms slowly rotating around the centre.

We call it a SPIRAL GALAXY. The Milky Way is a special type of spiral galaxy. It has an elongated centre that looks somewhat like a beam or a bar of light. Galaxies like this are called BARRED SPIRAL GALAXIES. Spiral galaxies contain a lot of cosmic dust, so a lot of new stars can form there.

But there are also galaxies, which contain very little dust, because most of it has been used up to form new stars. That’s why such galaxies are believed to be extremely old. Most of them don’t look like spirals but rather like stretched-out spheres - or ellipsoids. We call these ELLIPTICAL GALAXIES. They vary in shape, some of them resemble a basketball, some are stretched out and look like a hot dog sausage.

Elliptical galaxies are the largest in the universe. There are also galaxies that don’t have any regular shape, there are no spiral arms and no definite centre. We call them IRREGULAR GALAXIES. The irregularity might appear when two galaxies get too close and one of them “steals” stars away from the other. Sometimes galaxies crash and this can also change their shapes.

Galaxies are pulled together by gravitational forces. Several galaxies drawn together form bigger or smaller structures - CLUSTERS. Our Milky Way belongs to the so called Local Group, which also contains a big spiral Andromeda Galaxy, and more than fifty smaller galaxies. The Local Group itself is a part of a larger collection of galaxies - the Virgo Supercluster, which is just one of millions of galaxy superclusters in the universe! The Milky Way we see in the sky might not be a glowing river of milk or a mythical creature but the truth about it and other galaxies is as fascinating as the stories our ancestors told.