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Home , NGC 1427A, Wonders of the Universe

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Stars are an important component 10 of galaxies.

Outcomes What you will learn: • Early cultures and civilizations recorded observations of star life stages By the end of this chapter, and explanations of the universe. you will: • The formation and life cycles of stars, including the Sun, help us understand • examine how various cultures, past and present, the formation of not only the solar system, but galaxies as well. including First Nations and • Different types of stars are classified according to mass and life cycle. Métis, understand and represent astronomical • Galaxies have specific shapes and contain star clusters, black holes, and phenomena dark matter. • inquire into the motion and characteristics of astronomical bodies in our solar system and the universe • analyze scientific explanations of the formation and evolution of our solar system and the universe

A collision between two spiral galaxies NGC 6050 and IC 1179 in the Hercules constellation

Before Reading

Key Terms Determining Importance • binary system • black hole • dark matter • galaxy When information seems far beyond your experiences, you must determine • magnitude • protostar its importance to you. Skim the bulleted items on this page and the next. • quasars • supernova Then, write two statements about how the solar system and the formation • universe of stars are important to you.

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Stars 10.1

Here is a summary of what you will learn in this section:

• First Nations and Métis understandings explain the origins of the universe and astronomical phenomena such as the Milky Way and supernovas. • A star forms inside a nebula as gravitational forces pull dust and gas together, creating a spinning, contracting disk of material in which nuclear fusion Figure 10.1 This 14th century begins. Stars are classified illuminated manuscript shows according to their colour, an astronomer using a sighting luminosity, and temperature. tube, a precursor to the • Stars have life cycles during telescope, used to sight which they form and then Polaris to aid in telling time. evolve in one of three main ways. Looking Back in Time Every major culture has astronomers, people who investigate the universe and the objects in it. The universe refers to everything that physically exists—the entirety of space and time, and all forms of matter and energy. Long ago, astronomers had only three aids to help them understand the wonders of the universe: sharp eyesight, their current understanding, and an ability to make detailed observations (Figure 10.1). Today, highly powerful and sensitive instruments allow astronomers to peer farther and farther into the universe and to gather information about the celestial bodies and phenomena in it. Supercomputers can analyze the incoming data from 100 000 stars at the same time. When astronomers observe a faraway astronomical body, the distance they are looking across is so vast that they are really looking back in time. Light takes time to travel. When you look at your hand, for example, you see it not as it is, but as it was a few billionths of a second ago. At short distances, this delay is insignificant. However, when you are looking far into space, the delays begin to add up. For example, it takes about 1.5 s for the light from the Moon to reach Earth. Therefore, we always see the Moon as it was 1.5 s ago. The planet Jupiter, farther from Earth than the Moon is, appears to us as it did 45 min before.

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All the thousands of distant twinkling stars we can see from Earth even without a telescope are part of the galaxy we live in, the Milky Way. A galaxy is a collection of hundreds of billions of stars that are held together by gravitational forces (Figure 10.2). The distance from the Milky Way to the other galaxies is extremely great. Scientists have also found that, with a few exceptions, all galaxies are moving farther away from ours and from each other. These are incredible thoughts, yet scientific evidence

Figure 10.2 The Andromeda galaxy. In 3 billion years, our supports them and many other intriguing descendants will see a galaxy rise in addition to a sunrise conclusions about the nature of our universe. when Andromeda eventually collides with the Milky Way.

D13 Quick Science A Map of the Universe

The universe is so large that it can be difficult to know about). If you know the shape of each comprehend all the astronomical bodies and their object or a symbol to represent it, draw that. relationships to each other. A map can help you Do not concern yourself with trying to make visually show objects’ relationships to each other your map to scale. Label each object. and their relative positions in space. A Map of the Universe Purpose (not to scale) Earth Most Distant To list all the objects you know that exist in the from Earth universe and show their relationships to each other by arranging them on a map

Figure 10.3 Step 2 Materials & Equipment 4. Post your map on a wall in the classroom. • poster paper or newsprint • felt pens Questions 5. Compare your map with that of other groups. Procedure How do they differ? How are they similar? 1. Working in pairs or a small group, brainstorm a list of all the different kinds of astronomical Pose New Questions bodies you know about. 6. What new objects did you learn about during this activity? 2. Copy the labels shown in Figure 10.3 onto the sheet of poster paper. 7. Compose a question about an object that is new to you, or one you find interesting, that you want 3. Arrange the objects from your list in the order to have answered by the end of this chapter. you might encounter them on a trip that begins at Earth and continues to the most distant 8. What challenges are there in using a piece of reaches of the universe (or as far out as you paper to map space?

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More than a Ball of Plasma infoBIT Stars are hot balls of plasma that shine because nuclear reactions The Study of Stars are taking place at their core. When stars die, they explode and Astron is the Greek word for blow most or all of their matter into space. Some of that matter star, and the suffix -nomy means science or study. comes together in a way that makes a solar system. This means So, astronomy is literally that the atoms in every human being today came from earlier the study of the stars. stars that once existed in our part of the Milky Way. You are made of atoms that are very, very old. Elders also tell about the stars in the night sky. Some First Nations and Métis creation stories talk about human spirits coming from the stars, and when a human dies their spirit returns to the night sky as a new star. The idea that humans come from the stars is shared by many scientists and some Elders, although the details differ. Another First Nations and Métis idea is that we are all related to everything in Mother Earth. This idea has new meaning when we think about the scientific idea that our Earth and everything on it comes from the same earlier star in the Milky Way—we are all related to an early star by sharing the particles produced in that star.

Measuring a Star’s Brightness It is easy to think that all the stars forming a constellation or infoBIT asterism lie at the same distance from Earth, as though drawn on Moon Magnitude a celestial sphere. In fact, the stars in a constellation vary greatly Earth’s Moon has a magnitude in their distances from Earth, with some being many times of Ϫ12.7 and our Sun has a Ϫ farther away than others. They only appear to be twinkling magnitude of 26.75. from a flat surface because they are of similar brightness. The brightness of a star is known as its magnitude. Some of the brightest stars in the night sky will have a magnitude of Ϫ1, and as the numbers get larger, the stars are less bright (Figure 10.4). A star with a magnitude of 0 will be brighter than a star with a magnitude of 1. This system was created by ancient Greek astronomers Hipparchus and Ptolemy. They divided the stars they could see into six magnitudes, but as technology changed, so did the divisions of magnitude. The Hubble Space Telescope can detect stars as faint as 30 magnitude! The use of magnitude allows astronomers to determine how far away a Figure 10.4 This time-lapse image shows supergiant Polaris, the star is and the current stage of its life cycle. North Star. It is more massive than the Sun and 1000 times brighter.

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Other than our Sun, the nearest star to Earth is Proxima Centauri. It is part of a group of three stars that orbit each other, called the Centauri system (Figure 10.5), located about 4.3 ly away from our solar system. Although Proxima Centauri is the closest star to Earth after the Sun, it isn’t the brightest star we can see at night. The brightest star visible from Earth is Sirius, even though it is nearly twice as far from us as the Centauri system. Sirius is brighter because it is a different kind of star than the Centauri stars.

Figure 10.5 The Centauri system. Proxima Centauri, Earth’s nearest star Binary Star Systems after the Sun, is part of this system. The Centauri system, aside from being our closest star neighbour, is interesting in another way. It is a multiple star system. Well over half of the star systems that astronomers have observed have two or more stars. A system with two stars is called a binary system (Figure 10.6). (Centauri is a trinary system because it has three stars.) If the stars are close enough together, it might be possible for planets like ours to orbit all the way around both or all of them. Some astronomers suggest that Earth-like planets orbiting around tightly bound binary stars might be more common than our one-star system.

How a Star Is Born

Figure 10.6 Albireo is an example of Compared with the life span of humans, the life span of stars is a binary star system. When viewed extremely long. All stars form from the dust and gases inside a by the unaided eye, Albireo looks like a single star. A telescope shows collapsing nebula (Figure 10.7). A nebula’s collapse can be triggered that it is really made up of two stars. by a disturbance such as the gravitational attraction of a nearby star or the shockwave from an exploding star.

Figure 10.7 Stars are “born” in nebulae, such as the Eagle nebula shown here, with its aptly named Pillars of Creation region (inset).

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Inside a collapsing nebula, the region with the greatest amount infoBIT of matter will start to draw material toward it through gravitational Nuclear Fusion Reactions forces. This is where the star will form (Figure 10.8 (a)). Material Recall from Unit C that falling inward to the core has excess energy. This energy causes thermonuclear power plants the central ball of material to begin to spin (Figure 10.8 (b)). use fission, the splitting of Extremely high pressures build up inside the ball, which in turn atoms, to produce energy causes the tightly packed atoms to heat up. As the temperature to generate electricity. The climbs, the core begins to glow. This is a protostar (Figure 10.8 (c)). nuclear fusion that occurs A protostar is a star in its first stage of formation. in a star is the opposite process. In fusion, hydrogen Eventually, the temperature of the spinning protostar rises atoms are combined to to millions of degrees Celsius. This is hot enough to start nuclear create helium atoms. fusion reactions, in which the hydrogen nuclei combine to form helium. Over tens of thousands of years, the energy from the core gradually reaches the outside of the star. When that occurs, the fully formed star “switches on” and begins to shine.

(a) (b) (c)

Figure 10.8 (a) As a region of a nebula collapses in on itself, gravitational forces start pulling dust and gas together into small masses. (b) As a mass grows, it begins a cycle of heating up, spinning, contracting (pulling inward), heating, and so on. (c) The result of this process is a protostar.

The Life Cycle of Stars A century ago, astronomers could tell that many different kinds of stars existed. What they had not yet discovered was that stars have a predictable life cycle just like all living things do. How a star evolves in its lifetime depends on the mass it had when it originally formed. Astronomers describe stars in three general mass categories: low, medium, and high. Low mass stars burn more slowly than high mass stars, so their life span is longer and their temperatures are cooler. Red dwarf stars, such as Gliese 581, are low mass stars. They emit dim, reddish light. Conversely, high mass stars burn hotter, brighter, and bluer than low mass stars. Supergiant stars, such as Polaris and Betelgeuse, are examples of high mass stars. Medium mass stars are in between. Our Sun is an example of a medium mass star.

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Suggested Activity • Life Cycle Comparison of Different Star Masses D14 Problem-Solving Activity A low mass star advances through different phases than a high on page 359 mass star does. Look at Figure 10.9 and Table 10.1 to examine how the mass of a star affects its life cycle.

white dwarf red dwarf black nebula dwarf white dwarf protostars

red black hole giant

main sequence star = 1 solar mass

neutron star

supernova supergiant

massive main sequence star = 100 solar masses

Figure 10.9 The three main life cycles of stars. What cycle a star goes through is determined by what mass the star first develops after its formation in a nebula. Table 10.1

Star Mass Example Life Span (years) After the Hydrogen Runs Out Becomes . . . low Gliese 581 100 billion collapses and cools white dwarf medium Sun 10 billion collapses, but the collapse causes the temperature and red giant pressure to increase enough to start nuclear fusion again, this time using helium as fuel. Once the helium runs out, the new star collapses and burns out. high Betelgeuse < 7 billion similar to a medium mass star, but it goes through many supergiant cycles of collapse, and nuclear fusion re-starts. Eventually, new elements, such as iron, are formed in its core.

Learning Checkpoint

1. Using diagrams, describe the process of the "birth" of a star from collapsing nebula to when it begins to shine. 2. Betelgeuse has a magnitude of Ϫ7.2. Sirius has a magnitude of ϩ1.4. Explain which star is brighter.

Classifying Stars As the life cycle of stars shows, there are many types of stars. Some differences among them include what colour they are, Figure 10.10 The stars in this binary how bright (or luminous) they are, and even what their surface system differ in colour, luminosity (brightness), and surface temperature. temperature is (Figure 10.10).

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The Hertzsprung-Russell Diagram Suggested Activity • In 1919, two astronomers, Ejnar Hertzsprung and Henry Norris D15 Inquiry Activity on page 360 Russell, sorted and plotted thousands of stars according to three characteristics: colour, luminosity, and surface temperature. During Reading They wanted to find out whether any patterns might emerge that would tell us more about the nature of stars. Comparing Important Ideas The plotted data revealed that very clear relationships existed As you read about the life among star properties. Figure 10.11 shows a version of what is cycle of stars, create a table called the Hertzsprung-Russell diagram. In it, the stars are to compare the different arranged as follows: types. Note the types of stars, their names, examples, and • by colour — Red stars are plotted on the right, and blue two important facts about stars are plotted on the left. Other stars, such as the yellow each type. Which type of star Sun, are plotted in between. has the longest life? Which type always comes to a • by luminosity — The brightest stars are plotted at the top, violent end? and the dimmest stars are plotted at the bottom. A star with a luminosity of 100 is 100 times brighter than the Sun. • by surface temperature — The hottest stars are plotted on the left, and the coolest stars are plotted on the right.

supergiants Betelgeuse Deneb Rigel Polaris 10 000 Antares Arcturus red giants Vega Procyon A

100 Spica m a Aldebaran i 1 n Sirius A

s e Sun

Luminosity (Sun = 1) q u e 0.01 Sirius B n c e white dwarf Procyon B 0.000 1 Proxima Centauri

0.000 001 15 000 12 000 9000 6000 3000

Surface Temperature (°C)

Figure 10.11 The Hertzsprung-Russell diagram represents thousands of stars based on colour, luminosity (brightness), and surface temperature.

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The Hertzsprung-Russell diagram shows many patterns based on the three properties of colour, luminosity, and surface temperature. For example, the star data forms a distinct band that stretches from the top left of the diagram to the bottom right. This is called the main sequence. The Sun is a main sequence star. These stars are thought to be in the stable main part of their life cycle. They have evolved to this stage since formation, but will gradually either cool and die out or expand before exploding. Groups of stars that do not appear along the main sequence are often near the end of their lives. At the bottom centre of the diagram are white dwarfs, such as the star Procyon B. They are white because they are hot, but dim because they are small. White dwarfs are cooling and will eventually become black dwarfs. At the top right of the diagram are red giants, such as Aldebaran, and supergiants, such as Betelgeuse (Figure 10.12) and Antares. The outer layers of these stars are cool and appear red, but they are bright because they are so large. All of these giants will eventually explode.

Figure 10.12 Betelgeuse is a red supergiant. It is so huge that if it were in the solar system where the Sun is, it would reach nearly all the way to Jupiter’s orbit.

infoBIT Supernovas: The Violent End of High Mass Stars Supernovas A star might exist for millions or even billions of years before it goes Stars capable of becoming supernova. A supernova occurs when a high mass star explodes. supernovas are rare right now The explosion releases huge amounts of energy, some of which is in the region of space nearest visible as bright light. This light fades over time. When a supernova Earth. This is fortunate because occurs, the explosion forms the heavier elements, such as iron and a supernova explosion anywhere nitrogen. These heavier elements create not only the celestial bodies, within 100 ly of Earth would but life itself. Humans and other animals need iron to make blood roast the planet! cells, and Earth’s soil needs nitrogen to support plant life.

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The amount of heavier elements in a star’s core will affect how long a supernova will produce light. For example, the fusion of iron does not release energy. If too much of the core of a star is made up of iron, the star—which may have been shining continuously for more than 7 billion years—will “turn off” in minutes. With no fuel left, the star collapses one final time. This collapse is so fast and intense that the core of the star heats up to many hundreds of millions of degrees and explodes (Figure 10.13).

Figure 10.13 In 1987, Canadian astronomer Ian Shelton photographed the explosion of the brightest supernova seen by anyone since the invention of telescopes. The images here show supernova 1987A before the explosion (a) and after (b).

(a) (b) The blast is directed both outward and inward. The outward blast sends these heavy elements far out into space. Some of the debris and elements from the old star create new nebulae. Nebulae are often called star nurseries because stars develop from their dust and gas. The inward blast causes the atoms in the star’s core to compress and collapse. The remaining core after a supernova explosion faces one of two outcomes: • Neutron stars — A star 10 to 40 times the Sun’s mass will become a neutron star. The compression and collapse of the core forms neutrons, particles that are at the centre of most atoms. When the star’s core becomes little more than a ball of neutrons about 15 km across, it is called a neutron star. Neutron stars are made of the densest material known (Figure 10.14). • Black holes — A star greater than 40 times the mass of the Sun will become a black hole. A black hole is a region of space where gravitational forces are so strong that nothing, not even light, can escape. After exploding as a supernova, Figure 10.14 A neutron star. Imagine Mosaic Stadium, Regina, being filled the star’s core is under so much gravitational force that with steel and then the steel being nothing can stop its collapse, not even the formation of compressed to fit inside a 20-L fish tank. That represents the density of neutrons. In this case, the effect of gravitational forces is the matter in a typical neutron star. so great that all nearby matter and energy, including light, start to fall into a single point and form a black hole.

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In 1054, Chinese and Arab astronomers recorded a celestial event four times brighter than Venus that lasted 23 days. In the southwestern United States, observations of this supernova were recorded by the Anasazi people. They illustrated this celestial event in petroglyphs in Chaco Canyon (Figure 10.15). The remnants of this supernova now form one of the best-known nebulas, the Crab Nebula (Figure 10.16).

Figure 10.15 These petroglyphs (rock drawings) on the walls of Chaco Canyon in Arizona record the supernova event that created the Crab Nebula.

Figure 10.16 The Crab Nebula (M1), in the Taurus galaxy, is one of the best-known remnants of a supernova.

10.1 CHECK and REFLECT

1. In your own words, define the words 8. Explain how the colour of a star is related universe and galaxy. What relationship to its exists between the two? (a) luminosity 2. Astronomer Carl Sagan once said: “We (b) surface temperature are star stuff.” In your own words, explain 9. How are stars classified according to the what he may have meant by this statement. Hertzsprung-Russell diagram? 3. What do scientists mean when they discuss the magnitude of a star? 10. Explain the important concept about stars revealed by the Hertzsprung-Russell diagram. 4. What would be known about two stars, one with a magnitude of 2 and the other 11. What is a supernova? Why might astronomers with a magnitude of 4? be interested in this astronomical event? 5. Why are astronomers interested in 12. Design your own star. In your design, studying binary star systems? provide details about your star’s colour, mass, and life cycle. Include diagrams 6. What is a protostar? in your explanation. 7. What process must occur inside a forming star before it begins to shine?

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SKILLS YOU WILL USE D14 Problem-Solving Activity Toolkit 7 ■ Making technical drawings ■ Carrying out a plan Life Cycle of a Star

In some ways, a star is like a living thing. It is “born” 4. When you are confident your flipbook is complete, and eventually—and often dramatically—“dies.” place your pictures in order and staple them This process takes millions of years to complete, so together. Be sure to place the staple(s) only scientists can never observe the entire process in near one edge. You must be able to flip their lifetimes. When objects are extremely large or through your book. processes take huge amounts of time, scientists use 5. Exchange flipbooks with a classmate. Use his or models to try to understand what is happening. her flipbook to answer the questions below. In this activity, you will create a model of the life cycle of a star. To be able to observe this process, the Analyzing and Interpreting model you will create will be a flipbook. A flipbook is 6. What are the characteristics of a star? a simple animation technique. Pictures are drawn that show small, yet important, changes. When these 7. What is the relationship between the age of a pictures are placed in order and “flipped” through, star and its mass? it will appear as though the process is actually 8. How is the temperature of a star related to the happening. Using a flipbook approach, you will colour of that star? be animating the life cycle of a star. 9. When the temperature of a star decreases, then Initiating and Planning its gas pressure also decreases. Gravitational How can we model the life cycle of a star? forces are then stronger than the gas pressure. This makes the star unstable. As you look at the flipbook, what can you infer will happen next? Materials & Equipment 10. What do you think, based on your flipbook ϫ ϫ • 15–20 (7.62 cm 12.7 cm or 3” 5”) observations, will occur at the end of the star’s unlined recipe cards life cycle? • crayons, markers, or coloured pencils • heavy-duty stapler Communicate 11. How is the flipbook a good model of the life cycle of a star? Why is it not a perfect model? Performing and Recording 12. How could you make the flipbook a better model? 1. Choose a type of star and plan the illustrations you Is there a better way to model the information? will need to illustrate its life cycle. You may wish to do quick thumbnail sketches on a separate piece of paper. Remember, in a flipbook, the picture you draw is only slightly different from the previous picture and the picture that follows.

2. Once you have your plan in place, draw your pictures on separate recipe cards (Figure 10.17).

3. As you complete each drawing, place your pictures in order and flip through them to ensure the book is coming together the way you planned. Revise your plans as required.

Figure 10.17 Step 2

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DI SKILLS YOU WILL USE Toolkit 9 ■ Recording and organizing data

Key Activity Key D15 Inquiry Activity ■ Analyzing patterns Using a Hertzsprung-Russell Diagram

To make a Hertzsprung-Russell (HR) diagram, you Table 10.2 must use the luminosity and temperature of stars. Absolute Astronomers use a scale called absolute Star Name Magnitude b-v Colour magnitude to measure the luminosity, or brightness, Sun 4.8 0.63 of a star. The brighter a star appears, the lower the absolute magnitude. Sirius 1.4 0 To determine the temperature of a star, Canopus Ϫ2.5 0.15 astronomers use a telescope with two different filters, Arcturus 0.2 1.23 blue (b) and yellow (v). The filters block out all light Alpha Centauri 4.4 0.71 except for the blue and yellow. Astronomers then subtract the yellow magnitude (v) from the blue Vega 0.6 0 magnitude (b) to produce a value known as the Capella 0.4 0.08 b-v colour. Stars with higher b-v colour have Rigel Ϫ8.1 Ϫ0.03 lower temperatures. Procyon 2.6 0.42 Initiating and Planning Betelgeuse Ϫ7.2 1.85 How do luminosity (absolute magnitude) and Achernar Ϫ1.3 Ϫ0.16 temperature (b-v) relate to a star’s life cycle? Hadar Ϫ4.4 Ϫ0.23 Acrux Ϫ4.6 0.24 Materials & Equipment Altair 2.3 0.22 • pencil Deneb Ϫ7.2 0.09 • graph paper Regulus Ϫ0.3 Ϫ0.11 • three different coloured markers Castor 0.5 0.03 Spica Ϫ3.2 Ϫ0.23 Performing and Recording Becrux Ϫ4.7 Ϫ0.23 1. An HR diagram is similar to a traditional graph. Adhara Ϫ4.8 Ϫ0.21 Examine Table 10.2. On the sheet of graph paper, plot the luminosity, or absolute magnitude, Pollux 0.7 1.0 of the stars on the y-axis. 6. Compare our Sun to the other stars on the 2. Plot the temperature, or b-v colour, on the x-axis. diagram. How does its luminosity and colour 3. Using different coloured markers, circle any compare to the average star on the diagram? groups or patterns of stars. 7. How are supergiants, red giants, and white 4. Compare your graph with the HR diagram on dwarfs different from and similar to each other? page 355. Create a chart to identify which stars Create a Venn diagram to show your answer. are supergiants, red giants, main sequence 8. Is your HR diagram an accurate representation stars or dwarfs, and white dwarfs. of the majority of stars? Explain.

Analyzing and Interpreting Communication and Teamwork 5. How do giants differ from the dwarf stars in the 9. Brainstorm another way you could present the main sequence? Why do you think these stars information you found from the graph analysis. are different? Present your information to the class.

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Galaxies 10.2

Here is a summary of what you will learn in this section:

• Galaxies, which have four types of shapes, contain about 200 billion stars each and usually have a supermassive black hole in the centre. • Galaxies contain star clusters and black holes. • Quasars, which are the brightest objects in the universe, are created by energy being fed into black holes. • Dark matter makes up 90 percent of matter in the universe.

Figure 10.18 Spiral galaxy M81

Neighbours in the Universe As you learned earlier, a galaxy is a system of stars, dust, and gas held together by gravitational forces. Galaxies with more dust tend to produce more new stars, because stars form from dust and gases present in nebulae. Some galaxies, thought to be very ancient, have almost no dust because it has been used up in star-making. The farthest galaxies we can see may also be the oldest, because the light has taken so long to reach us. Astronomers think that the stars of these galaxies were possibly larger than the largest stars that exist today. If that were the case, then those stars had short, hot lives, usually ending in supernova explosions. Gravitational forces pulled the material together again, repeating the cycle of star formation, explosion, and spreading of new elements into space. Galaxies are commonly classified according to their shape: spiral, barred spiral, elliptical, or irregular.

Spiral and Barred Spiral Galaxies Spiral galaxies are named for the spiral-shaped arms that radiate out from the galaxy’s centre (Figure 10.18). About half of all spiral

galaxies, including the Milky Way, have what appears to be a bar Figure 10.19 The barred spiral galaxy across them (Figure 10.19). These are called barred spiral galaxies. known as NGC 1300

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A wave moving outward from the central regions of the galaxy causes the gas and dust to compress into arm-like bands that rotate around the central hub. A typical spiral galaxy completes a full rotation about once every 300 million years. New arms continually form as older ones disappear or change shape. Gravitational forces keep the spirals from flying apart.

Elliptical Galaxies Elliptical galaxies have shapes ranging from almost spherical to Figure 10.20 The giant elliptical football-shaped (Figure 10.20), or long and cylindrical, like a galaxy ESO 325-G004 pencil. Such galaxies are thought to result when two or more galaxies, such as spiral galaxies, merge. The largest galaxies in the universe are elliptical. Elliptical galaxies contain very little dust. This means they have fewer young stars than spiral galaxies do. Many of the stars in elliptical galaxies are extremely old.

Irregular Galaxies Galaxies without a regular shape are called irregular galaxies (Figure 10.21). The distorted form of an irregular galaxy may be

Figure 10.21 The irregular galaxy because the galaxy collided with another one or got close enough NGC 1427A to another galaxy so that its gravitational forces drew stars away.

D16 Quick Science Hunting for Galaxies in the Hubble Ultra Deep Field

The Hubble Ultra Deep Field image reveals several 2. Some galaxies will appear as a tiny dot, too thousand galaxies. In this activity, you will become more difficult to classify. Count as many of those as familiar with the variety of galaxy shapes that occur. you can and record them as unclassified.

Purpose Questions To identify and classify galaxies 3. Calculate the percentage of each type of galaxy you identified, including unclassified galaxies. Materials & Equipment 4. Based on your results, which type of galaxy is the • image of the Hubble Ultra Deep Field most common? Which kind is the least common? • handout showing galaxies of different shapes 5. Suggest why some galaxies were large enough for you to classify, while others were too small. Procedure 1. Study the image of the Hubble Ultra Deep Field. Pose New Questions Use the handout to help you identify as many 6. What other information would you like to learn about kinds of galaxies as you can. Count and record galaxies? How could you answer your questions? the number of each type.

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Our Galaxy: The Milky Way infoBIT Often, we can look up into the night sky and see so many stars that The Milky Way they are blurred together and look like spilled milk, thus giving The word “galaxy” comes our galaxy its name (Figure 10.22). This view of the Milky Way from an ancient Greek word is the result of the billions of stars that lie along a fairly flat disk galaktos, meaning milk. of stars revolving around the centre of the galaxy. The dark smudgy line along the band is dust. This dust obscures our view into the centre of the galaxy. However, we are able to view stars at the very centre of our galaxy by using telescopes that detect infrared light (heat) rather than visible light, because infrared infoBIT light is not blocked by the space dust. Spiral Galaxies Views Most of the images we have of spiral galaxies show them from a top view. This is because, from the side, a spiral galaxy looks like a thin disk.

Figure 10.22 The Milky Way

The Milky Way is often called the Spirit Road by many First Nations and Métis peoples of Saskatchewan. The stars are understood to be the spirits of ancestors that have passed on. – The Cree people call the Milky Way cipay-me–skanaw—the ghost, spirit, or skeleton road. They understand that this swath of stars is a path that spirits follow to the next world. In Cree, the word infoBIT ahca–hk or spirit is used synonymously with the word for star. Distant Galaxy The Lakota view the Milky Way as a pathway to Wanaghiyata, As better telescopes are the promised home of departed souls. The Lakota call the Milky developed, astronomers Way Wanaghi Tachanku or “trail of the spirits.” They understand continue to discover galaxies that where the Milky Way splits, or forks, is where the departed that are farther and farther are judged on the life they had led and how they will spend the away. The most distant galaxy discovered so far is 13.2 billion afterlife. Some First Nations communities understand that these light-years away. It is called ancestors journey on the Spirit Road toward Creator. UDFj-39546284. It is more – The Nakawe (Saulteaux) people have a different commonly referred to as understanding of the Milky Way. They call the Milky Way anango redshift 10 galaxy candidate. mikana, which means “flight of the birds” or “migration.” Nakawe– Elder Danny Musqua explains that the stars that make up the Milky Way help birds and insects migrate along the star track from north to south at specific times of the year.

10.2 Galaxies 363 ©P 10_sk_sci9_se_ch10:Layout 1 3/28/11 8:15 AM Page 364

100 000 ly The Milky Way is about 100 000 ly in diameter and about 2000 ly thick at its Milky Way widest point, near the core (Figure 10.23). Such a size is very difficult to imagine. The solar system, which is enormous compared to the size of Earth, is very tiny compared to the whole Milky Way galaxy. While light from the Sun takes about 5 h to reach the most distant planet in the solar system, Neptune, that same light would take 100 000 years to cross the

Sun core disk entire Milky Way. We cannot see the entire Milky Way directly because our solar system is inside it.

Figure 10.23 A top view and a side view of the Milky Way galaxy Galaxy Clusters Suggested Activities • Many of us have played the game of writing out our full address, D17 Inquiry Activity on page 368 from the street name and number to the city or town, province, D18 Inquiry Activity on page 369 country, Earth, and finally, Milky Way. In fact, your universe address does not end there. If you could get out beyond our own galaxy and look back at it, you would see that the Milky Way is part of a group of about 20 galaxies. Such a group is called a galaxy cluster, and the one containing the Milky Way is known as the Local Group (Figure 10.24). More than 2 trillion stars may lie inside the cluster. The Local Group is part of the Local Cluster of galaxies, and that in turn is part of the Local Supercluster.

Local Supercluster

solar system Local Cluster

Figure 10.24 Galaxies tend to occur in groups called galaxy clusters. Galaxy clusters in turn form groups called superclusters. According to Milky Way galaxy Local Group astronomers, there may be more than 100 billion galaxies in the universe.

Other Objects in Galaxies When astronomers look at galaxies, both near and far away, they are looking at more than just stars, dust, and “space.” There are other objects that exist that are components, or parts, of a galaxy. There are also objects, based upon collected data, that can only be inferred by astronomers.

Star Clusters Galaxies contain distinct groupings of stars known as star clusters. A star cluster is a concentration of stars in a relatively small region of space. Star clusters occur in two broad types. One is an open cluster, which contains a few hundred to a few thousand stars. Open clusters are among the youngest star groups in a galaxy. The other type of star cluster is a globular cluster,

which contains hundreds of thousands of stars, drawn together in Figure 10.25 A globular cluster of stars a spherical form by the stars’ gravitational forces (Figure 10.25). Globular clusters are the oldest star groups in a galaxy.

Black Holes Through studies of hundreds of thousands of galaxies, astronomers now believe that each galaxy contains at least one supermassive black hole at its centre (Figure 10.26). As you learned earlier in this chapter, a black hole is a region of space where gravitational forces are so strong that nothing, not even light, can escape. The evidence for the existence of black holes is strong. For example, at Figure 10.26 Artist’s concept of a the centre of the Milky Way, a number of stars can be seen rapidly black hole orbiting around a point in space that seems to have nothing in it. Scientists believe that this spot is almost certainly a black hole. A black hole mainly affects its surroundings through its tremendous gravitational pull. Its gravitational force is so strong that it can pull a star right into it (Figure 10.27). This completely collapses the atoms in the star. The mass of the star is added to the black hole’s original mass, increasing the mass and gravitational pull of the black hole. It has been estimated that the Milky Way’s black hole has been pulling stars in for at least 7 billion years. Currently, the black hole has a mass equal to about 3 million stars of similar size to our Sun.

Figure 10.27 A region of space containing a black hole, as photographed by the Hubble Space Telescope

Astronomers speculate that when galaxies collide, the black hole at the centre of each one gradually moves toward the other. After hundreds of millions of years, they will merge, with their masses combining into a single supermassive black hole. Figure 10.28 shows the two black holes at the centre of a galaxy that resulted from the collision of two smaller galaxies.

Quasars In the 1950s, astronomers began using radio telescopes. A radio telescope forms a sort of picture using the naturally occurring radio wave energy coming from objects. Figure 10.28 In the central region of galaxy NGC 6240, two black holes are visible (shown here in blue). This galaxy was formed by The radio waves give scientists clues as two small galaxies colliding. to the composition of astronomical objects. Radio telescopes provided scientists with data that enabled them to identify a quasar for the first time. Quasars are believed to be the brightest and most distant objects in the universe. In the 1960s, scientific astronomers believed they were actually stars because they emitted huge amounts of energy. On average, they burn energy equivalent to a trillion Suns. As technology improved, scientists soon learned that these objects were not stars. They began to believe that quasars were actually the centres of distant galaxies and that quasars themselves were supermassive black holes. The brightness of the quasar is due to energy being fed to the black hole. As it is “feeding,” energy is emitted. Because of the incredible distances between quasars and Earth, these objects are still a mystery to astronomers. During Reading All Ideas Are Not Equal Dark Matter As you read, think about Although there are billions of astronomical bodies in space, even which information is most more astounding is that astronomers speculate that those objects important to know and add up to less than 10 percent of the total matter in space. At least which is nice to know. In 90 percent of the universe may be filled with matter that is not your notebook, draw a web even visible. Scientific astronomers have named this dark matter. showing the most important Dark matter ideas in large circles and refers to matter in the universe that is invisible the “nice to know” ideas in because it does not interact with light or any other kind of smaller circles connected radiation. Because of this, dark matter is invisible to direct to the larger ones. observation by telescopes.

For a long time, astronomers have been puzzled by the unexpected motion of many galaxies. It appears as though gravitational forces are affecting them, yet the amount of visible mass (for example, stars, moons, gas, and dust) does not seem to be enough to cause it. For example, the stars in the Milky Way revolve around the galaxy’s centre at such high speed that we would expect them to be flung off, just like a spinning water sprinkler sends drops of water flying out in all directions. However, evidence shows that the Milky Way is not coming apart. Astronomers have concluded that it is being kept together by the gravitational force of an enormous amount of matter that we cannot see directly (Figure 10.28). Today, most of the gravitational force in the universe is thought to be produced by dark matter.

Figure 10.29 By observing how matter in this galaxy cluster bends light rays, astronomers were able to compute and map out where they believe the dark matter (shown by the dark blue ring) is distributed in the cluster.

10.2 CHECK and REFLECT

1. Why do galaxies with more dust than other 6. What are some differences between open galaxies generally produce more new stars? star clusters and globular star clusters?

2. How is the distance of galaxies from Earth 7. What is the relationship between black related to the age of the galaxy? holes and galaxies? Why is this 3. Compare and contrast the characteristics relationship of interest to astronomers? of different types of galaxies. 8. Why were quasars originally thought to 4. How did First Nations and Métis peoples be stars? What are they really? explain their observations of what 9. What is dark matter and why is it invisible scientists call the Milky Way? to direct observation by telescopes? 5. Are star clusters and galaxy clusters the 10. Write your full address, including all the same? Explain. astronomical pieces up to the universe.

DI SKILLS YOU WILL USE Toolkit 7 ■ Observing and measuring

Key Activity Key D17 Inquiry Activity ■ Drawing conclusions Modelling the Distances between Galaxies

The distance from the Milky Way to its nearest 8. Why do you think what we know about the neighbouring galaxy, Andromeda, is vast: 2.5 million ly. universe is so limited? Suggest a technology that Yet compared with the distances to other galaxies in could be invented to overcome this limitation. the universe, Andromeda seems right next door to us. In this activity, you will create a scale and plot the Communication and Teamwork distance to several galaxies on a local map, setting 9. How could you use your model to teach someone the distance from the Milky Way to Andromeda at 1 m. about the distances between galaxies? Initiating and Planning Table 10.3 Seven Galaxies and Their Real and Model How can we model the distance from the Milky Way Distances from the Milky Way to seven other galaxies? Model Distance Distance Materials & Equipment Appearance Galaxy (ly) (m) Andromeda 2.5 million 1 • paper or notebook • ruler galaxy • photocopy of a map of • markers the local area around • photograph of your school Hubble Deep Field • calculator Magellanic 12.5 million 5 galaxy NGC 2366 Performing and Recording

1. Copy the data from Table 10.3 into your notebook. Sombrero 38 million 15 2. Using the data provided in the first three rows, galaxy estimate the model distance for the remaining four galaxies and record these distances in the table. Antennae 90 million galaxies 3. Mark an X at any point on the map of the school area to represent the Milky Way galaxy. Label it.

4. Following the map’s scale, measure 1 m on the map in any direction and plot this second point. Seyfert’s 190 million Sextet Label it Andromeda galaxy.

5. Continue plotting all but the galaxies shown in the Hubble Deep Field photograph. The direction to Cartwheel 620 million each galaxy is not important, just the distance. galaxy Analyzing and Interpreting 6. In your own words, describe the universe composition using the map as an example. Galaxies 10 000 in Hubble million 7. Estimate how far your map would have to extend Deep Field to include the galaxies in the Hubble Deep Field.

DI SKILLS YOU WILL USE Toolkit 9 ■ Observing and measuring

Key Activity Key D18 Inquiry Activity ■ Drawing conclusions An Ever-Growing Crab

A supernova sends out a massive amount of energy. 4. On graph paper, plot the expansion of each knot This energy is released in knots or clumps. These versus its distance from the pulsar in the 1999 clumps can be 10 times the mass of our Sun and image. Draw a line of best fit for your graph. travel at a speed of thousands of kilometres per 5. To find the age of the nebula, you will first need second. From Earth, we can see these knots for a to find out how much it has expanded over time, really long time, sometimes centuries. As these knots or its expansion rate. Calculate the time elapsed move away from the centre of the explosion, the between the images to the nearest 0.1 years. cloud that is formed is a nebula. We can determine Divide the expansion amounts calculated in step 3 how much a nebula is growing and its age by by the time difference to determine how many tracking the energy knots. centimetres/year the nebula has expanded. Initiating and Planning 6. Using your expansion rate, you can determine To determine the age of a nebula and how much it the age of the nebula. If rate = distance/time, has expanded over time then time = distance/rate. Use the expansion rate you calculated in step 5 and the distance of each knot from the pulsar in the 1999 image Materials & Equipment to determine the age of the nebula. • two images of the Crab Nebula, one from 7. Calculate the average age of the nebula from the 1956 and one from 1999 ages determined from the 11 knots from 1999. • calculator • ruler Analyzing and Interpreting • graph paper 8. If the image of the nebula was taken in 1999, in what year did the nebula explode?

Performing and Recording 9. The images of the nebula were scaled to each 1. In a small group, examine the two images of other. This means that one centimetre was the the Crab Nebula. You should be able to see same physical distance in both images. Why the filaments and knots of gas throughout the would this be important? nebula. Note any differences you see between 10. What do you believe is the reason for the the two pictures. It is these changes that will relationship between the pulsar and the speed of help you determine the age of the nebula. expansion you may have found on your graph?

2. Near to the centre of the nebula is the pulsar, or 11. It is believed that the Crab Nebula exploded in the collapsed core of the original exploded star. 1054. How close was your answer? Measure in centimetres the distance of each knot from the pulsar. You should find 11 knots Communication and Teamwork on each picture. HINT: Measure each knot in 12. How much do you feel you can rely on your both images before measuring the next knot. group member(s) to complete the required 3. Calculate the difference in distances between task(s)? Did everyone just do tasks on their own, the knot and the pulsar in 1956 and in 1999. or did the group collaborate? The difference is the expansion of the nebula. 13. How effectively did you communicate with the Look at the numbers you calculated. Is the group? How would you change the way that you amount of expansion close (around 10 percent) communicated with your group member(s)? or is the difference greater?

10 CHAPTER REVIEW

Key Concept Review 10. Relate black holes to star life cycles and galaxies. 1. Identify one similarity among the understandings of First Nations and 11. How is the amount of dust within a galaxy Métis peoples and scientists. important to the structure and formation of a galaxy? 2. If two stars appear to have the same magnitude, are they the same distance 12. Why do some galaxies have a lot of dust in from Earth? Explain. them, while others have little or none?

3. Explain how the study of distances of stars 13. Describe some of the First Nations and and planets is really a study of the past. Métis explanations of the Milky Way. 14. Compare and contrast the two different 4. What is the relationship between the mass types of star clusters. of a star at its birth and its life span? 15. Why is dark matter so important to the 5. Briefly describe the life cycle of our Sun, formation of galaxies? according to scientists.

6. List the three key properties of stars used to Connect Your Understanding construct a Hertzsprung-Russell diagram. 16. Summarize how a protostar might become 7. High mass stars end their existence in an a black hole. explosion that can produce two possible 17. How would you use a Hertzsprung-Russell results. Name and describe the two diagram to determine where a star is in possibilities. its life cycle? 8. What are the differences between a 18. What would be the approximate colour, protostar and a supernova? luminosity, and surface temperature of the following stars on the Hertzsprung-Russell 9. Identify the type of galaxy in each of the diagram? You may wish to make reference photos below. to the larger Hertzsprung-Russell diagram (a) (b) in Figure 10.11 on page 355.

10 000

) 100

Sun (a)

0.01

Luminosity (L (b)

0.000 1 (c) 0.000 001 15 000 12 000 9 000 6 000 3 000 Temperature (b-v colour)

Question 9 Question 18

19. Suppose you can see two stars of equal Reflection brightness in the night sky. One star appears 32. Do you think that the use of the words to be yellow in colour, and the other star nursery, birth, life cycle, and death provides appears to be blue. Which star is closer to a reasonable analogy to the explanation of Earth? Explain your answer, making star formation? Explain. reference to the Hertzsprung-Russell 33. How have your ideas about the size and diagram in Figure 10.11 on page 355. structure of galaxies been changed by 20. Different cultures explained the appearance what you read in this section? Explain. of supernovas. Explain why these events 34. Do you think that events, such as might have had a cultural importance. supernovas, in neighbouring galaxies, 21. Technically, no photographs of black holes and even in galaxies far away, influence can be taken. Explain why this is the case. humans on Earth? Explain your ideas.

22. What information do scientists use to After Reading classify different types of stars? Reflect and Evaluate 23. How is the formation of stars similar to Work with a partner to list all the strategies you have the formation of galaxies? learned for determining or finding important ideas. Create a tip sheet for other students in the class on 24. Discuss the importance of gravitational how to find important ideas. Exchange your tips with forces in the creation of galaxies. another pair of students and then post your sheet in the classroom, with the teacher’s permission. 25. Imagine you are an astronomer and you are asked to identify a galaxy. What would be the characteristics you would look for? Reflection on Essential Inquiry Questions 26. Explain why scientists believe that the galaxies that are the farthest away from What is the relationship between a star’s colour, Earth are also the oldest. luminosity, and surface temperature and its life cycle? How has this understanding developed? SI 27. What do the distances between galaxies and Why is it important to understand astronomical their components reveal about the universe? bodies, such as stars and black holes, when developing technologies for exploring 28. Compare and contrast the formation of our our universe? TPS solar system to the formation of galaxies. As we explore the universe, who should have 29. In your own words, explain how quasars access to that knowledge? DM and black holes are related. What observations of the universe were made by 30. Dark matter is matter that is invisible. First Nations, Métis, and other cultures? CP Describe how scientists have inferred Unit Task its existence. In this chapter, you have learned about the life 31. How has our understanding of stars and cycle of different types of stars, and about the galaxies changed over time? Use specific different shapes and objects, such as black examples to support your answer. holes and quasars, of the galaxies found in our universe. How could this knowledge be important to the completion of your Unit Task?