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.