2-2 Galaxies

The Milky Way Galaxy

As long as humans have been looking up at the night sky, they have seen a faint, glowing band of light going from one side of the sky to the other. It's tough to see from the city, but on a clear night away from city lights, it's easily visible and quite beautiful:

This is, of course, an excellent picture of what you might see, but it gives you an idea about what's possibly up there.

A lot of objects in the sky are named for figures in various mythologies, and the Milky Way is no exception. In Greek mythology, a baby named Heracles is placed by his father, the god Zeus (whose name in Roman mythology was "Jupiter"), on the breast of a sleeping Hera to nurse. Since Heracles was not a god (his mother was a human), Zeus' idea was that if his son was to drink the milk of Hera, a goddess, he would become immortal. Hera woke up, saw she was nursing a baby that was not hers, pushed the baby away, and her milk spilled across the sky.

The ancient Greek name for the Milky Way was γαλαξίας κύκλος (galaxias kyklos, or "milky circle"). Thus, our word galaxy, which has come to mean a collection of stars, dust, planets and gases that is bound together by its own gravity, is tied to this Greek name. (Can you see that the name for the sugar in milk, lactose, comes from this Greek phrase as well? How about the word "circle," if you change the first consonant slightly?)

Hubble's Discovery

Telescopes were only invented in the early 1600s, and we will be examining them later in more detail. But, by the early 1900s, they had become quite good at gathering enough light to make faint stars much easier to see. Not long after their invention, astronomers like Galileo were able to see that the Milky Way was made up of countless individual stars.

Without any type of telescope, under good viewing conditions, you can see a small fuzzy patch of light high in the sky in the constellation Andromeda, highlighted here inside a square:

Tough to see, isn't it? As telescopes got better and better, astronomers could get a slightly clearer look at these fuzzy light-patches. They called them spiral nebulae, as "nebula" is the Latin word for "cloud," and a lot of them seemed to have a shape like a spiral.

This would be what astronomers in the early 1900s would be able to see with their best telescopes: there's clearly some sort of structure, but what is this thing?

Edwin Hubble was an American astronomer who, in the 1920s, was making observations of the Andromeda Nebula, as it was called at the time.

He was studying a type of well-known star called a Cepheid Variable within the Andromeda Nebula, and he came to a stunning conclusion: This star was far outside the Milky Way.

At that time in cosmology, the study of the structure and evolution of the universe, there was a big debate: Is the Milky Way galaxy all the universe, or is the universe bigger than that? Hubble's discovery that the Andromeda Nebula was an entirely separate galaxy settled that debate.

We now call this the Andromeda Galaxy, and there are countless others out there. One of the first pictures ever taken by the new James Webb Space Telescope, which is a patch of sky about the size of a grain of sand at arm's length, shows hundreds of galaxies:

There are a few individual stars here, and they show up as six-pointed points of light. But everything else in this picture is an entire galaxy, containing billions or trillions of individual stars.

(The James Webb Space Telescope was launched in 2021 to replace the Hubble Space Telescope, which was launched in 1990. And, yes, the Hubble Space Telescope was named after Edwin Hubble.)

Types of Galaxies

Hubble's observations with the new telescope at Mount Wilson Observatory in California showed that galaxies fell into one of a few different types.

Elliptical

Elliptical galaxies range from roughly spherical to very elongated. But, these galaxies do not look like a spiral, unlike Andromeda that definitely has that type of shape.

Spiral

A spiral galaxy is probably what you think of when you try to picture a galaxy in your mind. There is a central bulge which contains a lot of the galaxy's material, plus there are spiral arms around the outside which contain most of the rest of the stars.

Barred Spiral

A barred spiral shares a lot of features in common with spiral galaxies. However, they have a straight "bar" across the centre, and the spiral arms come out from there.

We have evidence to show that, to the best of our knowledge, the Milky Way is a barred spiral galaxy.

Irregular

There are some galaxies that just don't fall into any of the above categories. They might have an unusual shape because they were the result of a previous collision between two galaxies, and the gravitational pull between stars as they pass close to each other -- but likely never actually touch -- twists and rips the galaxies into strange shapes. These are called irregular galaxies and they can be very interesting-looking.

Hubble's Tuning Fork Diagram

A tuning fork is a piece of metal with a distinctive shape. This shape lets it "ring" with a very clear sound that can be used to help tune a piano's strings.

Hubble started classifying galaxies based on their shapes, and was able to construct his tuning fork diagram as a way to easily categorize their shapes.

Other Large Objects

Galaxy Clusters

Galaxies tend to clump together into galaxy clusters, which are the largest structures in the universe that are held together by their own gravitational pull.

In the picture above, each smudge, smear or dot of light is an entire galaxy. There are several hundred galaxies shown in this cluster, called "Abell 370". The mass of the dust and gas in the cluster acts like a lens, which smears-out some of the galaxies into arcs of light instead of being a more conventional shape.

The Milky Way belongs to the Local Group galaxy cluster.

Galaxy Superclusters

If we go up a level of size, we get to a galaxy supercluster, which is a group of galaxy clusters. These are the largest objects in the universe of any type, and are so large that they expand as the universe expands. We are a part of the Virgo Supercluster.

On these large scales, the universe -- all the matter, space, time and energy we could ever detect -- is made up of either a supercluster or a void, an incredibly empty region of space where there is essentially no matter of any type at all. The matter is clumped together into the superclusters, then down into clusters, then galaxies, then solar systems like ours, then individual stars and planets. If this doesn't make you feel small, nothing else will!

Quasars

For a few decades, astronomers have found strange, galaxy-like objects in deep space that emit a lot of all kinds of radiation: radio waves, X-rays, visible light, and others. Not quite sure what to make of these, they were given the name quasi-stellar radio sources, later semi-abbreviated to quasars.

Above is an artist's idea about what a quasar might look like. Since they are so far away, it's difficult to get a good picture of them, but we can tell that they are extremely bright. They tend to have jets of material being ejected out of their poles, and a few years ago an extremely bright one was found:

Since looking outwards means looking back in time (due to the speed of light), it's possible that quasars are a kind of primitive galaxy that was around a long time ago. Or it could be that galaxies start off as quasars before settling down into something more stable. It's hard to say what they actually are.

Below is a video from the strange and wonderful Vsauce talking about the brightest object in the universe. Is it a quasar? Possibly.

Practice

The Basics

1. Arrange the following objects from smallest to largest: galaxy supercluster, galaxy, solar system, galaxy cluster.

2. What are the three main types of galaxies? How can you tell between the different types?

3. Which galaxy did Edwin Hubble find to be the first outside our own Milky Way?

Extensions

1. What is a pulsar, and how is it different from a quasar? How is it similar?

2. The Andromeda Galaxy is one of the few that appears to be coming closer to us. Read this article to get a sneak preview of what will be "the greatest show on Earth," a few billion years from now.

3. Look at the picture of Edwin Hubble. He's holding a piece of paper with a picture on it. Then look at the picture just below it. See the connection?