Galaxies

Have you ever stood on a clear night, looked up at the sky, and seen a faint glowing band stretching across it? What you saw is the Milky Way. The Milky Way gets its name from ancient Greece, where people believed it was the goddess Hera who had spilled milk across the sky while nursing the baby Hercules. You should not cry over spilled milk, but you should not believe everything you hear either: today we know that it is not milk but our very own galaxy that we can see in the sky.

Galaxy Classification

The universe is packed with galaxies, and they come in many different shapes and sizes. The most well known and most spectacular are spiral galaxies, probably the ones you picture when you think of galaxies. Their defining feature is that they are relatively flat and have spiral arms that make the whole galaxy resemble a kind of hurricane or whirlpool. We do not yet fully understand what creates these spiral arms, but there are many theories. One is that they are created by stellar winds from enormous stars exploding in supernovae! The rotation of the spiral arms also causes gas and dust to spread throughout the galaxy, continuously triggering the formation of new stars.

Our own galaxy, the Milky Way, is a spiral galaxy, and so is our nearest large neighbour, the Andromeda galaxy. Both galaxies are part of a larger collection of around 50 galaxies called the Local Group, bound together by gravity. Most of these are small dwarf galaxies, but the Milky Way and Andromeda are by far the largest and most dominant members of the group.

 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, so there are more stars and more stardust when you look through the disc of the galaxy.

Another type of galaxy is the elliptical galaxy. Unlike spiral galaxies, these do not have a flat structure but are instead shaped like a sphere, either perfectly round or somewhat flattened. Think of them as shaped like a regular football, an American football, or something in between. In the past it was thought that elliptical galaxies would develop into spiral galaxies over time, but today we know this is not the case at all. In fact, large elliptical galaxies are thought to be the result of a collision between two spiral galaxies! In such a collision, the gas and stardust from the two galaxies gets scattered, and the new galaxy loses its beautiful structure. This also means there are no regions with an abundance of gas and dust, so fewer new stars form in elliptical galaxies.

Other galaxies can be categorised as irregular galaxies. As the name suggests, these are galaxies whose shape simply does not fit neatly into the other two categories. Irregular galaxies are often small, and they are particularly useful for astrophysicists who want to study star formation, since you do not have to account for the movement of spiral arms, which can complicate the study of star formation in spiral galaxies.

The structure of the Milky Way

As established, the Milky Way is a spiral galaxy. It is flat, has spiral arms, and new stars are constantly being formed in those arms. All the stars in the Milky Way also orbit around the galaxy's centre, a little like Earth orbits the Sun. It comes natural to wonder then what are all the stars in the Milky Way orbiting, and the answer turns out to be quite remarkable: at the centre of the Milky Way lurks a supermassive black hole (read more about Black Holes).

The black hole, with the memorable name Sagittarius A* is so massive that it weighs 4 million times more than the Sun. You would need to gather 4 million stars the size of our Sun to match the gravitational pull of this giant. Sagittarius A* is also one of the first black holes ever photographed, a milestone achieved in 2022. Today we believe that all large galaxies harbour a massive black hole at their centre, holding the galaxy together.

You might think 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 compared to the stars, it is actually not massive enough to account for what we observe. The faster something rotates, the stronger the gravitational force needed to hold it in place. It is a little like a carousel: the faster it spins, the more securely you need to be held on to avoid flying off. And the stars in the Milky Way simply move too fast for Sagittarius A* to hold them alone. The reason they are not flung out of the galaxy is that they are also held in place by dark matter. Dark matter is a kind of matter that is completely invisible but has gravity, acting as a kind of glue that holds the Milky Way together, keeping all its stars from being hurled away as they orbit at 800,000 kilometres per hour (read more about Dark Matter and Dark Energy).

The evolution of galaxies through time

Galaxies are made partly from the gas that formed during the Big Bang 13.8 billion years ago. That gas was distributed in such a way that some regions of the universe had slightly more of it than others. Regions with more gas had more gravity.

We tend to think of gravity as something unique to large objects like Earth, but in fact all matter in the universe generates gravity. The more massive an object, the stronger its gravitational pull, which is what keeps us standing on the surface of Earth rather than floating off into space. And while Earth pulls on you, you also pull back on Earth just a tiny amount. In this way, everything with mass pulls on everything else.

Gravity pulled gas and dust together, and regions of the early universe that were slightly denser than their surroundings gradually attracted even more material, growing denser still, while emptier regions grew emptier. Over time, this subtle difference grew into the first galaxies. Thus, the early universe developed regions of abundant gas and dust that eventually became the very first galaxies. Galaxies are therefore not evenly spread across the universe but arranged in a kind of net or web. This is what we call the large-scale structure, a cosmic web made up of at least hundreds of billions galaxies.

Galaxies do not stand still and tend to change over time. This can happen when two galaxies drift too close to each other and collide. In such a collision the galaxies merge into one large galaxy, and this is probably how galaxies grew to the enormous sizes they have today. 

This is in fact also the future of our own Milky Way, which is on a collision course with our neighbour, the Andromeda galaxy, a collision that could begin in around 4 billion years. It all sounds rather dramatic and unsettling. But in such a collision, the chance of actually being hit by another star or planet is in fact very small, though we should perhaps prepare for the night sky to look completely different.

How are galaxies analysed?

Before we could study galaxies in detail, we first needed to catalogue them. In the 18th century, the French astronomer Charles Messier was hunting for comets when he kept being distracted by fuzzy objects in the sky that looked like comets but were not. Rather than ignoring them, he compiled a list of over 100 of these objects so he and other astronomers would not confuse them with actual comets. This catalogue, published in 1774, became one of the most important in the history of astronomy! Many of the objects turned out to be galaxies, and they are still known by their Messier numbers today. The Andromeda galaxy, our nearest large neighbour, is M31. The famous galaxy whose black hole was first photographed is M87 (learn more about this image in Telescopes and observatories). Next time you see a galaxy referred to by the letter M followed by a number, you’ll know where it comes from!

Everything we know about galaxies comes from observations. The distances in space are so vast that we cannot take physical measurements beyond our own Solar System. Therefore, when studying galaxies, astrophysicists work with light, using observations from Earth's many telescopes.

The light emitted by a galaxy can tell us an enormous amount about what is happening inside it. Researchers look both at the image of the galaxy itself, for example to determine what type it is, and at its light spectrum. A spectrum tells us what kinds of light the galaxy emits and how much of each kind, ranging from visible light such as blue and red, to forms of light we cannot see with our eyes, like ultraviolet or X-ray radiation (read more about Light).

What is absolutely essential for astrophysicists is that different elements emit uniquely characteristic light. For example, if a great deal of a very specific shade of blue light with a wavelength of 485 nanometres is detected, this tells us there is hydrogen in that galaxy, because that precise colour is unique to that element. A light spectrum therefore functions almost like a collection of fingerprints, revealing the ingredients hidden inside the galaxy. 

The light spectrum also allows astronomers to measure how fast galaxies are moving away from us. When a galaxy moves away from us, its light gets stretched to longer, redder wavelengths, an effect called redshift (indicated in astronomy with the letter “z”). The faster a galaxy is moving away, the more its light shifts toward the red end of the spectrum. Since more distant galaxies are moving away faster due to the expansion of the universe, redshift also gives us a powerful way of measuring how far away a galaxy is.

Light also has a speed. It is the fastest thing there is, travelling at 300,000 kilometres per second. The fact that light has a finite speed is incredibly important for astronomers to bear in mind, because it means that all the light we see from the universe has been travelling for some time before reaching us here on Earth. The further away you look, the longer the light has been travelling. In other words, when we look far away, we are also looking back in time. This is enormously useful for an astronomer who wants to know what the very first galaxies looked like and how they differed from those that exist today. You simply have to look far enough away to see them. It is also how we know how galaxies were distributed across the universe billions of years ago.