Astronomy: Galaxies & The Milky Way (#82)

I’ve done mainly history episodes recently because it’s what I’ve mostly been interested in, so I thought for today’s episode I’d go off in a direction that I surprisingly haven’t covered for a while. I’m going back to science - more specifically, back to space!

Yet again, this was going to cover a lot more than I’ve actually managed to cover in one episode, so I will definitely be doing a follow-up. I can see myself getting back into wanting to do only space-related episodes again!

In this episode, we’re going to learn about galaxies, and the galaxy we live inside - the Milky Way. I was also going to talk about stars, but after beginning to research it, I realised that’s going to be a whole episode on its own.

I should start off by saying that I got the inspiration for this episode, and a lot of the research actually, from a wonderful book called Fun Science by Charlie McDonnell. I’m sure I’ll be taking a ton more inspiration for future space episodes from this book, too. If you want to learn more about space in a very fun and accessible way, I highly recommend it - I’ll put a link to where you can buy the book in the notes for this episode at learnaboutpod.com.

What is a Galaxy?

So let’s go right back to basics: what is a galaxy? Simply put, a galaxy is a huge grouping of stars, planets, gas and dust, all held together by gravity. Galaxies are typically separated from one another by huge distances measured in millions of light years. Galaxies are sometimes said to be the building blocks of our universe. Their distribution isn’t random: galaxies are strung out along unimaginably long filaments across the universe, like a cosmic web of star cities.

Galaxies group together in clusters. Our own galaxy is part of what is called the Local Group, which is a cluster comprising 55 galaxies that we know of so far. In turn, galaxy clusters themselves group into superclusters. Our Local Group is part of the Virgo Supercluster.

There are several basic types of galaxy, each containing sub-types. Galaxies were first systematically classified, based on their visual appearance, by Edwin P. Hubble in the late 1920s and 30s, during years of painstaking observations. Hubble’s Classification of Galaxies is still very much in use today, although, since Hubble’s time, it has been updated and amended with each new observation.

Before Hubble’s study of galaxies, it was believed that our galaxy was the only one in the universe. Astronomers thought that the smudges of light they saw in their telescopes were in fact nebulae within our own galaxy and not galaxies in their own right. It was Hubble who demonstrated, by measuring their velocities, that they lie at great distances from us, millions of light-years beyond the Milky Way — distances so huge that they appear tiny in all but the largest telescopes.

He also demonstrated that, wherever he looked, galaxies are receding from us in all directions, and the further away they are, the faster they are receding. This means that Hubble was the one to discover that the universe is expanding.

It’s estimated that there are at least 100 billion galaxies in the observable universe, although thanks to improved telescope technology, this figure may go up to 200 billion. These galaxies range wildly in size and shape, and the smallest type that you can find is called a dwarf galaxy, which usually contains about 10 million stars. (In comparison, the truly massive galaxies out there can contain up to 100 trillion stars.) The tiniest dwarf galaxy ever found is called Segue 2, which only has 1,000 stars!

The oldest and most distant galaxy ever observed by astronomers has the slightly less catchy name of z8_GND_5296. This stupidly-named galaxy is about 30 billion light years away, and although quite small (only 1-2% the mass of the Milky Way), it’s producing stars at a very rapid rate. Astronomers hope that they can use this galaxy to increase their understanding of how the earliest galaxies formed.

Let’s quickly look at the different types of galaxies, and then I’ll quickly cover how galaxies are potentially formed before moving onto learning a bit more about the Milky Way.

Types of Galaxies

Spiral Galaxies

The most common type of galaxy is the one most people are familiar with: the spiral galaxy. In fact, we’re sitting in one right now - the Milky Way is a spiral galaxy. Spiral galaxies have majestic, sweeping arms, thousands of light years long, made up of millions upon millions of stars. Our solar system is situated about two thirds of the way out from the galactic center towards the periphery of the galaxy, embedded in one of these spiral arms. (I’ll come back to this when I talk in more depth about the Milky Way.)

Spiral galaxies are also characterised by having a bright centre, made up of a dense concentration of stars, so tightly packed that from a distance the galaxy’s centre looks like a solid ball. This ball of stars is known as the galactic bulge.

Astronomers believe that spiral galaxies may have formed over long periods of time by merging with smaller galaxies, and that it’s this process that triggers the ‘spinning’ motion which results in the galaxies’ typical disk shape and distinctive spiral arms. It’s also thought that most spiral galaxies have a super massive black hole in their centre but unfortunately they’re very hard to identify, even with their super-massiveness.

Lenticular Galaxies

These are disk-like galaxies that basically look like their spiral cousins, but without their arms. Astronomers think that these lenticular galaxies probably used to be spiral, but are now slowly turning into elliptical galaxies - over time, spiral galaxies lose their arms as they burn through their gas and dust supplies.

Elliptical Galaxies

Next up we have elliptical galaxies, and these are the largest galaxies in the universe. They’re distinguished by their smooth, egg-like shape, and are made up of old stars. Ellipticals don’t have a huge amount of gas or dust present. They’re bright at the centre, but that light tends to fade more and more towards the edges - if you look at one through a telescope, you’d be forgiven for thinking you were looking at a smudge on your lens. Also, while spiral galaxies are said to be the most common, some astronomers think it could actually be possible that there are more elliptical galaxies than any other type!

Irregular and Dwarf Galaxies

Now we get to the wild cards - so-called irregular as they don’t really have any kind of standard configuration. These are usually smaller, and astronomers believe they were born in the same way as larger galaxies like the Milky Way, but for whatever reason they stopped growing. 

Ensnared by the gravity of a larger galaxy, dwarf galaxies orbit its periphery. The Milky Way has around 20 dwarf galaxies orbiting it that we know of, although some models predict there should be many more.

Some irregular galaxies are known as starburst galaxies, which is when they contain loads of gas and dust, and form young, hot stars at an exceptionally fast rate.

How Do Galaxies Form?

Galaxies got their start nearly 14 billion years ago, with one unimaginably hot, dense and tiny pinpoint. According to the big bang theory (the actual theory, not the TV show), this singularity was the universe in its entirety. Then it exploded, cooling and expanding in the process. Imagine a balled-up piece of paper unfolding into a giant map, and you have a very basic model of what happened. (The thing that freaks me out and I can’t wrap my head around it is what was there before?? Where WAS this speck???)

Following the big bang, the universe consisted of only radiation and subatomic particles. So how did it evolve into more than 100 billion galaxies? Well, scientists have two kinds of theories. The first theory is that galaxies formed when vast clouds of gas and dust collapsed under their own gravitational pull, and the second theory is that the young universe contained loads of small lumps of matter, which clumped together to form galaxies.

Either way, it was probably caused by gravity, and the resulting clumps then collapsed into protogalaxies consisting of dark matter and hydrogen gas. The hydrogen then fell toward the center of the protogalaxy while the dark matter remained as an outer halo surrounding it.

Wait… What is Dark Matter?

(A quick side track here: I thought I should offer up a quick explanation about what dark matter actually is. It’s a phrase we hear a lot, but for ages, I had no idea what it actually meant - I was just sort of like, “ah yes, dark matter! I’ve never actually thought about what that actually means but it sounds cool!”

For the first 150 million years after the Big Bang, there were no galaxies or stars or planets. The universe was featureless. As I just explained, as time passed, the first stars formed, which collected into galaxies, which then began to cluster together. Those clusters are made up of the galaxies and all the material between the galaxies. Clumps of matter smashed into each other, and the planets in our solar system began to form around the sun. 

Something must hold our solar system, galaxies and clusters of galaxies together, and that “glue” is gravity.

In some clusters, the space between galaxies is filled with gas so hot, scientists can’t see it using visible light telescopes. The gas can only be seen as X-rays or gamma rays. Scientists look at that gas and measure how much there is between galaxies in clusters. By doing this, they discovered that there must be five times more material in the clusters than we can detect. The invisible matter that we can't detect is called "dark matter".

I’ll link to an article in the notes that talks about dark matter in more detail.

...Okay, Back to the Galaxies

It’s worth bearing in mind that the process of galaxies forming and changing is nowhere near finished; our universe is evolving all the time. Smaller galaxies are often gobbled up by bigger ones, and even our own galaxy probably contains the remains of several smaller ones that it has swallowed during its lifetime.

The reason these mergers often happen is because, on a galactic scale, the universe is actually quite crowded. The Milky Way, for example, spans about 100,000 light years, and our nearest major galaxy, Andromeda, is about 2.5 million light years away. To us, these distances obviously seem gigantic, but on a galactic scale, this actually means the distance between the two galaxies is only 25 times greater than their size (which in space terms is pretty small). To downsize this proportionally, the distance between you and your nearest galaxy would be a measly 12.5 metres, or 41 feet.

As galaxies are so massive, their gravitational force is also incredibly strong. This means that when you crowd them together, the attraction can be so strong that two galaxies latch onto each other and don’t let go. Not only that, but as the galaxies get closer and closer together, that attraction increases.

So, a collision between the Milky Way and Andromeda is pretty much inevitable. Right now they’re drawing towards one another at a rate of about 400,000km (248,000 miles) per hour, and eventually they’ll combine to form one giant city of stars. It’s going to take about 4 million years for them to get around to it though.

It’ll be pretty spectacular when they do finally come together though. When two massive spiral galaxies do have an encounter, huge, cold clouds of gas inside them will be compressed, resulting in millions of new stars bursting into life all at the same time. They’ll swing by each other first, and then plunge into one another, forming another burst of star formations. The two galaxies will repeat this plunging action again and again, possibly taking billions of years before they merge together to form a new elliptical galaxy.

The Milky Way

We’re almost done. Now, let’s learn more about the Milky Way! Our galaxy is 13.2 billion years old, give or take about 800 million years or so.

Remember earlier when I talked about Hubble and his Classification of Galaxies, and how the main types also have subtypes? Well, the Milky Way is, in fact, in one of Hubble’s spiral galaxy subtypes: it’s a barred spiral, which means it has a bar of stars protruding out from either side of the center. The ends of the bar form the anchors of the spiral arms, the place from where they sweep out in their graceful and enormous arcs. This is a fairly recent discovery: how the bar forms in a galaxy is not yet understood.

Also established recently is the fact that the disk of the Milky Way is not, as most diagrams depict, flat: it is warped, like a long-playing vinyl record left too long in the sun. Exactly why is not known, but it is thought to be the result of a gravitational encounter with another galaxy early in the Milky Way’s history.

Our solar system lives in one of the arms of The Milky Way, called the Orion arm. It’s a quiet part of the galaxy, about half way out from the centre. The Milky Way is shaped like a huge whirlpool that rotates once every 200 million years. It is made up of at least 100 billion stars, as well as dust and gas. It is so big that light takes 100,000 years to cross from one side to the other.

The centre of the Galaxy is very hard to see because clouds of gas and dust block our view. Scientists think that it contains a supermassive black hole, called ‘Sagittarius A’, that swallows anything passing too close. It has a rough mass equivalent of more than 4 million suns put together.

Outside the main spiral are about 200 ball-shaped clusters of stars. Each 'globular cluster' is very old and contains up to one million stars. As previously mentioned, the Milky Way belongs to a cluster of at least 40 galaxies, which also includes our galactic neighbour, Andromeda.

Astronomers have calculated that the Milky Way contains something between 200 and 400 billion stars, and might also contain up to 100 billion planets - pretty impressive when you consider the fact that it only forms about seven new stars every year.

A lot of it is populated by stars like our own Sun, but most of the stars in the galaxy are actually red dwarfs, a type of star that might be more important than we actually think. (Not to tease you, but I’ll come back to this in a future episode about stars and black holes.) Many of the planets in the Milky Way actually orbit red dwarfs, and they emit a heat that, in theory, may make some of those planets slightly more habitable for us humans.

Fun fact: Because of the Milky Way, you’re currently travelling about 1,000 times faster than a jet plane. Right now, it’s rotating at a speed of 270km (168 miles) per second. In the last hour, we’ve actually travelled about 972,000km (604,000 miles) through space.

Even given how fast that is, we never feel how fast we’re travelling. Another little sidetrack here but I wanted to make sure you’re not left with gaps in your knowledge: if you want to know why we don’t feel the Earth move? It’s because the speeds are constant. The spinning and orbital speeds of Earth stay the same so we don’t feel any acceleration or deceleration. You can only feel motion if your speed changes. For example, if you are in a car which is moving at a constant speed on a smooth surface, you will not feel much motion. However, when the car accelerates or when the brakes are applied, you do feel motion. The same goes for the movement of the Earth.

The Future of Galaxies

Now I come to the final little section of this episode: what’s going to happen to galaxies in the distant future? I mentioned before that the universe is expanding. What I didn’t mention is that this expansion is happening at an ever-increasing rate. The more time goes on, the faster all of the galaxies in the cosmos are moving away from each other.

What this means is that, eventually, all galaxies in the universe will be moving away from one another so quickly that they’ll actually reach the speed of light. If there are still astronomers around when this happens, they’ll have a very different picture of the universe to the one we have today - as it means that even light itself won’t be able to bridge the gap between the galaxies.

When those astronomers point their telescopes at the sky, all they’ll see is darkness. What’s more, if there are any extraterrestrials out there in galaxies beyond our own who might try to communicate with us - any signals that they might attempt to send out will never reach us.

While astronomers still know very little about exactly how galaxies formed in the first place, the study of galaxies is an endless voyage of discovery. Less than a hundred years after it was realized that other galaxies besides our own exist, we have learned so much about these grand, majestic star cities — and there is still much to learn.