Galaxies

Have you ever stood beneath a starry night, looked up at the sky, and noticed a faint, glowing band stretching across it? What you were seeing is the Milky Way. It got its name because, in ancient Greece, people believed it was the goddess Hera who had spilled milk across the sky while nursing the baby Hercules. One shouldn’t cry over spilt milk - but one shouldn’t believe every explanation one hears, either: today we know that it isn’t milk at all, but our very own galaxy that we can see in the sky.

Galaxies consist mainly of gas, dust, dark matter, and billions of stars! The gas and dust are found in what we call interstellar clouds, and it is within these clouds that stars are born. The glowing band we see across the sky comes from the light of stars, while the dust and gas appear as dark patches. Dark matter, on the other hand, cannot be seen - but as you can read further below, we know that it is there!

What kinds of galaxies are there?

The universe is packed with galaxies, and they come in many different shapes and sizes. The most well-known (and most beautiful!) are spiral galaxies.

These are probably the ones you picture when you think of galaxies. Their defining features are that they are relatively flat and have spiral arms, which make the whole galaxy resemble something like a hurricane or a whirlpool. We don’t yet fully understand what creates these spiral arms, but there are many theories. One idea is that they are formed by stellar winds from enormous stars that explode in supernovae! The rotation of the spiral arms also helps to spread gas and dust throughout the galaxy, leading to the continuous formation of new stars. You can read more about how stars are born and end their lives as supernovae on the pages Stars and Supernovae.

Our own galaxy, the Milky Way, is a spiral galaxy - and so is our nearest large neighbour, the Andromeda Galaxy. The reason you see the Milky Way as a band across the sky is precisely because it is a spiral galaxy. Spiral galaxies are flat, and we are viewing the Milky Way from within the galaxy’s disc.

To visualise this, imagine that you are living inside a pancake. If you look upwards, there isn’t much pancake batter in the way of your view. But if you look to the side, along the pancake, there is a great deal of batter in the way. In this (I admit, slightly silly) analogy, you should think of the batter as stars. So the band where you see many stars in the sky corresponds to looking along the pancake.

Another type of galaxy is the elliptical galaxy.

Unlike spiral galaxies, these do not have a flat structure, but are instead shaped more like a sphere, which may be perfectly round or somewhat squashed. Imagine shapes ranging from a standard football, to a rugby ball, or something in between.

In the past, it was believed that elliptical galaxies would evolve into spiral galaxies over time - but today we know that this is not the case at all. In fact, astronomers think that large elliptical galaxies may be the result of a collision between two spiral galaxies! In such a collision, gas and stellar dust from the two galaxies are scattered, and the new galaxy loses its distinct structure. As a result, there are no regions with especially high concentrations of gas and dust, which means that fewer new stars are formed in elliptical galaxies.

Other galaxies can be classified as irregular galaxies. As the name suggests, these are galaxies whose shapes simply don’t quite fit into the other two categories. Irregular galaxies are often small, and they are particularly useful for astrophysicists who want to study how stars form! In irregular galaxies, you don’t have to account for the motion of spiral arms, which can make it more difficult to study star formation in spiral galaxies.

The structure of the Milky Way

As already established, the Milky Way is a spiral galaxy. This means it is flat, has spiral arms, and that new stars are constantly being formed within those arms. In addition, all the stars in the Milky Way orbit around the centre of the galaxy - rather like the Earth orbits the Sun. This might lead you to wonder: what exactly are all the stars in the Milky Way orbiting? The answer turns out to be quite extraordinary: at the centre of the Milky Way lies a supermassive black hole. You can read more about black holes here: Black Holes.

This black hole (with the rather unwieldy name Sagittarius A*) is so massive that it weighs four million times as much as the Sun. In other words, you would need to gather four million stars the size of our Sun to match the same gravitational pull as Sagittarius A*.

Sagittarius A* is also one of the first black holes ever to have been photographed - a milestone achieved in 2022. Today, astronomers believe that all large galaxies harbour a massive black hole at their centre, helping to hold the galaxy together.

One might think that this would be enough to explain why the stars in the Milky Way move as they do. But even though the black hole is enormous, it is actually not large enough to account for what we observe.

The faster something rotates, the greater the gravitational force required to hold on to it. It’s a bit like a fairground ride - such as a carousel: the faster it spins, the more securely you need to be strapped in to avoid being thrown off. The stars in the Milky Way are simply moving too fast for Sagittarius A* to keep hold of them on its own.

The reason the stars are not flung out of the galaxy is that they are held in place by dark matter. Dark matter is a form of matter that is completely invisible, yet still exerts gravity. Its gravitational pull acts like a kind of “glue” that holds the Milky Way together, preventing the stars from being hurled away as they orbit the galaxy at speeds of around 800,000 kilometres per hour. You can read more about this here: Dark Matter and Dark Energy.

The evolution of galaxies

Galaxies are made up, among other things, of the gas that was formed during the Big Bang 13.8 billion years ago (see our page on the Big Bang for more). This gas was distributed in such a way that some regions of the universe contained slightly more gas than others. Regions with more gas therefore had stronger gravitational pull.

We often think of gravity as something the Earth has - but in fact, all matter in the universe has gravity. The Earth has a strong gravitational pull, which is what keeps us on its surface rather than drifting off into space. At the same time, while the Earth pulls on you, you are also pulling ever so slightly on the Earth with your own gravity. In this way, everything that has mass exerts a gravitational pull on everything else.

This means that gas and dust also attract one another through gravity. As a result, regions of the universe that already had a lot of dust attracted even more, while regions with very little accumulated less.

The regions of the early universe that contained the most gas and dust eventually developed into the very first galaxies. This is why galaxies are not spread evenly throughout the universe, but instead form a kind of network or cosmic web. This structure is known as the large-scale structure of the universe - a vast “web” made up of at least 200 billion galaxies!

Galaxies do not remain completely still, and they tend to change over time. This can happen, for example, if two galaxies come a little too close to one another and end up colliding.

In such a collision, the galaxies merge and become a single, larger galaxy. This is likely how galaxies have grown to the enormous sizes we see today.

In fact, this is also the future of our own galaxy, the Milky Way. It is on a collision course with our neighbour, the Andromeda Galaxy - a collision that may begin in around 4 billion years.

It may all sound rather dramatic and alarming - but in reality, it isn’t! The distances between stars within a galaxy are so vast that the chance of any star or planet actually colliding with another is extremely small. However, we may need to prepare for the night sky to look completely different.

How are galaxies analysed?

Everything we know about galaxies comes from observations. The distances in space are so vast that we cannot send space missions or probes beyond our Solar System to investigate distant stars and galaxies directly. Astrophysicists therefore rely entirely on light, using observations from the many telescopes on Earth to study galaxies. We explain much more about this on the pages Light and Telescopes and Observatories.

The light emitted by a galaxy can reveal a great deal about what is happening within it. When observing a galaxy with a telescope, scientists examine both the image of the galaxy itself - for example, what type of galaxy it is - and its light spectrum.

A spectrum shows what kinds of light are emitted by the galaxy and how much of each type there is. This might include visible light, such as blue and red light, but also types of light that humans cannot see, such as ultraviolet light or X-rays.

What is crucial for astrophysicists is that different elements emit different kinds of light. For instance, if a particular shade of blue light (with a wavelength of 468 nanometres) is observed, it indicates the presence of helium in the galaxy. This colour is unique to helium. In this way, a light spectrum works almost like a collection of fingerprints, revealing the ingredients hidden within a galaxy.

Another tool we can use to study galaxies is our knowledge of the speed of light. Light is the fastest thing there is, travelling at a speed of 300,000 kilometres per second. The fact that light has a finite speed is incredibly important for astronomers.

It means that all the light we see in the universe has been travelling for some time before it reaches Earth, where we can observe it. The farther away we look, the longer the light has been travelling to reach us. In other words, when we look far out into space, we are also looking back in time.

This is extremely useful for astronomers who want to understand what the very first galaxies looked like, and how they differed from those we see today. By simply observing galaxies that are very far away, we can see what they looked like a long time ago. This is how we know how galaxies were distributed throughout the universe billions of years ago.