Welcome to SES4U, Earth and Space Science, Grade 12.

This course provides you with the opportunity to study in-depth the concepts and processes that occur in astronomy and geology. This will give you fundamental knowledge and skills that relate to these fields and an understanding of the process of science. The course draws on biology, chemistry, physics, and mathematics to help you understand geological and astronomical processes.

This course is a journey through space and time. Your journey takes you from the infinitely large (the universe) to the incredibly small (the atom). In the first half of the course you will explore a range of topics, including the basic structures and processes involved in the formation of the universe, galaxies, stars, and planets. You will learn how to take astronomical measurements and interpret them. In the second half of the course, your focus switches to a detailed examination of one planet in the solar system – the earth. You will examine the history of the earth and investigate the structures and processes that provide the rocks and mineral resources which are necessary to human societies.

Acknowledgements (Opens in new window)

Introduction

What you will learn

After you have completed this learning activity, you will be able to

• analyze a major milestone in astronomical knowledge or theory (the discovery of the redshift in the spectra of galaxies), and explain how it revolutionized thinking in the scientific community
• analyze how and why advances in telescopes were developed and how they have improved over time
• describe the theoretical and evidential underpinnings of the Big Bang theory and their implications for the evolution of the universe

Hubble’s discovery

What is the universe like? Is it big or small? Is it made of the same material as seen on Earth or of some exotic type of matter? Can we even begin to comprehend the processes going on in it?

Staring up at a night sky full of stars, you may have wondered if it is even possible to know anything about those mysterious points of light.

It turns out that it Italic text startisItalic text End possible, and the discovery of answers to some of these questions over the past few centuries have sent shock waves through the scientific community.

This image was taken with NASA’s Hubble Space Telescope, named after American astronomer Edwin Hubble.

Two of Hubble’s greatest observations were first published in the 1920s. He observed the following:

• Stars, galaxies, and other objects in space are moving away from us in all directions.
• The farther away the object, the faster it is moving away from us.

Can you draw any conclusions from these observations?

Student Answer Show Suggested Answer

Olbers’ Paradox

As you will discover throughout the next few learning activities, most major breakthroughs in astronomy occur when new technologies are invented. Sometimes, however, they come about when someone simply asks a question that no one had thought to ask before.

In order to understand Hubble’s methods, you must go back nearly a hundred years before Hubble’s observations. In 1823, a German astronomer named Heinrich Wilhelm Olbers asked why the night sky was mostly dark. It was this simple question that led Olbers to start one of the first revolutions in modern space science.

This view is called the steady-state model of the universe, and was favoured by thinkers as diverse and brilliant as Isaac Newton (in the seventeenth century) and Albert Einstein (early in the twentieth century). However, this model troubled Olbers, who had been thinking about what an infinite, static universe would have to look like. He reasoned that if the universe were infinite in size, no matter where in the sky you looked, your line of sight (that is, the line out into space along which you’re looking) would eventually have to end at the surface of a star.

If every line outward from your eye eventually ended up on the surface of a star, the sky would be extremely bright all the time.

Since the surface of a star is bright, it stands to reason that the entire night sky should be as bright as the surface of a star. In other words, if the universe were both static and infinite in size, the night sky should be evenly bright white, not mostly black with points of light as it actually appears.

When an observation doesn’t agree with a theory, as in this case, we have what is called a paradox. Astronomers call the fact that the universe appears dark, not bright, the dark-sky paradox: if the universe really were static and infinite, there would be no such thing as a dark sky. As a result, a new model of the universe had to be constructed.

Olbers concluded that for there to be any darkness in the night sky, your line of sight had to not intersect the surface of a star. For this to be the case, the universe could not be infinite in size: it had to be finite, which means it must have a boundary of some sort. In addition, the fact that we see changes in the universe all the time – such as the formation of new stars (novae) or the disappearance of comets – suggests that the universe is not a static place. These two conclusions led to a profound question: if the universe is not infinite and is changing, was there a point in history when the universe began?

One way to answer that question is to examine the characteristics of light reaching Earth from other parts of the universe. One of the most important characteristics of light in this case is caused by the Doppler effect.

The Doppler effect

Have you ever heard the siren of an ambulance or police car pass you at high speed? As the vehicle passes by, changing from moving toward you to moving away from you, the sound goes from relatively high-pitched to lower-pitched. This is called the Doppler effect. It occurs when waves – such as sound waves – are sent out from a moving source. As an object moves forward, it pushes the sound waves in front of it, which tends to bunch them up. At the same time, the waves behind it are stretched out.

Depiction of the Doppler effect. The source (red dot) is moving to the left. This stretches out the waves behind it, and compresses the waves in front of it.

Waves have a property called wavelength: the distance between two successive crests, or high points, on a wave. If sound waves have a short wavelength, they sound high-pitched; if they have a longer wavelength, they produce lower-pitched sounds. Note that in the video, as one object approaches another, the wavelength decreases or shortens. As the waves are compressed, a higher-pitched sound results. Conversely, as the objects move away from each other, the wavelength increases or widens, resulting in a lower-pitched sound.

Now let’s explore the Doppler effect as it relates to light and vision.

Doppler effect in light

Light can sometimes act like a wave. Waves of light travel at the speed of light, which is given the symbol c, and is 3.00 × 10⁸ m/s (metres per second).

In theory, nothing can travel faster than this speed. Light is a form of electromagnetic radiation. It appears in a variety of ways, based on its wavelength. The variety of ways that light can appear is called the electromagnetic spectrum.

You will now begin your self-guided tour of the electromagnetic spectrum.

Examples of long-wavelength light are radio waves used to carry radio and TV signals. Examples of very short-wavelength light are X-rays and gamma rays.

Visible light encompasses all the colours your eyes can see, from red to violet. The wavelengths of visible light put them roughly in the middle of the electromagnetic spectrum. Within the visible light spectrum, red has the longest wavelength (700 nm) and violet has the shortest (400 nm). You will learn more about the characteristics of the electromagnetic spectrum in the next learning activity. (The symbol nm stands for nanometres, or 10^–9 m.)

As with sound, light waves also experience a Doppler effect. Light coming from a source moving toward you appears to have its wavelength compressed. Similarly, if the light source is moving away from you, the wavelength will be stretched out. When light waves get compressed, their wavelength decreases so they appear more blue in the visible light spectrum. By comparing the light spectrum from distant stars to that from the Sun, you can see if the light from the distant stars is compressed or stretched relative to what would be expected.

When light waves get stretched out, their wavelength decreases so that they appear more red. Astronomers often talk about the light coming from a distant star as having a redshift if it is moving away from Earth, and having a blueshift if the star is moving toward Earth. The Doppler effect in light provides a very useful way to determine if a light-emitting object, such as a star or galaxy, is moving toward or away from Earth.

Astronomers use the letter z as the variable representing the amount of redshift. A star showing a redshift would have a positive z value, while one showing a blueshift would have a negative z value. The absolute size of the z value indicates how fast the object is moving relative to the Earth.

If you detect a redshift (top), the star is moving away from you (positive z value).

If you detect a blueshift (bottom), the star is moving toward you (negative z value).

Try it!

Explain how you can tell if a light source is moving toward you by examining its wavelengths.

Student Answer Show Suggested Answer

Edwin Hubble’s observations

In 1919, a new telescope, 100 inches (2.5 m) in diameter, was installed at the observatory at Mount Wilson, California. A young American astronomer named Edwin Hubble had just started working there. At the time, the prevailing view of the universe was that the Earth’s galaxy, the Milky Way, made up the entire universe and that there was nothing beyond it.

Hubble had access to the largest and most powerful telescope in the world at that time. He turned it toward a curious type of object called a spiral nebula, of which several were known.

The 100-inch telescope at Mount Wilson, California, that Edwin Hubble used in the 1920s.

Spiral nebulae

A spiral nebula (plural “nebulae”) looks like a cloud in space formed into a spiral shape. Spiral nebulae are very faint. Astronomers at the time believed they were located within the Milky Way.

Hand-drawn image of a spiral nebula from 1850.
It was initially believed to be within the Milky Way, but proven by Edwin Hubble to be much more distant.

Using new information about a type of star called a Cepheid variable, and relating how the brightness of spiral nebulae varied, Hubble was able to prove that spiral nebulae were much farther away than previously believed. In fact, the spiral nebulae were so far away that they could not possibly be within the Milky Way galaxy.

Hubble concluded that spiral nebulae are actually entire galaxies completely separate from the Milky Way. This meant that the universe had to be much larger than anyone had previously thought.

This 2011 image shows the Cepheid Variable Star V1. It gives a photographic view of the spiral nebula, with distinguishable colours depicting the colour and brightness of the stars in view.

V1 is a special class of pulsating star called a Cepheid variable that can be used to make reliable measurements of large cosmic distances.

Patterns in redshifts

When Hubble began his work in the early 1920s, astronomers were accustomed to looking at objects that were, relatively speaking, fairly close to the Earth. They could see only objects within the Milky Way, since this was the limit of their telescopes. They did not have much experience observing objects that were as far away as spiral nebulae.

Using the new Mount Wilson telescope, Hubble was able to observe nebulae (now called galaxies). He became aware of two peculiar features:

• Light from most galaxies appeared to be shifted toward the red, or longer-wavelength, end of the visible light part of the spectrum.
• The farther away the galaxy was, the larger the redshift was.

One day, when he compared his photograph with previous exposures of the novae, Hubble made a startling discovery. One of the so-called novae dimmed and brightened over a much shorter time period than seen in a typical nova.

Hubble obtained enough observations of V1 to plot its light curve, determining a period of 31.4 days, indicating the object was a Cepheid variable. The period yielded the star’s intrinsic brightness, which Hubble then used to calculate its distance. The star turned out to be one million light-years from Earth. This was more than three times the diameter of the Milky Way, as calculated by astronomer Harlow Shapley.

Taking out his marking pen, Hubble crossed out the “N” next to the newfound Cepheid variable and wrote “VAR,” for variable, followed by an exclamation point.

Based on what he knew about the Doppler effect of light, Hubble was able to draw two important conclusions from his many observations:

• Light from most galaxies is redshifted, which means that most galaxies are moving away from Earth. Very few galaxies have been found that give off light that is blueshifted.
• Light from more-distant galaxies is redshifted more. This means that the farther away a galaxy is, the faster it is moving away.

From these conclusions, Hubble determined in 1923 that the distance and speed of most galaxies appear to obey a simple mathematical relationship. This relationship is now called Hubble’s law:

Bold text startspeed that a galaxy is moving away from Earth ∝ distance it is away from EarthBold text End

The ∝ symbol means “proportional to,” so if galaxy speed were plotted on a graph against distance, the data points would follow a straight line. If symbols are used for the two variables (v representing speed and d representing distance), the proportionality statement is:

Bold text startv ∝ dBold text End

While proportionalities are good at showing general relationships between two related variables, they become really useful only when turned into equations. To turn a proportionality into an equation, you need to know exactly how the variables are related. Hubble came up with this equation relating a galaxy’s speed to its distance away from Earth:

Bold text startν = H₀dBold text End

The number H₀ is a constant, called the Hubble constant, while v and d are variables. If you look at a graph showing the speed of galaxies (v) plotted against their distance from Earth (d), the result is generally close to the straight line predicted by Hubble’s equation. (Bold text startNote:Bold text End You will learn more about megaparsecs, the unit of measure for astronomical distance, in a later learning activity.)

An exception to the law was discovered in recent years when observations were made of some galaxies in the Virgo cluster, which deviates from Hubble’s law. The relationship generally holds everywhere in the universe. Interestingly, the galaxies around these two deviant clusters are all moving towards a central point, as if drawn there by a massive but invisible object. This object is sometimes referred to as the Great Attractor.

Implications of Hubble’s discovery: The Big Bang

Edwin Hubble concluded that most galaxies are moving away from the Earth, and from each other, at incredibly fast speeds – sometimes several thousand kilometres per second. Not only that, but all galaxies are also moving away from all other galaxies.

There are a couple of exceptions to this, most notably Andromeda. Andromeda is actually moving toward the Milky Way at about 300 km/s.

It’s easy to picture two objects moving away from each other, but to picture every object moving away from every other object is more difficult. It helps to build a model to simulate the situation.

Andromeda is one of the few galaxies moving toward Earth. Light from Andromeda is blueshifted, not redshifted.

Activity: Building a model of the universe

Content

In this activity, you will investigate how the major features of Hubble’s model of the universe can be shown using a simple model.

Materials

• a large round party balloon that you can blow up
• a felt-tip pen to mark the balloon
• measuring tape (if you don’t have a measuring tape, you can make your own using a strip of paper or string and a ruler)

Procedure

Perform the following sequence of steps.

Results

Create a table like this one and record your results in it.

Size of balloon	A–B distance (cm)	A–C distance (cm)	B–C distance (cm)
Small
Medium
Large

Analysis

Did the results match your prediction?

Student Answer Show Suggested Answer

In one sentence, write a general statement describing the relation between the speed at which objects move away from each other on the surface of the balloon, and the distance between the objects.

Student Answer Show Suggested Answer

The expanding-balloon model illustrates Hubble’s two key conclusions:

• The universe is expanding.
• The rate of expansion increases as the balloon (universe) gets larger.

The dots on the balloon are like Hubble’s galaxies. The difference is that the galaxies themselves are not expanding in size (unlike the dots on the balloon, which stretch out as the balloon expands) because they are held together by gravity.

Of course, the universe is not exactly like a balloon. The shape of the universe is not fully understood, but scientists have a good idea about some of its aspects.

The geometry of the universe

The surface of the Earth can be described as two-dimensional and unbounded. It is two-dimensional because any point on the surface of the Earth can be found using two coordinates, latitude and longitude. It is unbounded because it has no edge. You can’t fall off the edge of the Earth; if you keep walking, you’ll eventually go all the way around and end up at your starting point.

Life on Earth is not purely two-dimensional, of course, since you also live in three-dimensional space. For example, any point in the room in which you currently sit can be located using three “rectangular” coordinates (often called x, y, and z). Your position in the room can be exactly determined by assuming the opposite corner of the room, where it meets the floor, is the “origin” point (0, 0, 0). You can measure your exact position from there.

Lines of latitude and longitude act as grids to reference any location on Earth.

What is the real challenge in picturing how the universe is shaped?

It’s thinking about whether it is unbounded (continuous, like the surface of a sphere or balloon) or bounded (with an edge that could be crossed, like a parking lot). You will return to this question later, after examining how the universe was born and how it is expanding.

Galactic coordinate systems often use the centre of the solar system as an origin point. As shown below, galactic coordinate systems have three spatial dimensions: left-right, forward-backward, and up-down.

Big Bang

The implications of Hubble’s discoveries revolutionized astronomy. Scientists asked the following questions: if galaxies are currently travelling away from each other, as if they were riding on the surface of an inflated balloon, what would happen if the clock were run backwards and you could journey back in time? What would the universe look like then? The three answers they came up with stunned the world:

• The universe was once infinitely small.
• The universe was once much hotter.
• The universe began with a giant explosion.

Let's explore each of these in turn.

The universe was once infinitely small

Since the universe as a whole is expanding, this means that in the past it must have been smaller. In fact, at some point in time it was infinitely small. (Scientists call this point a “singularity.”)

The universe was once much hotter

If you compress a gas, it gets hot; if you expand it, it cools off. This is a well-understood phenomenon in chemistry and physics, described by the ideal gas laws. Combustion engines use this phenomenon to heat up a fuel-air mixture to the point of combustion in order to drive a piston up and down.

The same laws applied to the early universe. Since solid objects are relatively rare in the universe, the universe can largely be considered a gas. This means that as the volume of the universe decreases, it should get hotter. As you look back in time, the volume of the universe decreases so its temperature increases, until it approaches infinite heat at the singularity.

The universe began with a giant explosion

Given that the universe started out infinitely dense and infinitely hot, all the matter and energy in the universe today exploded outward from the singularity. As the universe expanded, it cooled down. It eventually cooled down enough for stars and galaxies to form and produce the universe seen today. The explosion that gave birth to the universe is called the Big Bang.

Scientists calculate that the Big Bang occurred approximately 13.7 billion years ago, or 1.37 × 10¹⁰ a (where “a” is the abbreviation for years, derived from the Latin annum, meaning “year”). They determined this based on the speed at which galaxies are moving away from each other today, and their distance from the Earth and each other. From this, it is possible to calculate how long the expansion has been going on, and thus how old the universe is.

What caused the Big Bang? This is a huge unsolved question. Some scientists think a quantum fluctuation in matter caused it, but current theories of physics can’t prove this with any certainty. Discussion of this topic is beyond the scope of this course – all you need to remember here is that the Big Bang occurred 13.7 billion years ago and that the universe has been expanding ever since.

The Kelvin scale of temperature

As a result of all this expansion, the universe has now cooled down so that the temperature of deep space is about 2.7 K above absolute zero, where absolute zero is the coldest temperature theoretically possible. Expressed in the Celsius scale, the temperature of deep space is about –270°C. Since most astronomers record temperature measurements in Kelvin rather than Celsius, it helps to know more about this temperature scale.

While degrees Celsius (°C) is the most common metric unit of temperature, the actual units of temperature used by astronomers come from a related scale, called the Kelvin (K) scale. Unlike other temperature scales, temperatures in Kelvin are not expressed using the degree unit. They are just expressed as K (for example, “200 K” not “200°K”).

To familiarize yourself with the Kelvin scale, you will note some important temperatures in Celsius and their conversion into Kelvin.

Wait patiently for the next screen to load, then raise the temperature to see ice melt into water.

Structure of the universe today

Matter is distributed fairly evenly throughout the universe. However, in some places it is localized into clumps such as stars and galaxies. You happen to live within one such clump, called the Milky Way galaxy.

The question about the shape of the universe – bounded or unbounded – is still unresolved. Telescopes let you see that there is a visible edge to the universe, but that does not mean it is a boundary. You can see why this is true by looking at the Earth. If you’re standing on its surface, the Earth has a visible edge, called the horizon. However, still more Earth lies beyond the horizon, and it eventually curves back in on itself. This means the Earth’s surface does not have a boundary, even though it appears to have an edge.

The same could be said for the universe. For reasons that will be discussed later, there is a limit to how far you can see with telescopes, and this visual wall is stretched around in every direction. It appears as if you are on the inside of a giant bubble. For this reason, it can be said that the universe is unbounded, just like the surface of the Earth, and is three-dimensional.

Try it!

Check your overall understanding of Big Bang theory by answering these questions.

The expansion of the universe has been described as similar to a loaf of raisin bread rising as it is baked. On a separate piece of paper, draw a diagram to show how four raisins change their positions relative to each other as the dough expands from small to medium to large.

Show Suggested Answer

Any gasoline engine operates on the principle that, if a gas composed of fuel and air is squeezed, it gets hot. How is the squeezing of the fuel-air mixture inside an engine similar to running the expansion of the universe backwards?

Student Answer Show Suggested Answer

Before the Big Bang theory was developed, scientists believed the universe to be in a steady state that had remained the same forever. Using this table, compare the steady-state model of the universe with the Big Bang model. Some information has been completed for you as a guide.

Feature	Steady state	Big Bang
Size of universe	Constant	Empty cell to be completedRevealed answerExpanding
Distance between objects	Empty cell to be completedRevealed answerConstant	Increasing
Age of universe	Infinite	Empty cell to be completedRevealed answerFinite

Suggested Answers

Cosmic microwave background radiation

The Big Bang theory for the origin of the universe explained Hubble’s findings, but for many years it remained just a theory – no one knew how to prove it. It was thought that the Big Bang, like all explosions, should have made a lot of noise in the universe, some of which might still be detectable today as the “Big Bang echo.” But what signal should astronomers look for? Many tried to find it and failed.

The scientific detective story behind this problem shows that in science, hard work and a little luck can be a potent combination. But to understand this story, you need to first review some basic thermodynamics.

Expansion of an adiabatic system

Thermodynamics is a branch of physics that deals with how heat and temperature change as systems of objects change.

An important idea in thermodynamics is the adiabatic system – one that is completely isolated from any other objects so that heat cannot be transferred into or out of the system. All adiabatic systems obey a simple rule: as an isolated system expands, it cools down.

The universe can be considered isolated – what could exist outside it? It can thus be treated as an adiabatic system, and obeys the rule.

As you will learn in the next learning activity, the type of the electromagnetic radiation given off by an object can be related to its temperature. Since the universe has expanded so much since the Big Bang, scientists in the 1940s predicted that the electromagnetic echo left by the Big Bang should have a temperature that is very cold, at slightly above absolute zero. This means there should be a specific type of background electromagnetic radiation associated with that. Finding this background radiation proved very difficult, however, because it was so faint relative to the bright lights of the galaxies and stars.

As expansion of the space occurs, the universe cools and becomes less dense.

First evidence of the Big Bang

In 1964, two astronomers, Arno Penzias and Robert Wilson, were working together at Bell Laboratories in New Jersey, U.S.A. They were just completing the design and build of a new type of highly sensitive radio telescope (antenna) to detect a wavelength of electromagnetic radiation called microwaves. It didn’t look like a typical antenna at all, but rather like a big speaker horn with one end open to the sky.

The antenna was designed to pick up very weak radio signals in the microwave band for use in basic research in radio astronomy. Unlike traditional optical telescopes that gather light in the visible part of the spectrum, radio astronomy observes invisible wavelengths of light such as radio waves and microwaves. These wavelengths of light can give new insights into the structure of stars, galaxies, and the universe by letting you see things that are invisible to your eyes.

When Penzias and Wilson put their antenna into operation, there was a small amount of static noise that would not go away. Listen to a static noise similar to what they heard:

The 18-ton horn-shaped antenna that Arno Penzias and Robert Wilson used at Bell Labs
in the 1960s to detect microwave radiation from space. For scale, look at the two men
standing on the platform underneath it.

At first, Penzias and Wilson suspected that the static was caused by droppings from pigeons perched inside the horn. But even after the antenna had been cleaned and the pigeons removed, the static persisted. After testing for all possibilities, they concluded that the static was real.

Penzias and Wilson determined the temperature of the source of the radio static and found it was close to absolute zero, at around 2.7 K. They also found that the static came from every direction in the sky, which ruled out an earthly origin for the noise. They had no explanation for their results, but still suspected that it might be some sort of meaningless noise.

Connecting the Big Bang and the microwave static

The COBE satellite and CMB anisotropy

This background static was given a formal name: cosmic microwave background (CMB) radiation. It appears almost exactly the same in all directions and is not associated with any star, galaxy, or other object. The CMB radiation (or CMB for short) is the earliest light given off by the universe just after its formation, when it was about 380,000 years old. The light is known to be this old because light travels at an incredibly fast but finite speed (about 3 × 10⁸ m/s). This means that as you look outward at more distant objects, you are looking farther back in time because it has taken the light so long to reach the Earth.

For clumps of matter such as planets, stars, and galaxies to form, the early CMB must not have been perfectly uniform. Theoretical predictions from the Big Bang model proposed that the variation in the evenness of the CMB should show a very specific pattern of tiny differences in temperature between regions of space. Finding the predicted pattern of variation in the CMB would help prove the Big Bang model.

In the 1980s, an instrument called the Cosmic Background Explorer (COBE) was formed to measure tiny fluctuations in CMB temperature. If the Big Bang theory were correct, the spatial pattern of the fluctuations in the temperature in the earliest light from the universe (the CMB) should correspond to the spatial pattern of galaxies and other structures seen today.

The COBE satellite and CMB anisotropy

When first released in the mid-1990s, the extremely small variation results of the COBE experiment generated a lot of media attention, as these results closely matched the theoretical predictions from the Big Bang model. Some called it evidence for the birth of the universe. The success of the COBE project led to a Nobel Prize in Physics for George Smoot in 2006. It also popularized astronomy in the public imagination again. George Smoot even made guest appearances on popular television shows like “The Big Bang Theory” and “Are You Smarter Than a 5th Grader?,” where he won the million-dollar first prize.

Refinements to COBE’s findings

A more precise instrument, the Wilkinson Microwave Anisotropy Probe (WMAP), was launched by NASA in 2001. It has produced a much more detailed picture of the CMB, at a variety of different temperatures.

Note the similarity to the COBE picture, especially the large cold (blue) spot near the middle. However, the WMAP picture has much finer detail.

In this learning activity, you learned about Edwin Hubble’s methodical and insightful work made possible by a newly built telescope. His research revolutionized astronomy and our view of the cosmos. You also examined evidence used to test predictions of the Big Bang theory. Finally, you considered the structure of the universe from its birth to the present.

In the next learning activity, you’ll learn about the nature of light: what it is, how it is created, and how it is absorbed. Then you will then explore how astronomers use different types of electromagnetic radiation, including the types that cannot be seen, to observe different objects in the sky.