What do galaxies look like? How do they move? What happens when they crash? Exploring our Milky Way and the galaxies beyond.
The Milky Way
In the early 20th century, a great debate about our place in the universe arose. Was our galaxy all that was out there? Some astronomers, such as Harlow Shapley, certainly thought so. Or was it just one of many clusters of stars, as the ‘island universes’ hypothesis suggested by Heber Curtis?
The question was settled for good by the work of Edwin Hubble in 1925. He used a powerful telescope to identify pulsing stars – Cepheid variable stars. By observing these, he was able to calculate how far away the Great Andromeda Nebula was. He found that, in fact, the Andromeda galaxy was a separate cluster of stars to our galaxy, and a vast distance away.
Our Milky Way is a spiral galaxy, and we orbit the sun on one of its spiral arms. But Hubble’s work helped prove that the universe stretches much further than that – 100 billion to 200 billion galaxies further.
Types of galaxies
We can classify galaxies into one of 4 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.
The arms of the galaxy, spiraling out from the center, are bright with the light of young, hot stars. They vary in appearance. Spiraling arms can be well-defined, as in grand design spiral galaxies, or a bit choppier in flocculent spiral galaxies. The arms can be tightly wound or trail more loosely, but around two-thirds have a bar of stars across their center.
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.
Peculiar galaxies are oddly shaped, likely due to interacting with other galaxies. The cartwheel galaxy, with its 2 distinct rings, is a good example. Irregular galaxies are shapeless and chaotic. The best known are our companion galaxies – the Large and Small Magellanic clouds.
The motion of galaxies
Galaxies move. In our galaxy, the Milky Way, there is movement within the galaxy as the outer regions orbit the center. Our sun is carried round the center of the galaxy about once every 200– 250 million years.
Galaxies move relative to each other in local groups – the Milky Way is currently traveling towards Andromeda at about 70 miles (113 kilometers) per second. In about 5 billion years, the 2 galaxies will probably collide. These objects are being moved around in clusters and superclusters of galaxies like our supercluster Laniakea. All of this is happening within a universe that continues to expand.
Dark matter
From the number of stars we see, our galaxy’s mass should be concentrated at its center. The speed at which objects rotate should therefore slow down as you travel further away from the center. However, this doesn’t happen. In fact, if anything, rotation speed gets slightly faster as you travel out. It remains high to the edge of the visible galaxy, upsetting Newtonian physics completely.
It is this curious behavior that led us to speculate about dark matter. Dark matter is thought to make up 85% of the matter of the universe, a halo surrounding every galaxy. It doesn’t interact with light, but has a gravitational pull.
At the speeds we see galaxies rotating, they should be shredded – and dark matter is the theoretical glue holding them together. Although scientists widely accept the existence of dark matter, its identity remains a mystery. If dark matter doesn’t exist, as some scientists have suggested, we would need to tweak our understanding of how gravity works.
What’s in the middle of the Milky Way?
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, around the region of the Sagittarius constellation. He wanted to further 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 the 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.
Supermassive black holes
Thanks in large part to observations made by the Hubble Space Telescope, 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, they’re truly enormous – millions to billions of times the mass of our sun.
Our own 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. The bigger the galaxy, the larger the SMBH, but we have no evidence to link these structures to galaxy formation. One idea is that they are primordial black holes from the birth of the universe, perhaps formed even before the first stars.
Quasars
When we first observed quasars in the 1950s, they looked like stars. Hence the name, a contraction of ‘quasi-stellar radio source’. But they’re not stars. They are young galaxies that are incredibly far away from us. We can only see them at such enormous distances because they are also remarkably bright – up to 1000 times brighter than the Milky Way.
Quasars are a type of active galaxy nucleus, powered by SMBHs. Active galaxy nuclei are regions at the center of galaxies giving off large quantities of radiation. The radiation from a quasar is generated by material in the accretion disk surrounding the black hole. As this material swirls around the event horizon, particles collide, and clouds of superheated dust generate vast amounts of energy.
Because we can see quasars at vast distances, some of the quasars we see are very old, their light reaching us from the infancy of the universe. One of the oldest is ‘J0313-1806’. It is around 13.03 billion light years from us – meaning that we see it as it was only 670 million years after the Big Bang.
When galaxies collide
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 2 stars colliding in merging galaxies is very low. There is so much space between stars within galaxies that they pass each other, like 2 swarms of bees. But the gravitational forces produced by 2 colliding galaxies rearranges stars and their orbits, changing both galaxies dramatically.
What emerges from a galactic collision depends on what the galaxies looked like before they crashed. If 2 galaxies of a similar size merge, they can form a new elliptical galaxy. Gases combine to create regions where new stars can be born. And if they have SMBHs at their center, these will eventually merge, sending gravitational waves rippling into the universe.
The Milky Way is currently on a collision course with the slightly larger Andromeda galaxy. This is due to happen in around 5 billion years, but some scientists have suggested that the haloes at the edge of the galaxies might be colliding already.
Galaxy clusters
Groups of galaxies can be bound together by gravity into clusters – the largest gravity-bound structures in the universe. Typically, a galaxy cluster is a few million light-years across and contains thousands of galaxies. Our nearest is the Virgo cluster, consisting of somewhere around 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. On the highest scale, they seem to follow filaments in space, punctuated by vast, empty, interstellar voids.
Our galactic neighborhood, containing the Milky Way and Andromeda galaxies, is known as ‘the local group’. Along with the Virgo cluster – a mere 55 to 65 million light-years away – we form part of the Laniakea supercluster. Around 100,000 galaxies share this supercluster with the Milky Way. Superclusters are not gravitationally bound, but will eventually drift apart.
