Galaxies

A galaxy is a massive system of stars, planets, gas and dust, all bound together by the force of gravity. When we say 'massive', we really mean it.

Our own galaxy – the Milky Way – contains at least 100 billion stars. It also contains at least as many planets, and possibly significantly more.

In the early 20th century, there was a lot of debate about galaxies. Was our giant galaxy all that was out there? Or was it just one of many clusters of stars?

The question of galaxies was finally settled by Edwin Hubble in 1925. He used a powerful telescope to identify a type of pulsing star called a cepheid variable star. By observing several of these, he managed to calculate how far away the Great Andromeda Nebula was.

He found that, in fact, the Andromeda Nebula was not a nebula at all, but another galaxy a vast distance away from our own. The Milky Way wasn't alone in the universe. It had a galactic neighbor.

Not just one, either. Nowadays, astronomers generally believe that the universe is home to between 200 billion and 2 trillion galaxies. These numbers are so large that they stretch beyond the limits of our comprehension.

Groups of galaxies can be bound together by gravity into clusters. Typically, a galaxy cluster is a few million light-years across.

The Milky Way is part of the Local Group cluster, which contains less than a hundred galaxies. But our nearest neighbor is the Virgo cluster, which consists of approximately 2000 galaxies.

These huge clusters of galaxies can form into even bigger groups, coming together to form superclusters. These superclusters can be hundreds of millions of light years across.

Together, the Local Group and the Virgo cluster form part of the Laniakea supercluster. Around 100,000 galaxies can be found here. However, unlike clusters, a supercluster is not gravitationally bound. Over time, it might drift apart.

We can classify galaxies into four different types: spiral, elliptical, peculiar, and irregular.

Spiral galaxies include our own Milky Way. They are flat spinning disks of stars, gas, and dust with a central bulge.

Older, redder stars populate their central bulge, while the arms of the galaxy, spiraling out from the center, are bright with the light of younger, hotter stars.

Spiral galaxies can vary in appearance. The spiraling arms might be well-defined – we call these grand design spiral galaxies – or they might be a bit choppier – we call these flocculent spiral galaxies.

Elliptical galaxies can range from nearly spherical to elongated to the shape of a cigar. They are smooth and featureless to look at, characterized by a lack of dust and gas, and populated by older stars. We think they are formed by spiral galaxies colliding. An example is Cygnus A.

Peculiar galaxies are similar to spiral galaxies, but slightly oddly shaped, probably due to interacting with other galaxies. The cartwheel galaxy, with its two distinct rings, is a good example.

Last but not least, we have irregular galaxies, which are utterly shapeless and chaotic. The best known examples are our companion galaxies – the Large and Small Magellanic clouds.

In 1931, Bell Telephone Laboratories employed Karl Jansky to help them hunt for the static interference playing havoc with their telephone communications.

To do so, he built the first radio telescope. While he found local sources of static, he also found something else more puzzling: a powerful radio signal coming from the center of our galaxy. He wanted to investigate this signal – which he called star noise – but he couldn’t find the support to do so.

Years later, in 1974, Balick and Brown published work identifying the source of these radio waves: a supermassive black hole. Named ‘Sagittarius A’ in 1982, this enormous black hole sits at the center of the Milky Way.

Vast amounts of energy are released at the edge of the event horizon, where superheated gas gathers in an accretion disk. Currently, 'Sagittarius A' is quiet, but it’s been through active phases. In 2022, an image taken by the Event Horizon Telescope allowed us to see this monster at the heart of our galaxy for the first time: a dark contrast against its bright accretion disk.

We now believe that almost all large galaxies have a supermassive black hole (SMBH) at their center. Most, but not quite all. The elliptical galaxy A2261-BCG, for example, is 10 times bigger than the Milky Way, but doesn’t appear to have a SMBH.

Where they do exist, SMBHs are truly enormous – millions to billions of times the mass of our sun.

Our own galaxy's SMBH is currently a calm, gentle giant. But in the past, it’s gone through feeding frenzies: taking in vast amounts of material and shooting out the leftovers as superheated jets of gas. These created Fermi bubbles – huge bubbles of x-rays and gamma-rays ballooning from either side of the galaxy's core.

We don’t know why so many galaxies have a SMBH at their center. One idea is that they are primordial black holes from the birth of the universe, perhaps formed even before the first stars.

The SMBHs at the center of most galaxies are often accompanied by something called an active galaxy nucleus (AGN).

AGNs are regions of space that give off vast amounts of radiation. This radiation is released when dust and gas starts to fall into the SMBH. As this material is drawn in, particles collide, and generate vast amounts of energy.

An extreme kind of AGN is something called a quasar. These can shine up to a thousand times brighter than the Milky Way – they're so bright that we're able to see them even when they're incredibly far away.

One of the furthest quasars is called J0313-1806. It's about 13.03 billion light years from us – that's right at the edge of the universe.

Galaxies move. In our own galaxy, the Milky Way, there is movement within the galaxy, as the outer regions orbit around the center. Our sun is carried around the center of the galaxy about once every 200–250 million years.

Galaxies also move relative to each other in local groups – the Milky Way is currently traveling towards the Andromeda Galaxy at about 113 kilometers per second. In about 5 billion years, the two galaxies will probably collide.

On top of all this, these galaxies are being moved around in clusters and superclusters of galaxies, like our supercluster Laniakea. All of this is happening within a universe that gradually expands.

Galaxies collide in slow, violent dances. Driven by their combined gravity, they swing around each other, swirling closer until they lose their shapes.

The likelihood of any two stars colliding in merging galaxies is very low. There is so much space between stars within galaxies that the stars will usually drift past each other, like passing swarms of bees.

But, even if there aren't any direct collisions, the gravitational forces produced by the two galaxies will rearrange stars and their orbits, changing both of the galaxies dramatically.

What emerges from a galactic collision depends on what the galaxies looked like before they crashed. If two galaxies of a similar size merge, they might form a new elliptical galaxy. If they have SMBHs at their center, these will eventually merge.

There's one big mystery with galactic motion. In theory, all the stars that orbit around the SMBH should orbit more quickly towards the center of the galaxy, and more slowly at the outer edges.

This is how gravitational systems are meant to work. In our solar system, the planets orbit more and more slowly the further you get from the sun. But this doesn’t happen in a galaxy. Instead, rotation speed gets slightly faster as you travel further from the center.

This curious behavior has led us to speculate about the existence of something called dark matter. This matter, if it exists, doesn't interact with light, so we cannot see it – but it exerts a gravitational pull.

If there was a halo of dark matter around every galaxy, its gravity could explain why objects rotate more quickly towards the edge, rather than the center. This is only a theory, but most scientists agree that dark matter exists – otherwise, our understanding of physics must be broken.