The Hitchhiker’s Guide to Galaxies

Cities of stars

You can think of galaxies like cities of stars, scattered across the universe. Galaxies contain stars, planets around those stars, dust, dark matter, interstellar gas, and the remnants of dead stars, all gravitationally bound around a massive black hole at the centre.

Our galaxy, the Milky Way, is just one of billions of galaxies within the universe. Small galaxies can contain between 100 million and several billion stars. The largest galaxies can contain trillions of stars.

A Zoo of Galaxies

Historically, galaxies are classified based on what they look like.

There are four main types of galaxy: spiral, lenticular, elliptical, and irregular. Elliptical galaxies are smooth and relatively featureless with a bright bulge. Lenticulars have a bright bulge with a disk like structure around the bulge. Spiral galaxies have a bright bulge and a disk of spiral arms. Between one- and two-thirds of spiral galaxies also have a bar structure across their centres.

Elliptical, lenticular, and spiral galaxies can be classified using the Hubble sequence, also known as the tuning fork diagram. Galaxies that do not fit into this sequence are irregular.

The human eye is much better at spotting patterns than a computer, but professional astronomers can only classify so many galaxies. So there’s a great citizen science project out there called Galaxy Zoo where you can have a go at classifying galaxies yourself, and help out with real research. In the past, citizen scientists have even discovered a completely new type of galaxy! To get started, check out the Galaxy Zoo website.

Galaxies are usually named after their catalogue entry or set of coordinates, but sometimes they are also given names based on what they look like or who discovered them. For example: the Sombrero Galaxy, the Sunflower Galaxy and the Tadpole Galaxy.

Our Milky Way galaxy is a barred spiral galaxy, containing between 100 billion and 400 billion stars. Orbiting around these individual stars, there could be 100 billion planets! As we live inside the Milky Way, we cannot take pictures of the whole thing, just parts of it. So we have to make our best guess as to what the Milky Way looks like, based on observations from the inside.

Milky Band of Stars

All of the stars that we can see in the sky are part of the Milky Way galaxy. But when we look along the denser disk of the galaxy, on a clear night sky, we can see a milky-looking band of clouds and stars curving across the sky, which gives the galaxy its name.

Ancient civilisations created many stories about what this band of light could be.

The Babylonians thought that it was the severed tail of a dragon. The Greeks believed that it was a trail of milk from the goddess Hera unknowingly breast feeding the hero Hercules. In many Asian traditions, the Milky Way was a great river in the sky. And to the Maori people, the Milky Way was the wake of a canoe.

Although we now know that the Milky Way is not any of these things, this band of light is one of the things that tells us that the galaxy is a spiral galaxy. If our galaxy was an elliptical galaxy, all the stars would be evenly scattered across the sky instead of in a bright band.

Galactic Neighbours

Collections of galaxies are called groups or clusters.

In our local group, the Milky Way is the second largest galaxy. The largest is Andromeda. This is another spiral galaxy that is 2.5 million light years from Earth. It is the nearest major galaxy to Earth and lies in the direction of the Andromeda constellation, which is where it gets its name from. The Andromeda Galaxy is visible to the naked eye on clear nights when there is no Moon and is the most distant object that you can spot without a telescope. It has a diameter of approximately 220,000 light years.

In approximately 4.5 billion years, the Milky Way and Andromeda are expected to collide, creating a very large elliptical galaxy.

The Cartwheel Galaxy is a lenticular galaxy, but it can also be classified as a ring galaxy, due to its shape. It has a diameter of 150,000 light years.

It is thought that the Cartwheel Galaxy was originally a spiral galaxy, until it collided with another galaxy. This caused a shockwave which pushed out lots of dust and gas from the centre creating the ring appearance It is slowly returning to back to a spiral galaxy.

Another ring galaxy is known as Hoag’s Object. Hoag’s Object has a more perfect ring than the Cartwheel Galaxy, but it is unlikely that it was formed in the same way as there are no remnants of the colliding galaxy. Its formation remains a mystery.

Seeing the Invisible

Galaxies do not just emit visible light; they also emit light on all parts of the spectrum.

For example the Cartwheel Galaxy has very strong X-ray emissions due to the large amount of massive stars and blackholes in the ring. UV and infrared light are very useful for understanding the formation and evolution of galaxies.

Galaxies that emit strong radio waves are usually massive elliptical galaxies with active supermassive black holes in their cores. As the black hole swallows up nearby gas and dust, the whirlpool of material around the black hole can emit huge amounts of radio light and even jets of superfast plasma. These black hole cores are called active galactic nuclei or AGN.