Unit 6: The Birth of Stars and

Planets

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 “How has the solar system changed over time (birth, formation of planets).” This unit covers that information. ● Math objectives:use ratios.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 1 ​ Learning Outcomes ● Students will learn about the formation of solar systems: what they form out of, how long it takes, the theory about it, and see images of solar systems at different stages. ● What our own solar system formed out of and how it has evolved over time.

Materials and tools needed for the activities ● Activities 1 and 2 : Stellarium installed

Time Required ● Lesson time - 90 minutes ● Activity time ○ Activities 1 and 2 : 5 minutes each

Contents The activities are marked in yellow. ​

● Stars and Planets have a beginning ● Early ideas about the formation of solar systems ● Clouds where stars and planets form ○ Activity 1: Find star forming regions on Stellarium ● Protostars and Proplyds ● Making planets ● Why do different types of objects form? ● The nebula becomes a star cluster, and star systems wander away ○ Activity 2: Observe the differences between open and globular star clusters in Stellarium ● How a star system changes over time (Summary)

S tars and Planets have a beginning

In this unit you are going to learn about the formation of solar systems, and, more generally, about stars and planets.

By solar system we mean something like our own solar system with at least one star and planets going around the star (or stars). At the time of writing (August 2nd, 2020), astronomers know of 3,176 solar systems other than our own, and new ones are being found all the time. Were they always there? Where did they come from? We’ll show you what we know so far.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 2 ​ In our own solar system we have one star, the Sun, and eight major planets and five smaller dwarf planets like Pluto. But in some solar systems, the planets orbit dead stars, two stars, or one star in a binary star system. There are even “rogue planets” that do not orbit stars at all and move by themselves through the Galaxy. We’ll talk about some of those combinations in upcoming units. When we talk about solar systems we’ll be referring to at least one star and a planet.

E arly ideas about the formation of solar systems

In the previous unit (Unit 5 - Galaxies and the Universe), we talked about how the 18th-century mathematician and astronomer, Pierre-Simon Laplace, had proposed how solar systems were formed. He was trying to understand why there are planets orbiting our Sun and how they got to be there.

Astronomers saw mysterious swirly objects in telescopes and realized they looked something like what Laplace had proposed, objects they called the Spiral Nebulae. Hubble showed in 1925, however, that the spiral nebulae were galaxies similar in size and composition to the Milky Way. Astronomers still hadn’t seen evidence of how a solar system forms.

Laplace’s ideas were based on his understanding of Sir Isaac Newton’s theories of gravity that were published 110 years earlier in 1687. One of his most important realizations was that gravity existed between all types of matter.

Gravity’s pull also occurs between matter that is not in a solid state, meaning things in a gaseous or liquid state. The Sun, for example, is in a gaseous state and it certainly has a strong gravitational pull.

Laplace was correct that even clouds create and experience a gravitational pull because they are composed of matter, just as Newton explained. Each atom or molecule in a cloud has its own gravitational pull, and pulls on all the others in the cloud. If a cloud is big enough, he reasoned, that should start pulling all the pieces together.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 3 ​ Figure 1: Gravity would pull all pieces of the cloud to the centre if the cloud wasn’t turning (credit: FDAO)

Laplace also understood that the pieces of the cloud were likely moving around, and that it could be turning slightly. As one collapsed down it would start to spin faster. If you’ve ever seen a figure skater pull his or her arms in as they start spinning you know what happens: they start spinning faster. If you’ve ever spun around in a chair, you’ve probably noticed that you start spinning faster if you pull your arms in (try it yourself but be careful). Similarly, as the materials of the solar nebula were being pulled towards its center, the nebula started spinning faster.

Most of the material would fall to the centre and become a star, this is why almost all the mass of our own solar system is in the Sun. But because the cloud is spinning slowly some parts don’t fall there and instead spread out into a flat disk. Why would some matter go into orbit?

One of the things Newton imagined was shooting a cannonball horizontally. You can see his drawing in Figure 2.This is Newton’s famous “Thought Experiment”. He didn’t actually do the experiment, he just predicted what would happen. If the cannonball wasn’t fired with much speed it would quickly fall to the ground. But if it was fired faster it would go farther before it hit the ground. Eventually it would be fired so far it would land out of view over the curve of the Earth. And if you fired it hard enough from a mountaintop, it would go around the Earth and never come down (assuming air resistance wasn’t there and no mountains were in the way). It would then be in orbit. Every time it is fired it is still pulled by the Earth’s gravity, but eventually it is fired so hard that it never lands on the Earth, even though it is still falling.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 4 ​

That’s the same thing that happens to astronauts when they are “weightless” in orbit of the Earth. They are falling in the Earth’s gravity but are going so fast sideways that they always “miss” the Earth. That’s what being in orbit means.

Figure 2: Newton’s cannonball thought experiment

If the matter in the cloud didn’t fall directly into the star that was forming it would have some horizontal speed (at a 90 degree angle to the pull of gravity), and if it has enough speed would go into orbit around the new star, just like Newton’s imaginary cannonball went into orbit around the Earth.

This motion at a right angle to the gravitational pull is also the same thing that keeps the Moon in orbit around the Earth, and the Earth in orbit around the Sun.

Another way to think of it is a spinning ball. The points located near the equator of that ball will move much faster than those near the poles. If you have an Earth globe at home, try spinning it and look at different countries. Greenland will spin much slower than India for example. In a similar fashion, the matter near the poles of the solar nebula moved much slower than the equator. Just as with a slower moving cannon ball, matter near the poles of a spinning cloud has more trouble escaping the gravitational pull and falls into the middle. Thus, the nebula flattens into a disk, with most of the material in the centre.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 5 ​ Ideas for extending this section: ● Learn about Sir Isaac Newton. What else did he accomplish?

Resources and references: ● Sir Isaac Newton ● How objects spin faster when ‘collapsed’ (watch from 1:35 to 3:18) ​

Review and discussion questions: ● Newton’s idea of shooting cannonballs was something people could relate to in the 1600s. One problem with his idea is that the speed the cannonball would have to go to be in orbit just above the surface of the Earth would be so fast that it would burn up in our atmosphere. This is why orbiting spacecraft have to be above our atmosphere. But rockets must also go around the Earth quickly. Why do you suppose rockets are usually launched from a location closer to the Equator of the Earth than the North Pole? Think about our globe example. ● Why don’t clouds in Earth’s sky collapse and form stars? Give as many reasons as you can.

C louds where stars and planets form

Let’s take a look at the clouds where this happens.

In our unit on the Milky Way Galaxy we saw that there are big clouds of gas and dust in space that astronomers have nicknamed “Stellar Nurseries” as that’s where you’ll find baby stars. These clouds contain mostly hydrogen and helium from the birth of our universe some 13.8 billion years ago, but also contain material from previous generations of stars that died. The universe was already 9.2 billion years old when our solar system started to form!

These stellar nurseries, called molecular clouds, can be hundreds of light years across and have as much mass as a million Suns. They are often uneven in density inside, and if it starts to become dense enough those areas can start pulling together other bits of gas and dust to start forming a new star system.

In the earliest stages, before the stars start shining, the clouds can be very dark and dense. In the photo of the Running Man Nebula in Figure 3, you can see there are dark regions we can’t see through. The bright pink/red parts are where a baby star has lit up brightly. You can see in the zoomed-in image, that a picture taken from the Spitzer space telescope in 2004 shows a very dim star, but a picture taken in 2008 shows that same star became as bright as its neighbour! The pink/red colour in the images is infrared light, something we couldn’t see with a normal telescope.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 6 ​ Figure 3: Running man Nebula in the Orion Molecular Cloud Complex showing a new star appear. (courtesy JPL NASA)

Once stars start to shine within them the whole gas cloud lights up, typically with a red colour due to hydrogen gas. In the photo of the Whirlpool Galaxy in Figure 4 you can see these red star-forming gas clouds along the spiral arms close to star clouds that include many bright blue stars. A photo of our Milky Way from a distance would also show many red star forming regions.

Figure 4: The Whirlpool Galaxy, M51 (courtesy Hubble Space Telescope/NASA)

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 7 ​

A ctivity 1 - Find star forming regions on Stellarium

● Open Stellarium and go through the usual steps to minimize light pollution (see activities 2 and 3 from Unit 3 if you have forgotten how to do it). ● Disable the ground to avoid having to change the date every time you look for a different object. ● We don’t want to mix up the newly forming stars with other stars that just happen to be in the way of the nebulas, so press S to deactivate all the stars in the program. ● Press F3 and search for the Great Orion Nebula, as the name might suggest, you’ll find it inside the Orion Constellation. This nebula is a very popular deep sky object among amateur astronomers as it’s rather bright and can be spotted with binoculars or a small telescope if the sky conditions are good. ● Next we’ll be looking at the Eagle and the Carina Nebulae, you’ll find them close to the Scutum and Carina constellations respectively. ● Look for bright spots within these clouds of dust, that’s where the newly formed stars are located! ● Here’s a Wikipedia article on Molecular Clouds (the clouds from which solar systems ​ ​ form). Pick a few names from the article, read more about these clouds and look them up on Stellarium.

Ideas for extending this section: ● Look at more spiral galaxies (googling them is fine) to look at star-forming regions. Include the word “galaxy” to make sure you look at galaxies. ○ NGC 3982 galaxy ○ M81 galaxy ○ M83 galaxy ○ Triangulum Galaxy

Resources and references: ● What is a nebula? (NASA Space Place) ​ ● Star Formation (Wikipedia) ​

Review and discussion questions: ● What colour are the star-forming clouds when bright new stars light them up? ● Why do big groups of new stars tend to look blue?

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 8 ​

P rotostars

When the star is still forming out of the cloud - before it has started to shine - we call it a “protostar”. Material is still raining in on it and the disk is forming around it. This can be a very turbulent time in the formation of the star. As the gas and dust falls there are collisions and the protostar and disk get hotter.

As the material swirls around the baby solar system the new solar system can develop a strong magnetic field. Some of the hot, swirling material can be shot out of the north and south pole above the stars, creating huge streamers into space. You can see just such a “jet” of material, shown in red, shooting into space from a protostar and disk in Figure 5. The disk is in silhouette and is seen from the side so it looks like a dark line in the middle of the green area. The protostar can’t be seen and is where the dark line of the disk and red line meet. Astronomers think this might have happened with our solar system about a million years into its formation.

Figure 5: A jet from a newly forming star system

In figure 6 you can see two protostars - a binary protostar - that are blowing huge amounts of material into space. The binary protostar is at the bottom left. Look for the animation in the resources and references section.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 9 ​

Figure 6: A binary protostar (bottom left) blowing material into space

Once stars start igniting and blowing away excess gas and dust you can see what’s inside better. Figure 7, the Orion Nebula, which you saw earlier, is at that stage. It’s only about 24 light years across, so it isn’t a big one, but it is relatively close to us. At first glance, it’s hard to see what’s happening there.

Astronomers finally confirmed that baby solar systems looked like what Laplace proposed when they turned the sharp vision of the Hubble Space Telescope on the Orion Nebula. They found many objects there that they named “Proplyds”, short for “Protoplanetary Disk”, shown in the ​ ​ ​ ​ ​ ​ close-ups of Figure 7. We see this cloud lit up as there are bright stars inside causing the whole cloud to shine. Astronomers have found 180 proplyds there. It’s important to notice that solar systems are born in batches. There are typically dozens or thousands of new solar systems born out of the same cloud. This means that it is likely that there were many solar systems that formed out of the same cloud our own solar system formed in.

Some of the proplyds were discovered to be tear-drop shaped as dust and gas was blowing past the disk where the new star system is. You can see that better in figure 8.

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

Figure 7: Proplyds in the Orion Nebula (courtesy NASA)

Figure 8: A close-up of proplyds (courtesy Hubble Space Telescope)

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 11 ​ It appears to take a 100,000 years or more to go from the time when parts of the cloud start collapsing to forming a proplyd. In the closeup shot above you can see five proplyds. While they look different from each other, each is a star system that is forming that may or may not have planets. There may be more than one star forming in each disk. We can see most of these because there are bright stars that have already turned on nearby and are shining on the proplyds around them. The black one is not lit up like the others, likely because we are seeing it backlit from behind, but we see it because it is preventing us from seeing the bright gas cloud behind it.

Figure 9: ALMA, the millimeter/sub millimeter telescope array in the Atacama Desert (courtesy ALMA) ​

Recent observations by the ALMA (Figure 9) telescope in South America have revealed proplyds in much more detail. This type of telescope uses even redder light than what an infrared telescope can see and can peer through dusty clouds to see what’s going on inside.

Figure 10 is a view of a proplyd 450 light years away. You can see what looks like the spiral arms of a galaxy. Astronomers were right one hundred years ago to be confused about whether the “spiral nebulae” were newly forming star systems or distant galaxies. As it turned out, their telescopes weren’t nearly good enough to see proplyds as they were very tiny and usually hidden in dusty clouds that required a different type of telescope to see properly.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 12 ​

Figure 10: Elias 2-27 (courtesy ALMA)

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 13 ​ Figure 11: Many images of proplyds (courtesy ALMA)

In Figure 11 above you can see a range of recent images of proplyds, some of which look a bit like galaxies and some of which do not.

Sometimes a proplyd will form too close to a hot star that has already started to shine. The intense ultraviolet light from those nearby brilliant stars will make the proplyd too hot and cause it to lose most of its material, which astronomers think may prevent new planets from forming.

These proplyds only last for the first ten million years of a solar system’s life.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 14 ​ Figure 12: New Stars in the Orion Nebula causing the cloud to shine (courtesy NASA) The image on the left is from the Hubble Space Telescope and the image on the right is from a telescope that sees infrared light

Ideas for extending this section: ● Find images of galaxies and compare them to the proplyd disks.

Resources and references: ● Proplyds in Orion (Vimeo) ​ ● Formation of Proplyds (Science Source) ​ ● Hot gas ejected by a binary protostar (Animation) ​ ● XZ Tauri (Wikipedia) ​

Review and discussion questions: ● In unit 5 we discussed the “Great Debate” in astronomy about whether the “spiral nebulae” were solar systems forming or whether they were galaxies. Now that we know what proplyds look like, what do you think of this debate?

M aking planets

The proplyd closeups that you saw above often showed gaps in the disk. This is what happens when planets start to form. Simulations show that orbiting dust and ices first start to stick together to form tiny pieces called planetesimals that gradually grow over time as more material crosses its path. When these chunks start getting to be the size of a large Earth mountain,

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 15 ​ gravity starts to help by pulling pieces in that might not have run into it. As more pieces stick, the gravity created by the lump increases, and the process - called accretion - accelerates.

These solid grains started joining together into bigger and bigger chunks whose sizes could reach a few tens of kilometers in size. In our own Solar System, some of these still exist today, like the asteroid belt between Mars and Jupiter, or the Kuiper belt beyond the orbit of Neptune, we’ll talk more about these in a later unit. Astronomers believe that the Solar System formed many planetesimals as described above. While the details aren’t quite well known to scientists, we know that some of these planetesimals got bigger, becoming what we call protoplanets, and later formed the planets of the solar system.

You can see a closeup in Figure 13 of a proplyd where planets are starting to clear out their orbits. Those dark rings have no gas because a planet has collected it a bit like a snowball rolling down a snowy hill and also pulled it in with its growing gravitational pull. It is much harder to see the planets themselves, though!

Figure 13: This is HL Tauri, a proplyd

In the outer parts of the flattened solar nebula, which were colder (because it is further away from the warming protostar), protoplanets were much larger because more rocks and ice were available. They became so large they could attract nearby gasses. These giant gassy planets

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 16 ​ were getting hotter than their terrestrial counterparts because of contraction, but not anywhere near the point of becoming a star. Eventually, they cooled down and became known as Jupiter, Saturn, Uranus and Neptune. Those giant planets were large enough, that some planetesimals would orbit them and combine to form the moons.

Interestingly, thanks in part to studies of the moon rocks brought back by the Apollo astronauts, Earth’s Moon was most likely formed when the proto-Earth collided with another planetesimal that was roughly the size of Mars, called Theia. The collision launched vaporized rock which cooled and clumped together and started orbiting Earth, becoming the Moon. A big collision similar to that is showing in figure 14. Our present day Earth is a mix of the proto-Earth and Theia, and so is our Moon.

Figure 14: In the early history of the solar system there were large collisions between planets and asteroids and even planet-sized bodies (courtesy NASA/JPL)

There are millions of planetesimals in orbit at first. As some start to become big and also pull in gases and turn into planets, their gravity can change the orbits of smaller chunks, flinging them away into new orbits, and maybe even away from the solar system altogether. The new solar system gets cleared out faster and faster as the planets get more mass.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 17 ​ Figure 15: After planets form the extra planetesimals are thrown into new orbits. The streaks show where the planetesimals are thrown. (artist conception / simulation courtesy CalTech)

Some of the ejected planetesimals are flung completely out of the new solar system. Others simply change orbits but don’t go far, and continue orbiting in a way where they don’t get flung out further. Those turn into what we know as asteroids. Still other pieces are flung far out into the solar system, past the new planets, and become the dwarf planets and comets.

There are thought to be giant clouds of many billions or trillions of comets in the distant parts of our solar system, for example, that were flung there when our solar system was young. The biggest collection of them is called the Oort cloud. Sometimes a comet will wander in from the Oort cloud at random. We can see them when they get close to the Sun. Roughly two dozen comets visit the inner solar system from the Oort cloud every year.

Ideas for extending this section: ● We learned a great deal from the rocks brought back by the Apollo astronauts, which led in part to understanding where our Moon came from. Read about the Apollo missions.

Resources and references: ● Oort Cloud (wikipedia) ​ ● What was the Apollo program? (NASA) ​ ● Steps of the formation of the solar system (Lumen Learning - Astronomy) ​

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 18 ​ ● An animation showing how the Moon could have formed (Youtube) ​

Review and discussion questions: ● Where do the leftover planetesimals go after the gravity of the planets fling them out of the inner solar system?

W hy do different types of objects form?

In Unit 2 - Properties of Stars, we talked about how multiple stars systems are common. That means that you could have one or more stars orbiting each other, possibly with planets as well. As we’ll see in the next couple of units, there are big differences between planets in our solar system, and the range of planets found elsewhere is even bigger. There are many questions that continue to challenge astronomers. Why do multiple stars form and sometimes not? And why do stars of different masses form, with higher mass stars being rare?

We don’t have complete answers yet, but with modern telescopes that sense heat, infrared waves, and radio waves, we can look inside some of these star forming regions and see how things develop.

As you saw in the photo of the Orion Nebula, the gas and dust is not distributed evenly. This means that some areas have more gas or denser gas than others. It seems reasonable that if you start with more gas and dust you can get a more massive system of stars. The same goes for planets.

One thing we also know about proplyds is that they are closer together than stars in our solar system’s neighbourhood. That means that they can sometimes interfere with each other when they are forming. This could lead to one star going into orbit around another, for example. In other cases, though, it appears that two or more stars will form together.

We also saw that if massive new stars start up nearby, that can stop new stars and planets from getting more massive and also stop them from forming altogether.

The jets that come off of new stars also push material away.

There are also rare events like a nearby exploding star or galaxies crashing together, and these may also change how things develop. In particular, those events compress the gas clouds and trigger star and planet formation. It’s thought that a collision with a smaller galaxy may have triggered the formation of our solar system, but astronomers don’t have enough information yet to be sure. It may also have been triggered by an exploding star.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 19 ​ Exactly why different types of star systems form and why different types of solar systems form is still poorly understood but it is an exciting area of research that astronomers will learn much more about over the coming years.

Ideas for extending this section: ● Review collisions between galaxies and look at how they make new stars.

Resources and references: ● Colliding galaxies are hotbeds of star formation (Earth and Sky) ​

Review and discussion questions: ● Review Unit 5 on galaxies to read about colliding galaxies. Has the Milky Way collided with galaxies in the past? ● Are there mostly massive stars that are formed when stars are made or are there mostly low mass stars? (Review Unit 2 if you are unsure)

T he nebula becomes a star cluster, and stars wander away

How many star systems usually form together? It’s typically up to a few thousand at a time. This is a type of star cluster known as an “Open Star Cluster” and is what is left after the newborn stars that formed in the cloud start shining. The light from each star has a slight pressure, and they also have strong winds that come off the star and push out against the clouds. By the time the stars form, only 10% of the dust and gas is turned into stars, planets, and other celestial bodies. The remaining 90% gets blown away.

When the new stars start to blow away the excess gas and dust, the gravity of that region of space also drops, so newborn stars near the edge of the cloud will wander away. These stars won’t be part of the cluster.

Stars that are close enough together and near the center of the cloud will tend to stick together and be part of the cluster that is left over. The stars move around in the cluster, and if they get too close to the edge and have the right speed and direction they can wander away completely. Eventually, all of the star systems will wander away into the Milky Way. This usually happens in the first billion years of a star’s life. It’s often caused by a star cluster wandering too close to another giant gas cloud in space.

This means that for part of the first billion years our solar system existed, it was inside of a star cluster. Because the stars in a cluster are closer together compared to our current neighbourhood in the Galaxy, there would have been many brilliant stars in our night sky.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 20 ​ This is a big difference between stars in a globular star cluster, which we learned about in other units, and open stars clusters: in a globular star cluster, stars usually stay there for their whole lives, but in an open cluster, most star and planet systems leave early and spend most of their lives on their own.

There are about 1,100 open clusters that have been catalogued in the Milky Way Galaxy but astronomers estimate that there may be ten thousand or more. They are found in the galactic disk as that’s where the star forming clouds are, unlike the bigger, ancient globular star clusters. The old globular star clusters are usually close to the age of our galaxy, 10 billion years or older, while the open star clusters range up to a couple of billion years old, although most are a few hundred million years old.

Figure 16: An open cluster in the Small Magellanic Cloud (courtesy Hubble Space Telescope)

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 21 ​

A ctivity 2 - Observe the differences between open and globular star clusters in Stellarium Let’s try to find the differences between the two kinds of star clusters by looking at them in Stellarium!

● Open Stellarium. As usual, minimize light pollution and remove solar system objects. ● Disable the atmosphere, as well as the ground because we will be looking at very different areas of the sky. ● First we shall look at open star clusters. Press F3 to open the search window, and look for the Hyades, the Pleiades, and the Beehive clusters. Look at the clusters’ shape and count the number of stars forming them. ● Now we’ll move to globular clusters, search for M13, Omega Centauri, and 47 Tucanae. Once again take a good look at their shape and try counting the number of stars in each. ● Do you notice any differences between open and globular clusters? Do they have a similar structure? Which type of cluster contains more stars ? ● Pay attention to the open clusters: do you see any of them far from the galactic equator? Compare that with what you learned about globular clusters in the Milky Way from unit 4.

Ideas for extending this section: ● Have a look at different open star clusters listed on NASA’s Astronomy Picture of the ​ Day ● Imagine what the night sky would like like on a young Earth in the star cluster like the ones you see in these pictures. Try painting one on the computer

Resources and References: ● Open star clusters (Wikipedia) ​

Review and discussion questions: ● Do you expect stars in the same cluster to have the same age? How similar in age? ● When you look at an open star cluster, at what stage of existence are the stars and planets in the cluster?

H ow a solar system changes over time

After a solar system forms, the orbits of the planets, asteroids, and comets don’t necessarily stay the same. In our solar system Jupiter’s gravity has a large effect on the orbits of other bodies. For example, it causes the Earth’s orbit to change between being quite circular to an ellipse then back to nearly circular, a cycle that is still happening and takes about 100,000 years (it is nearly circular now).

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 22 ​ Jupiter also changes the orbits of some of the smaller bodies going through the solar system like comets and asteroids. Some things it has flung out of the solar system, while other things get sent into the Sun. Still other objects have their orbitals made bigger or smaller than normal. Through this kind of change, it has also “collected” a group of asteroids in its orbit.

Although Jupiter is so massive that it can fling small bodies this way while not disturbing its own orbit, the smaller giants, like Uranus and Neptune can get moved ever so slightly with each interaction. Over the course of the Solar System’s formation, there were so many of these gravitational interactions that Neptune has moved into an orbit farther from the Sun from when it first formed.

Some planets have had large bodies smash into them. We saw in an earlier section that the most likely explanation of the existence of our Moon is that a Mars-sized planet smashed into the Earth when the solar system was young. The planet Uranus is tipped quite a bit, indicating that it too had something large smash into it. Venus is another planet with a weird spin; it actually spins in the opposite direction to all other planets, or it spins the same way but upside down. A few different explanations are theorized. It could have reversed its spin by being struck by a large object. Another hypothesis is that the Sun’s gravity could be pulling on its thick atmosphere, slowly, then eventually reversing the spin, or flipping it over. No one idea has been shown to be correct.

It’s thought that there was a period when many asteroids and comets crashed into the planets in the first few hundred million years of the solar system, called the Late Heavy Bombardment, which created many of the big features such as the dark areas that we still see on the Moon, for example. It is still uncertain when that happened and how long it lasted.

Figure 17: Late Heavy Bombardment (courtesy NASA)

Saturn’s beautiful rings are possibly the result of a small moon that was destroyed by a comet or asteroid that smashed into it, though this is uncertain.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 23 ​

While we know that collisions happen much less often since the early history of the solar system, we are fairly certain that an asteroid or comet caused the extinction of dinosaurs 65 million years ago. Big impacts still happen. In 1994 we saw a comet called Shoemaker-Levy 9 crash into Jupiter. Astronomers use big telescopes like the Plaskett Telescope near Victoria, BC, and Canada’s small space telescope, NEOSat, to track asteroids and comets to try to find which ones might crash into the Earth so we can prevent that from happening.

Figure 18: Comet Shoemaker-Levy 9 broke into pieces before it crashed into Jupiter in 1994 (Courtesy Hubble Space Telescope)

If you remember the previous unit, we mentioned that the Milky Way and the Andromeda Galaxy would collide and merge in about 4.5 billion years. You might have wondered what will happen to the solar system during that collision, and if you went through the Resources of that section, you already know the answer: our solar system would mostly be unchanged. Relative to their size, stars are much more far apart than galaxies, so even during a galaxy merger, it’s very unlikely that a given star will collide with another star.

As the galaxy’s structure will greatly change, the position of the Sun within the Galaxy will also be different. There’s even a small chance that we get ejected out of the Galaxy because of the collision. But if you simply consider our solar system by itself, it would be nearly unaffected by this; since the Sun is unchanged, the planets would still orbit it like nothing happened.

However, the Solar system won’t look like it does today. As we saw in Activity 2 of Unit 2 - Properties of Stars, when the Sun will have burned through nearly all of its fuel, in about 5 billion years from now, it will expand in size and become a Red Giant about 260 times wider than it currently is. In the process, it will engulf Mercury and Venus.

The Earth and the other planets will be spared, but that does not mean life would be unchanged on our planet. Not only will Earth be much closer to the Sun, but our star will also emit much

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 24 ​ more light as a Red Giant than it currently does. By then, Earth will be too hot for water to be in a liquid state, and life as we know it wouldn’t be possible on Earth anymore.

At some point while the Sun is growing and becoming more luminous, Mars might be able to sustain liquid water on its surface, but it won’t stay that way for very long. When the Sun will have become a Red Giant, we suspect that Titan, one of Saturn’s moons, would be at just the right distance from the Sun to be warm enough for humans.

After all the Sun’s fuel is consumed, the Red Giant will expel its outer layers into space, forming a planetary nebula, and the core of our star will contract into a dim and cold white dwarf. This is how most average and low mass stars will end.

It is also important to note that the planets themselves will continue to change over time. We often think of the orbits in the Solar System as moving precisely, like clockwork. However, things are changing slowly, on very long time scales.

For example, the Moon pulls on the Earth to create the ocean tides, and that pulling slows the rotation of the Earth. Each year, the length of a day increases 15 microseconds. That’s 15 millionths of a second. We aren’t able to perceive this change, but the world’s most accurate clocks can measure it. Eventually, the Earth will rotate as fast as the Moon orbits, but by that time, the Sun will be in the Red Giant stage of its life, as described above.

Stage Stage When it happens

Before the Earlier universe: stars are born and die From 13.8 billion years ago to collapse of the star and make chemical elements and dust, 4.6 billion years ago in the forming region. which collect into the clouds out of which case of our solar system. later generations of stars form. This is just happening now for some stars and planets!

Collapse of the cloud to start to form a Takes at least 100,000 years. protostar and proplyd

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 25 ​ Planets form until dusk and gas clears From 100,000 years to 55-60 out. The protostar is at the center. million years after the solar system starts forming

The Star turns on. A cluster of new stars 50 million years after the star called the Trapezium is shown in this starts to form image.

Material is ejected and blown away after It takes 3-10 million years for the star turns on. the extra dust and gas to be cleared out after the star turns on.

Late Heavy Bombardment (asteroids Takes place 300-600 million and comets crash into the planets) years after the solar system starts to form (curation and dates uncertain)

Star wanders away from open cluster 300-800 million years after the star starts to form

Earliest life on Earth starts (simple About 500 million years after single-celled life, like bacteria). Image the birth of our solar system. credit: Rocky Mountain Laboratories NIAID

Orbits of planets change, axis of planets Continues to happen to the wobble, comets and asteroid continue to solar system after it is formed collide with planets up to the present day and into the distant future.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 26 ​ Sun enters the Red Giant Stage. It will For a star like our Sun, about grow to roughly 260 times its current 11 billion years after the birth size. of the solar system.

The “Cosmic Calendar”, created by the astronomer Carl Sagan, is a good way to get a feel for the relative time spans for when things occur. In the Cosmic Calendar, the entire 13.8 billion year history of the universe up until present day is shown in one year. Therefore each “month” on the calendar is equal to 1.15 billion years. The beginning of the universe is on 12 a.m on January 1st and the present moment is when the clock strikes midnight on December 31st.

January February March

● Big Bang on January ● Milky Way Galaxy 1st at 12 a.m. finishes forming ● First Stars in the Milky around March 15 Way’s area of the universe form in globular clusters form January 25th

April May June

● The Milky Way had finished merging with other large galaxies in the early universe.

July August September

● Our solar system ● Our Sun turns on starts to form on around 5 a.m. on August 31st at 12 September 1st a.m. ● Life may have ● The collapse is appeared as early as complete and the September 14th protostar and proplyd ● Our solar system are completed around leaves it’s star cluster 12:04 a.m. on August by September 21st. 31st.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 27 ​

October November December

● First animals appear on December 13th. ● Homo Sapiens appear late on New Years Eve.

Next Year:

The Milky Way collides with the Andromeda Galaxy around April 1st of the next year. The Sun turns into a Red Giant the following week, and it becomes a White Dwarf by the beginning of May.

Ideas for extending this section: ● Learn about how stars change over their lives.

Resources and references: ● The Cosmic Calendar (Cosmos/National Geographic Society) ​ ● How our solar system will end in the far future (Forbes) ​ ● Formation and evolution of the solar system (Wikipedia) ​ ● Re-thinking a critical period in Earth’s history (NASA) ​ ● NEOSat: Canada’s Sentinel in the Sky (Canadian Space Agency) ​

Review and discussion questions: ● When life first appeared on the Earth could our solar system have still been in a star cluster? ● Name two Canadian telescopes astronomers are using to track asteroids and comets.

Friends of the DAO - ExoExplorations - https://centreoftheuniverse.org/exoexplorations 28 ​