Unit 5: Galaxies and the Universe

This material was developed by the Friends of the Dominion Astrophysical Observatory with the assistance of a Natural Science and Engineering Research Council PromoScience grant and the NRC. It is a part of a larger project to present grade-appropriate material that matches 2020 curriculum requirements to help students understand planets, with a focus on exoplanets. This material is aimed at BC Grade 6 students. French versions are available.

Instructions for teachers ● For questions and to give feedback contact: Calvin Schmidt [email protected], ​ ● All units build towards the Big Idea in the curriculum showing our solar system in the context of the Milky Way and the Universe, and provide background for understanding exoplanets. ● Look for Ideas for extending this section, Resources, and Review and discussion ​ ​ ​ ​ ​ questions at the end of each topic in this Unit. These should give more background on ​ each subject and spark further classroom ideas. We would be happy to help you ​ expand on each topic and develop your own ideas for your students. Contact us at ​ the [email protected]. ​ ​

Instructions for students ● If there are parts of this unit that you find confusing, please contact us at [email protected] for help. ​ ● We recommend you do a few sections at a time. We have provided links to learn more about each topic. ● You don’t have to do the sections in order, but we recommend that. Do sections you find interesting first and come back and do more at another time. ● It is helpful to try the activities rather than just read them. ● Explore the “Ideas for extending this section” and “Resources” sections at the end of each topic in this Unit - they aren’t just for teachers!

Learning Objectives ● The BC curriculum requires students to learn about the “The overall scale, structure, and age of the universe.” This unit covers that information. ● To use ratios to estimate distances.

Learning Outcomes ● To put our Milky Way into perspective in terms of scale with other galaxies.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 1 ​ ● To understand that galaxies are not distributed uniformly: they come in clusters and there is large scale structure ● To understand that the Milky Way is one of trillions of galaxies ● To understand that spacetime is created between galaxies, causing them to move apart ● To understand that galaxies move, and sometimes orbit and collide ● To know that the universe has a finite age, and that this is determined in a couple of different ways that agree reasonably well ● To understand that galaxies vary in size, number of stars, dark matter content

Materials and tools needed for the activities ● Activity 1: ○ Two different sized coins (like a dime and a loonie) and a ruler (30 cm) ● Activities 2 and 4: Stellarium ○ You should make sure that you’ve installed Stellarium and know some of the basics as we’ve described in our Stellarium Introduction document. We will use it ​ ​ frequently in this unit. ● Activity 5: ○ 3 sheets of paper and pencil crayons

Time Required ● Lesson time - 90 minutes ● Activity time ○ Activities 1 and 5: 10 to 15 minutes ○ Activities 2 and 4: 10 minutes each ○ Activity 3: Minimum of 5 minutes

Contents The activities are marked in yellow. ​

● Is the Milky Way the whole Universe? ○ Activity One: Things that are different sizes can look like they are the same size ● Finding stars in the fuzz ○ Activity Two: Find the Andromeda Galaxy in Stellarium ● Not all galaxies are like ours ○ Activity Three: Help researchers classify galaxies ● Where are galaxies in the sky? ○ Activity Four: Find the Virgo and Coma clusters in Stellarium ● Do galaxies move? ○ Activity Five: Model the expansion of the Universe ● How many galaxies are there? ● How old is the universe?

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 2 ​ I s the Milky Way the whole Universe?

We learned in the previous unit how enormous the Milky Way Galaxy is both in terms of size and the number of stars in it. As we also learned, that would seem like it would be enough to satisfy anyone’s imagination.

The word for “everything there is” is “universe” or “cosmos”.

In 1920, one hundred years ago from when this unit was written, two famous astronomers by the name of Shapley and Curtis debated whether some of the dim, fuzzy blobs they saw in telescopes could be other Milky Ways or new solar systems. There were thousands of them, whatever they were. Astronomers called this the “The Great Debate” as answering the question would say important things about the universe.

Many of these fuzzy objects had been discovered with telescopes about 150 years earlier and astronomers kept wondering what they were. They referred to them as nebulous, meaning cloudy, as they couldn’t see any detail in them, although they could tell some were spiral in shape. They called these the “Spiral Nebulae”.

Figure 1: A drawing of a “spiral nebula” in 1845 by Lord Rosse before astronomers knew what they were. Was it a solar system forming or another Milky Way?

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 3 ​ It’s surprising now that astronomers were wondering in 1920 if they were looking at baby solar systems because no one knew back then that other solar systems existed. But in 1796, an astronomer and mathematician named Pierre-Simon de Laplace proposed that when solar ​ ​ systems formed they might look like cloudy swirls at first.

One thing that they knew would be obviously different between solar systems and galaxies was size: solar systems, like ours, are puny compared to galaxies. Were these fuzzy spirals something as small as a solar system or something as huge as the Milky Way? That led to the question of whether these were small things nearby or big things far away.

You might wonder why figuring out whether these fuzzy blobs were near or far was such a tricky problem. If an adult appears tiny you usually assume they are far away. That’s because there is a limited range in the size of people. You can’t have people 100 meters tall, for example, or 5 cm tall. They are probably going to be around 1.7 meters tall, on average for an adult. But what if you don’t know what it is? You can see in Figure 2 that two unknown objects can appear to be the same size when in actuality they are just at different distances.

Normally, as we saw in Unit 1 with the parallax thumb activity, we can tell how far away something is using our eyes. But with these fuzzy blobs there was no parallax effect, and because they were fuzzy it would be harder to measure than with a star. Even if they had been as close as nearby stars it would be hard to tell. “Close” in astronomy can still mean way out in the Milky Way, many tens or hundreds of trillions of kilometers away.

Figure 2: Top: Two fuzzy objects in the sky, from an observer’s perspective look like they are the same size. Bottom: The same two fuzzy objects are actually different sizes, but one is closer and the other is farther.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 4 ​ Activity 1 - Things that are different sizes can look like they are the same size

If we do know the size of something, we can measure the distance to it by measuring how big it looks. Let’s try that using coins. Make sure the coins are a noticeably different size, like a dime and a loonie.

Measure the size of each coin. You can use a ruler to measure the distance from one side of the coin to the opposite side. It will help to write down your measurements.

Place the bigger coin on a surface so you can look straight down on it. One end of the ruler should be near the coin and the other end near your eye (be careful). Close the eye furthest from the ruler. Now hold the smaller coin above the bigger coin to see how it can look bigger when it is closer to you, and look smaller when it is farther away. Figure 3 shows a picture of how we did this. If you hold it at the right distance from your eye, the smaller coin will appear to be the same size as the larger coin.

Figure 3: The bigger coin (loonie) on a table 30cm from our eye, and a dime held closer.

To find out what that distance is, we will need to do some calculations. A ruler is usually about 30 cm long, so we’re going to make that the distance (the ruler will point straight up from the big coin). If you divide the size of the coin by the distance to it, you will get a size-to-distance ratio. This new number tells you how big the coin looks from that distance. That means if another coin appears to be the same size, it will have that same ratio! In fact, you can see this at work in the diagram in Figure 2.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 5 ​ To find out how far away from your eye the small coin needs to be to appear the same size, we just take the size of the small coin, and divide it by the size-to-distance ratio. Now, when you hold the small coin at that distance you just calculated, you will see that it will appear to be the same size.

For example, if you used a loonie and a dime, and put the loonie 30cm away, you would have the following information written down:

Loonie size 26mm

Dime size 18mm

Loonie distance 300mm (30cm)

Then dividing loonie size by loonie distance ( 26 ÷ 300 ) gives:

Size-to-distance ratio (apparent size) 0.0867

If you want the dime to have that same apparent size, you use that ratio to find the distance from your eye. Divide the dime size by the apparent size ( 18 ÷ 0.0867 ) to get the dime distance:

Dime distance 207mm (20.7cm)

You can see in Figure 3 that the dime appears to be as big as the loonie. That dime is held roughly around the 20cm mark. Note how it is not important to be too precise.

You can try this with some other object too, like the lid of a jar, although you may need a longer ruler, like a metre stick or a tape measure. Measure its size, then divide it by the size-to-distance ratio. Will you have to hold that object closer to you than the big coin, or farther away?

Ideas for extending this section: ● Can you think of anything else that you had trouble judging the distance of by its size, e.g. birds that you don’t recognize?

Resources and References: ● Lord Rosse (Wikipedia) ​ ● Lord Rosse’s telescope (Wikipedia) ​

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 6 ​ Review and Discussion Questions: ● Why was it hard for early astronomers to know that the “spiral nebulae” were other galaxies loaded with stars? ● Calculate how many times bigger our Milky Way galaxy is than our solar system. Our solar system out to Pluto’s orbit is about 4.9 billion kilometers. That’s about 1/2,000th of a light year. The disk of our Milky Way is about 170,000 light years wide. Find the ratio in size. How many times bigger is our Galaxy than our solar system? ● A Canadian dime is 18 mm wide. If it represented the width of our solar system, how big would the Milky Way be, in kilometers? There are a million millimeters in a kilometer. If you put one edge on Vancouver, where would the other edge be in Canada?

F inding stars in the fuzz

Do you remember when we said in Unit 3 how Galileo discovered that the fuzzy clouds of the Milky Way were made up of stars when he looked at them with his little telescope? To figure out what galaxies were, astronomers in the 20th Century investigated whether the fuzz of the “Great Nebula in Andromeda” would also look like stars in a bigger telescope or whether they would just look like gas clouds in a baby solar system..

The astronomer on one side of the debate in 1920, Heber Curtis, thought he had spotted a known type of star in the Great Nebula in Andromeda that flared in brightness. But since it got as bright as the rest of the Great Nebula, the astronomer taking the other side, Harlow Shapley, said it must be something else as no star could be as bright as hundreds of billions of stars. They were both wrong: that can happen with a different type of star that no one knew about yet, and we call them supernovae. Even after the debate was over, no one was sure who won.

That’s why an astronomer by the name of Edwin Hubble - the person the Hubble Space Telescope was named after - went looking for a different type of star in the Great Nebula in Andromeda. Like Curtis, he picked that nebula to look at as it was the largest “spiral nebula” in our sky, about four times the appearance of our Full Moon in size, as he figured it might also be the closest one.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 7 ​ Figure 4: An 1899 photo “Great Nebula in Andromeda” / The Andromeda Galaxy (photo by Adam Isaac Roberts) All of the individual stars you see in this photo are in the Milky Way. The fuzzy disks are other galaxies

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To attempt this Hubble used the biggest telescope in the world at the time: the 100-inch Hooker telescope (named after the man who donated the money to build it, John D. Hooker). In 1925, just five years after the Great Debate, Hubble found some stars that were a special type called “Cepheids” (pronounced “Seph-eeds”) that would go dim then bright again in a regular pattern. Years earlier, the astronomer Henrietta Swan Leavitt (pronounced “Love-it”) discovered that these types of stars have a special property: Bright ones would change slowly, dim ones would change quickly. This is now called “Leavitt’s Law” after her.

Figure 5: Astronomer Henrietta Swan Leavitt

Hubble recorded how long it took to go from bright to dim and back to bright again, then used Leavitt’s Law to find out how bright they truly were. Comparing the true brightness to the brightness they appeared to be through the telescope allowed Hubble to measure the distance

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 8 ​ to these Cepheids, and thus the Andromeda Nebula. As we said in an earlier unit, it’s like seeing two streetlights at different distances: if they are the same type of streetlight the further one will be dimmer.

Using Leavitt’s Law, Hubble had discovered that the Andromeda Nebula was far away, and that it must be huge. It was a galaxy but with twice as many stars as the Milky Way. This galaxy is also the farthest thing you can see without a telescope. We now call the “Great Nebula in Andromeda” the Andromeda Galaxy.

Hubble realized that the thousands of spiral nebulae then known must also be distant islands of stars, many much further away than the Andromeda Galaxy. Each one contained so many stars that it would take a human being thousands of years to count them all up, just as with our Galaxy. The universe was much, much bigger than just the Milky Way.

While they were all in the same universe (remember that “universe” means everything there is), astronomers realized each one was so big and had so many possibilities that you could think of it as a universe. This led to galaxies poetically being called “Island Universes”, a name first suggested by Immanuel Kant in the 1700s. Hubble stuck with the name “nebulae” because that was what he was used to, and never called them galaxies like we do today.

A ctivity 2 - Find the Andromeda Galaxy in Stellarium

We’re going to find the Andromeda Galaxy in the night sky. As its name might hint to, this galaxy can be found in the Andromeda constellation. In Canada, it can technically be seen all year, but it is much easier to spot it during Fall. If you live in the southern hemisphere, you will only be able to see Andromeda from mid-Spring to early-Summer.

● Open Stellarium and set the month to October, and make sure it’s night. ● Then press F4 to open the “sky and viewing options” window. As usual, minimize light pollution and deactivate solar system objects so moonlight doesn’t affect the view. ● Close the window and activate constellation lines and labels using the bottom toolbar ● You should remember how to find the Cassiopeia constellation from unit 3 (if you don’t, simply press F3 and use the search bar), we will be using it to locate Andromeda. ● We will be using Caph and Shedar, the rightmost stars of Cassiopeia’s W, as pointer stars. Draw a line between the two and extend it from the bottom. Following that line, you’ll find the rather bright star Almach, which corresponds to the feet of Andromeda in the constellation. ● Follow the shape of the Andromeda constellation, you’ll see some sort of “branch” that starts from the star Mirach. Let it guide you and you should see a disk shape at the end of it: it’s the Andromeda galaxy! ● If you’re having trouble with this method, Figure 6 should help you follow along.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 9 ​ ● Select your newfound galaxy and zoom in on it to see it with much more detail. Can you find Andromeda’s two largest satellite galaxies ? They’re called M32 and M110 and you should be able to spot them rather easily. ● Reactivate solar system objects and play around with your location and time to get the Moon as close as you can to Andromeda. Then zoom on the Moon until you can’t see it shining anymore and measure it with a ruler. ● Without zooming in or out, move your view to Andromeda and measure it as well. You’ll find that it’s about 4 times bigger than the Moon! You can also see a comparison in a link below in the Resources and References.

On moonless fall nights, you can spot the Andromeda galaxy in the sky with your naked eyes (if you’re far from city lights of course) using the method explained above. And if you’re using binoculars or a small telescope, you could even find M32 and M110

Figure 6: Finding the Andromeda galaxy (circled in red) in the night sky

Ideas for extending this section: ● The type of star that Hubble used is called a Cepheid variable star. You already know of one such star: Polaris, the North Star, is a Cepheid. Learn more about Polaris and find it in Stellarium. Click on Polaris in Stellarium to find out how far it is. Now find Delta Cephei, the star “Cepheids” are named after. ● In this activity, we used the Cassiopeia constellation to find the Andromeda galaxy, but some people prefer to use the Square of Pegasus to find it. Read the EarthSky article in the Ressources below to learn how to do it.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 10 ​

Resources and References: ● A modern image of M51 (Canada France Hawaii Telescope) ​ ● A virtual flyby of M51 Made with modern images, we fly over the galaxy that Lord Rosse ​ sketched (NASA) ● The Andromeda Galaxy with the Moon next to it for scale (how big they look) ​ ● Cepheids (StarChild NASA) ​ ● Use the Square of Pegasus to find Andromeda (EarthSky) ​ ● Zooming in on the Andromeda Galaxy video (NASA) ​ ● Zooming in on the Andromeda Galaxy (NASA) - this is a photo you can zoom in on to ​ see individual stars in the galaxy. Try zooming in on different areas. ● Henrietta Swan Leavitt biography

Review and discussion questions: ● Read the biography of Henrietta Swan Leavitt. What do you think it was like for her working as an astronomer back then as a woman, and as a person who was also deaf? ● If you were to represent the Milky Way and Andromeda galaxies by dinner plates, how far apart would they be? Hint: find the ratio of the distance to Andromeda, 2.2 million light years, to the diameter of the Milky Way - we’ll use 200,000 light years. Now measure the width of one of the plates. Multiply the width of a plate representing the Milky Way by the number representing the ratio you just calculated in order to find how far the plate representing Andromeda should be.

N ot all galaxies are like ours

Hubble started to compare the galaxies and found that while some were spirals, many were not. Later astronomers learned that some types of galaxies don’t have as much gas and dust, while some had more, and that the amount of dark matter also varies. As you might remember, Dark Matter is matter that has mass but which isn’t visible. It doesn’t block light like dust or reflect or give off light: it’s invisible.

Some galaxies don’t seem to contain any new stars, unlike our Milky Way, which is still making new ones. In Unit 4 we talked about how the spiral arms of the Milky Way stand out because that’s where batches of new stars are, and groups of new stars have many bright blue stars among them. When you look at spiral arms, notice how blue they look. The colour of the star clouds of a galaxy tells you something about whether it is still making stars.

Galaxies are split into different types based on what they look like. There are spiral galaxies; The Milky Way, and Andromeda are both spirals. While some have many arms, some galaxies can have as few as two. In Unit 4 we learned that the Milky Way has a football-shaped bulge, which makes it a type called a barred spiral. Barred spiral galaxies are slightly more common

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 11 ​ than spiral galaxies without bars. Figure 7 shows a spiral galaxy in the left image, and a barred spiral in the right image.

Figure 7: The Pinwheel spiral galaxy (left), and the NGC 1365 barred spiral (right) (courtesy CFHT)

Elliptical galaxies are those that don’t have arms. They aren’t making any stars, and are ball shaped, rather than being a disk. Some are round and others are shaped more like a football or rugby ball. These galaxies may look boring without those bright blue star-forming arms, but some of them can be 10 times the size of the Milky Way, and contain hundreds of times more stars. Figure 8 shows the elliptical galaxy Messier 60. It is twice as big as the Milky Way.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 12 ​ Figure 8: Elliptical galaxy Messier 60 (courtesy CFHT)

Irregular galaxies are weird. They don’t have much shape to them. Some have just the tiniest inkling of a spiral structure, suggesting that they might be spiral galaxies that got distorted by another nearby galaxy, like the Large and Small Magellanic Clouds. Like those two galaxies, most irregular galaxies are smaller than the Milky Way. Figure 9 shows an irregular galaxy about a third the size of the Milky Way.

Dwarf galaxies are the smallest, only containing hundreds of millions of stars. They can have qualities of the other types mentioned. The Magellanic Clouds are often also considered dwarf galaxies (see unit four for photos).

Figure 9: IC 4710 irregular galaxy (courtesy Wikipedia)

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 13 ​

Figure [size-comparison]: Size comparison of some different galaxies

A ctivity 3 - Help researchers classify galaxies

Computers have trouble accurately identifying the type of a galaxy in an image, and because of that it would take scientists forever to classify all of them by themselves. They have millions of galaxy pictures to classify, so they need all the help they can get. You can help!

After reading this section, you have enough knowledge to identify galaxies. That means you can participate in the citizen science project called “Galaxy Zoo” on Zooniverse. In the Galaxy Zoo you sort galaxies by their shape. It’s a great way to develop and make use of your knowledge of the types and properties of galaxies.

● Follow this link to the Galaxy Zoo project. ​ ​ ● If you want to read more about the project before getting started, there’s information about why scientists need this data to be classified, how the galaxy images are taken, as well as a link to scientific papers that have been published thanks to this project. ● Go back to the previous page when you’re ready to classify galaxies. You’ll have the choice between two modes: “classic” and “enhanced”. ● If you choose “Classic”, you will be given a random picture among all the galaxies that need to be identified. “Enhanced” will let you see the galaxies that robots have the hardest time classifying, so scientists need more help with these.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 14 ​ ● Whichever you decide to choose, when you start classifying, a tutorial will pop up and will explain how this works and some basic principles such as “the answer may not ​ always be obvious - just take your best guess” ● You will see a picture of a galaxy on the left of your screen, and some characteristics to choose from on the right. Choose the feature that best matches what you see and click “next”. ● As the tutorial should have said, if you’re unsure about a certain characteristic of the galaxy, click on “Need some help with this task?” to see examples or open the “Field guide” on the far right of your screen which can help you with questions that are often difficult.

Figure 10: Screenshot of the Galaxy Zoo interface, you can access the Field Guide on the far right of the page

Ideas for extending this section: ● Learn more about types of galaxies and discover Hubble’s classification of galaxies in the “Types of galaxies” document in the Resources below. ● Try classifying galaxies in this excellent study experiment from the Sloan Digital Sky ​ Survey.

Resources and References: ● Types of galaxies (nineplanets.org) ​

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 15 ​ ● Galaxy size comparison (Universe Today) ​

Review and discussion questions: ● Elliptical galaxies have no blue stars. What does that say about what’s going on in those galaxies (or rather, what isn’t going on)? ● What are the three main types of galaxies? What are some of the different properties that they each have?

W here are galaxies in the sky?

There are both good and bad places to look for galaxies in the sky. In fact, astronomer Richard Proctor noticed in 1878 that the “nebulae” we know today as galaxies weren’t found near the Milky Way in our sky, and said this was the “Zone of few Nebulae”. This later came to be called the “Zone of Avoidance”.

In Unit 4 on the Milky Way we discussed how dust was scattered around in the plane of the Galaxy, and our Solar System is in the plane, so as we look around we see that dust blocks our view of stars in the Milky Way. But the dust also blocks our view of anything behind it, including galaxies, at least with the kind of light our eyes use.

In figure 11 you can see a map of 30,926 galaxies. Note that there is a big blank area along the middle where no galaxies are found. Now compare this to the map of stars in the Milky Way from Unit 4, which we’ve reproduced here for comparison, and look at how well they match up: galaxies are found where we don’t see things in the Milky Way.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 16 ​ Figure 11: Map of 30,926 galaxies in our sky showing Zone of Avoidance (Harvard Smithsonian Center for Astrophysics)

Figure 12: Map of stars and dust in the Milky Way for comparison (GAIA/ European Space Agency)

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 17 ​ Astronomers have been able to detect some galaxies in the Zone of Avoidance, though, using infrared and radio waves instead of visual light telescopes.

Figure 13: A nearby galaxy hidden by the dust of the Milky Way, visible in infrared light. This is in the constellation Cassiopeia (Spitzer Infrared Space Telescope/ NASA)

If you want to see galaxies, the best thing is to look away from the Milky Way and its dust. You’ll need a telescope for all but a couple of galaxies. Summertime in Canada is not the best time to see many galaxies as the Milky Way is overhead.

Let’s assume you are looking away from the Milky Way, say in the springtime in Canada when the Milky Way is on the horizon and we are looking out into deep space when we look up. Are galaxies scattered evenly across the sky?

Most galaxies are found in groups, called clusters. Some clusters are loose and don’t have many galaxies in them, while others have thousands of galaxies and the galaxies orbit and run into each other more often. In springtime in Canada there are two large clusters of galaxies overhead, in the constellations Virgo and Coma Berenices.

The Virgo Cluster contains about a thousand galaxies and is 50 million light years away. You can see some of the brighter galaxies in binoculars on moonless nights in light pollution free sites in the spring.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 18 ​ Figure 14: The Virgo Cluster (NASA)

Our Galaxy is part of a smaller group of galaxies (called the Local Group) that, along with the Virgo Cluster, are part of something called the Virgo Supercluster.

There are many remote clusters of galaxies. There’s one not far from the handle of the Big Dipper called the Coma Cluster, called that because it is in the constellation Coma Berenices (“Bernices Hair”). The Coma Cluster is about six times as far as the Virgo Cluster and also has about a thousand galaxies. Figure 15: The Coma Cluster

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 19 ​ Figure 16: A distant cluster of galaxies 1.5 billion light years from us. Abell 3827 (Gemini ​ Telescopes) ​

A ctivity 4 - Find the Zone of Avoidance and some galaxy clusters in Stellarium

● Open Stellarium. As usual, minimize light pollution and deactivate all solar system objects to get a better view of the sky. ● In the sky and viewing options, under the DSO tab, drag the “Labels” sliding button all the way to the left, and the “Hints” one all the way to the right. There are now markers for all the deep sky objects, in the catalogues, but no labels to avoid crowding the view. ● Now go in the “Markings” tab and display the galactic equator, as well as the galactic poles. ● Close all windows and deactivate constellations and their labels, then look at the sky. You’ll notice there are a lot less galaxies (which are marked in red) near the galactic equator. That’s the Zone of Avoidance ! ● Now we’ll go looking for the Virgo and Coma clusters. First, reactivate constellations and their labels, and bring back the DSO “hints” and “labels” buttons to their usual position (restart Stellarium if needed).

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 20 ​ ● The clusters we will be looking at are near the Virgo and Coma Berenices constellations, as their names indicate. These constellations are higher in the sky in the month of April, so change the date accordingly. ● These galaxy clusters are really faint and hard to see, so press “S” on your keyboard to deactivate the stars, (and “D” to add hints to find deep sky objects if you restarted the program 2 steps ago). ● Zoom on Coma Berenices, it’s the closest constellation to the North Galactic Pole. You should already be able to see a few galaxies such as the Needle galaxy (also called Berenice’s hair clip). If you look even closer, near the top right of the pyramid shape of the constellation, you’ll find the Coma cluster. ● If you’re having trouble locating it, use the search window (F3) to find it. Don’t forget to press “D” again to remove the labels if you want to admire those galaxies, ● To find the Virgo cluster, look at the area of the sky that’s between the Virgo and Leo constellations. ● You can’t use F3 to search for the Virgo cluster as it isn’t classified as one object on Stellarium, but if you search for the “Virgo Galaxy” you’ll land right inside the cluster.

Ideas for extending this section: ● Read about the constellation Coma Berenices. Notice that it is not only the area in the sky where there is a supercluster of galaxies, but it is also where the north pole of our Milky Way is in the sky. What do non-western cultures call this constellation? Read about these cultures if you are not already familiar with them.

Resources and References: ● Zone of Avoidance (Wikipedia) ​ ● Quasar and a deep galaxy field (included as this image always blows my mind) ​ ● Abell 3827 Cluster ● Coma Berenices

Review and discussion questions: ● If the Virgo cluster is 50 million light years from us and the Abel 3827 cluster is 1.5 billion light years from us, how many times farther is the Abel 3827 cluster? Hint: find the ratio of the distances.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 21 ​ D o galaxies move?

Galaxies are so big and so far away that it’s impossible to see one move. But by studying the light they emit, astronomers were able to show that they do move around. Galaxies that are located in the same cluster feel the gravity of each other. Big galaxies like the Milky Way often interact with their satellite galaxies, and it often makes the spiral pattern look more defined. .

Collisions between two galaxies are spectacular, and they are most likely to collide if they are neighbours in a group or a cluster. It can lead to the formation of a bigger galaxy; this is called a galaxy merger. When galaxies collide they can cause new stars to form because the gas clouds inside combine to create star-forming regions. It is thought that most elliptical galaxies we see were formed by the collision of two smaller galaxies of similar sizes, which quickly used up the gas clouds by turning them into stars.

Astronomers have estimated that the Milky Way and Andromeda will collide in about 4.5 billion years. As both galaxies are about the same size, they think their collision will form a giant elliptical galaxy, which has been nicknamed Milkomeda or Milkdromeda. You can see a simulation of that collision in the Resources section below. ​ ​

Figure 17: Two spiral galaxies colliding (NASA)

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Figure 18: Galaxies in a cluster after several collisions - NGC 4410 (CFHT)

Edwin Hubble’s other big discovery was that galaxies that are not in clusters are all moving away from each other. This is different than simply moving in opposite directions, though: that’s because there is more space being made between the galaxies. The galaxies each feel like they are at rest. This is called the expansion of the universe. This discovery even seemed strange to Hubble after he found evidence that space is doing that, although Albert Einstein’s General

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 23 ​ Theory of Relativity explained how it could happen. The Hubble Space Telescope was used, in part, to try to improve measurements of how fast space is expanding.

A ctivity 5 - Model the expansion of the Universe

Required tools and materials : ● Three sheets of paper ● Pencil Crayons

First, take two sheets of paper and draw as many galaxies as you want (try to have at least five on each paper). Be creative: draw galaxies of every type you know. If you want you can even try to reproduce some of the images you saw in Activity 3. You can even try drawing two galaxies colliding, and merging!

Place the third blank sheet down on a surface. Place the two sheets with galaxies on them side-by-side on the blank one. Pretend this is somewhere in the universe: the paper is space, and the galaxies are, well, galaxies in that space. Space is expanding, so slowly move the two sheets of paper with galaxies on them away from each other.

There are two things to notice: First, the galaxies aren’t moving through space (you drew them right on the page, they can’t move!), and yet they are getting farther from the galaxies on the other page. Second, there is more space now, that third blank sheet is empty space.

So the two groups of galaxies aren’t truly “moving”, but they are getting farther apart. The galaxies within one cluster are not moving away from each other, though: They are held together by gravity, as you learned in this section. Although, in reality, they should still be moving around each other on that same page, just not getting super far apart.

If you keep pulling the clusters apart you will eventually run out of space; you will have moved past that third piece of paper to reveal the surface below. But what would happen in reality? Would it keep making more space, or would the expansion eventually stop? Read the section “How old is the universe?” to find out what scientists know so far.

Ideas for extending this section: ● You might be wondering what will happen to the solar system when Andromeda and the Milky Way collide. Read the “Andromeda-Milky Way collision” Wikipedia article in the Resources below. ● As an alternative to the paper model we mentioned, make an expanding universe on a ​ balloon. This is a well-known experiment, but note that galaxies don’t actually get bigger ​ due to the expansion of the universe, nor do clusters: gravity has a bigger effect than the expansion within galaxies and clusters. Please have a look at page 6 of this document, ​ ​

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 24 ​ which gets around this problem by gluing small bits of cotton on a balloon (the document is in french but you can just look at the photos if you don’t read french).

Resources and References: ● Look at a modern image of M51, the spiral nebula that Lord Rosse sketched that is ​ actually two galaxies colliding. ● Collision of Andromeda and the Milky Way (Video simulation) ​ ​ ● The Andromeda-Milky Way collision (Wikipedia) ​ ● Two spiral galaxies colliding (NASA) ​ ● Interacting Galaxies (Gemini Telescopes) ​ ● Starburst after galaxy collision (Hubble) ​ ● Hubble captures colliding galaxies A combination of simulations and actual images. ​

Review and discussion questions: ● How is the expansion of space different from a firecracker going off?

H ow many galaxies are there?

Astronomers knew by the mid 1980s that there were clusters of galaxies, but no one had made a complete map. The situation was as if people had made maps of Canada showing the major cities and a few other towns, but everything else was left blank.

Astronomer Margaret Geller is a pioneer in filling in that map.

Figure 19: Astronomer Margaret Geller (Harvard)

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 25 ​ The first map, made in1986 and shown in Figure 20, represents a small slice of the universe, sort of like cutting out a wedge of cheese (Figure 21), with us at the pointy part of the slice. The map is about as wide as your field of vision if you look up at the sky. Each yellow dot is a galaxy. Our own galaxy, the Milky Way, is the lowest dot. Think about that for a second: our entire Milky Way, if shown in an alien map from another galaxy, would just be a dot in this image.

How much of the universe is this? It’s about the same as the size of Greater Vancouver compared to the surface of the entire Earth. They’ve made more complete maps since then, which you can see in the References and Resources area below.

The Coma Cluster you learned about earlier is the group of dots in the middle, and the shape it makes has been named “The Stickman”. The furthest galaxies in the image are the ones along the top edge.

Notice that there are big areas with few galaxies. You can see bunches of galaxies but also voids where there aren’t any galaxies or even dark matter (and there is no dust blocking our view of those areas, in case you are wondering). These gaps in this image were a big surprise to the astronomers who made it, especially when you consider that the empty areas are bigger than large clusters like the Virgo Supercluster.

Figure 20 : The first good map of the universe in 3D (Harvard)

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 26 ​ As you can see, there are a lot of galaxies out there. If you stack other slices on this you can see that it is like galaxies are on the edges of bubbles. That’s more obvious in the videos we linked to, or if you use our Swiss Cheese example.

Figure 21: The universe is like Swiss Cheese. Galaxies are where the cheese is, with empty areas scattered around.

To try to figure out how many galaxies there are, astronomer Robert Williams used the Hubble Space Telescope to take many pictures of a single spot in the sky near the Big Dipper. He chose that spot because it was near the North Pole of the Milky Way, meaning that the telescope would look straight out of our Galaxy and have less dust in the way.

The piece of sky he looked at was tiny, much narrower than this map, but it stretched much further into the universe. The first “Stickman” map made by Dr. Geller stretched out about 600 million light years, but the one Dr. Williams recorded galaxies out to 12 billion light years, about twenty times as far as Dr. Geller's original map was made 15 years earlier (Dr. Geller made much deeper maps since the 1980s, and we’ve linked to some below).

Astronomers thought the picture might be blank, or show one or two galaxies, but they were surprised to see that this tiny piece of sky showed over 3,000 galaxies,

Astronomers later repeated this study by taking an even longer exposure of a different part of the sky, in the southern constellation called Fornax. It was a slightly tinier piece of sky. To get an idea of how tiny it was, look for the square near the Moon, in Figure 22. Hold a ballpoint pen up at arm's length. The piece of sky the ball covers is the same size of the piece of sky they photographed: very tiny.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 27 ​ Figure 22: Size of the Extreme Deep Field (NASA)

The Hubble Space Telescope took a total of 23 days to gather up enough light to make the photo shown in figure 23. It contains about 5,000 galaxies: almost everything you see is a galaxy, each containing billions of stars like our own Milky Way. This one stretches out further than the original deep field image: it goes out to 13.2 billion light years. Some of the galaxies even look like our Milky Way: see if you can find one. Look at the galaxy you’ve picked and imagine that it takes 10,000 years or more to count up all the stars in it.

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Figure 23: The Hubble Space Telescope’s Extreme Deep Field image

If that’s just a tiny piece of the sky, how many galaxies are there? Deeper photos show 10,000 or more galaxies. If we wanted to repeat this photo for the whole sky we’d have to take 32 million more photos. If they all contained a similar number of galaxies, as we think they do, then that would mean there would be 160 billion galaxies, roughly as many galaxies as there are stars in the Milky Way Galaxy. But some studies by astronomers suggest that with better telescopes we’ll see ten times as many galaxies. That would mean there are 2 million times a million galaxies (2 trillion galaxies).

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 29 ​ That means there are more stars in the universe as there are grains of sand on all the beaches of planet Earth. And as we’ll see in the next few units, because planets are so common, then that could mean that there are more planets out there than all the sand on Earth.

Ideas for extending this section: ● Listen to the Hubble Deep Field. If you put your pointer over a galaxy you will hear a low ​ or high tone. The low tones mean the galaxy is further away than the high tones. What do you hear when you find a star in our own Milky Way? Are the galaxies that look small always far away? What does that mean about the range of galaxy sizes? This may not work in all browsers but we tested it in Chrome and Firefox. ● The Millennium Run Simulation shows how galaxy clusters and voids form. You must ​ ​ download the video clips to watch them, but they are fascinating. For example the video called “color scheme 1” shows a timeline of the simulated universe, starting with matter being spread out, and ending with a pattern you might see in Figure [Geller].

Resources and References: ● Margaret Geller (Wikipedia) ​ ● How to map a galaxy’s position - with Margaret Geller (PBS) ​ ● A recent 3D map of the universe (Wikipedia) ​ ● Flying through the Universe (shows the empty areas between groups of galaxies) ​ ● Laniakea: our local Supercluster (video) ​ ​ ● Deep Field (Whitacre) The Hubble Deep Field image inspired a beautiful film and piece ​ of music, bringing together scientists, filmmakers, musicians, and thousands of singers from all over the world. ● The Extreme Deep Field (NASA) ​ ● Atlas of the Universe Starting with the nearest stars, zoom out to see the entire ​ observable universe.

Review and discussion questions: ● Are galaxies spread evenly throughout space?

H ow old is the universe?

Early scientists had differing ideas as to the age of the universe. Some believed that it is infinitely old and never had a beginning, while others thought it could have a beginning and an end. There was very little evidence available to them, until Hubble’s discovery. If the galaxies are moving away from each other, then they were closer in the past. At some point in the past, all the galaxies (and thus all matter in the universe) were in one location. This is evidence that the universe had a beginning. Originally, astronomer Fred Hoyle, who thought the universe had been around forever, was making a joke when he called it “The Big Bang”, but the name stuck.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 30 ​ How old the universe is now depends on how fast space is expanding. If the universe expands quickly, then the time since all the galaxies were in the same spot is short. If it expands slowly, then the time is longer. That’s one of the reasons astronomers measured how fast the universe is expanding. Using those measurements and other methods they estimate it is about 13.8 billion years old. That means the universe existed for about 9.3 billion years before our Sun, Earth, and the rest of our Solar System formed.

An important thing to do in science is to check that a discovery actually makes sense with reality. For example, imagine if we found a star that is older than 13.8 billion years. It would likely mean that either our estimate of the age of the universe is incorrect or the estimate of the age of the star is incorrect. This is one of the reasons astronomers have been looking for the oldest globular star clusters. Stars live different lengths of time depending on their mass, as we learned, so the astronomers try to find which stars that were born together are still shining. Based on that work, they come up with an age where there are no globular star clusters older than the universe. The oldest stars are only 70% (7/10) as massive as our Sun.

Figure 24: Globular cluster NGC 6397, estimated at 13.4 billion years in age (courtesy HST)

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 31 ​ So we are pretty sure the universe had a beginning, and we think we know the current age. But will the universe end? Or go on forever? It is possible that the expansion of the universe will slow, and gravity will pull everything back together again in what is called a Big Crunch. It is also possible the universe keeps expanding forever, which is what astronomers currently think. Those galaxies that are not held in the Milky Way’s cluster will eventually get so far away that we will never be able to see them. Over time, stars would die out, leaving little matter to form new ones. This fate would be called a Big Chill, as there would be no stars to produce much heat. But that wouldn’t be for a very long time.

Ideas for extending this section: ● Try making a Cosmic Calendar. January 1st corresponds to the birth of the universe. Add in significant dates using the links below.

Resources and References: ● Cosmic Calendar (from Cosmos: Possible Worlds) ​ ● Cosmic Calendar (Wikipedia) ​

Review and discussion questions: ● Would you prefer that the universe expands forever or not? (There is no right or wrong answer).

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