Intro to Astronomy Assignment

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Investigating Astronomy
Timothy F. Slater, Roger A. Freedman

Chapter 2

Decoding the Hidden Messages in Starlight

Welcome to week 2. This week you’re going to learn about how astronomers are able to decode the information that comes to us from objects in space by starlight. To start with, one of the most fundamental concepts in astronomy is the fact that light takes time to travel. We don’t notice this in our everyday lives. When we turn on a light switch, it doesn’t take a noticeable amount of time for the light to reach our eyes. Some scientists centuries ago did think that it might have a “speed limit,” but weren’t able to calculate it (they even tried setting up experiments on nearby hilltops, but the light appeared seemingly instantaneously). They needed much greater distances to be able to detect the speed of light. Let’s look at how this was first discovered.

Light Takes Time to Travel

When close to Jupiter, the moons appeared to eclipse “too early.”
When far from Jupiter, the moons appeared to eclipse “too late.”
Light takes time to travel the extra distance!
c = 300,000 km/s

Over 400 years ago, the astronomer Galileo Galilei pointed a telescope at Jupiter, and discovered four points of light that moved around it. He realized that they were moons orbiting the planet (they are now known as the Galilean moons of Jupiter: Io, Europa, Ganymede, and Callisto). Over the next few decades, astronomers were able to create carefully calculated tables of when each of the moons would disappear and reappear as they moved in front of or behind the planet (this is called an eclipse, though it’s not nearly as dramatic as the eclipses we talked about last week). These tables were so precise that navigators intended to use the eclipses of Jupiter’s moons as a clock of sorts (knowing the time is very important for being able to calculate your longitude on Earth). Danish astronomer Ole Rømer discovered that the tables were “slow” when Earth was close to Jupiter (in other words, when it was on the same side of the Sun as Jupiter), and “fast” when Earth was farther away (when Earth and Jupiter were on opposite sides of the Sun). The error would oscillate according to the relative distances between the planets. He deduced that light actually needed time to travel the extra distance (basically, the diameter of the Earth’s orbit, as seen in the diagram here). He calculated the speed of light as 300,000 km/sec, a VERY important number in astronomy. 300,000 km/sec is really fast. If you aren’t able to imagine metric units as well as English units, imagine instead 186,000 miles/sec. We use a lower case c to indicate this value in equations. In fact, you’ve probably heard of Einstein’s famous equation, E=mc2. That’s the same c.

Light Travel Time To Earth

Moon 1.5 seconds
Sun 8.3 minutes
Jupiter 43.2 minutes
Saturn 79.3 minutes
Pluto 5.5 hours
1 light-year ≈ 6 trillion miles ≈ 10 trillion km
Proxima Centauri 4.3 years
Andromeda Galaxy 2.5 million years
Furthest known galaxy 13.2+ billion years

At this speed, it takes light a second and a half to reach the Moon. That’s why there was a delay in the radio transmissions between Houston and the Apollo astronauts on the Moon! Light from the Sun takes about 8 1/3 minutes to reach us. What that means is that when we’re looking at the Sun, we’re not seeing it as it is right now, but rather as it was just over 8 minutes ago. If the Sun were to go out (which it won’t, but disregard the laws of physics for a moment), we wouldn’t know about it for over 8 minutes, because the light that had already left it would still be traveling toward us. Other objects in space are much more distant, and so their light has been travelling even longer to reach us. When NASA communicates with spacecraft around Jupiter or Saturn, those messages take around an hour. When the New Horizons spacecraft reached Pluto, it took about 5 ½ hours for its images to return to Earth. But those are all in our own solar system. Further out, the light travel time is measured in years. In fact, astronomers use the light travel time as a distance measurement. The distance that light travels in one year is called a light-year, and is equal to about 6 trillion miles or just under 10 trillion kilometers. The next nearest star to our Sun, Proxima Centauri, is 4.3 light-years away. The nearest large spiral galaxy to our own, called the Andromeda Galaxy, is 2 ½ million light-years away. This galaxy is just visible to the unaided eye in the northern autumn sky (from very dark locations), among the stars of Andromeda, and near the Autumn Square or Pegasus, the Flying Horse. So, if you are able to see that with your own eyes, you can think about the fact that the particles of light that are hitting your eyes have been travelling undisturbed since before the time when human ancestors were learning to walk upright. And just in case that doesn’t boggle your mind enough, the furthest known galaxy is over 13.2 billion light-years away!

ConceptCheck

Why has the speed of light been historically so difficult to measure?

For this first concept check, please pause the audio to answer this question.

Glowing objects, like stars, emit an entire spectrum of light.

The Sun emits energy that:
Your eyes can see
Your skin can feel
Can burn your skin

Now let’s start talking more specifically about light itself. All glowing objects, like stars, emit an entire spectrum of light. The filters used to take the images of the Sun here are used to see different types of light that the Sun emits. Some of this light, also called energy, can be seen by your eyes (called visible light), felt by your skin (called infrared light), or give you a sunburn (called ultraviolet).

Sunlight Is a Mixture of All Colors

Prisms don’t “add” colors to the sunlight.
Each color light “bends” as it passes through the material.

Visible sunlight (also called white light) is actually a mixture of all colors of the rainbow. When we pass sunlight through a prism, the prism refracts or “bends” the light. Different colors are bent by different amounts, so we see the light spread out into a rainbow. You can see a similar effect on the surface of a CD or DVD.

Light Travels in Waves

Water waves show diffraction, addition, and canceling.
So does light! A wave!

If you throw two rocks into water, you’ll see the waves rippling out creating a pattern, which is called a diffraction pattern. If two high points or two low points interact, they add together and get even higher or even lower. If a high point and a low point interact, they actually cancel out. Well, guess what? Light does the same thing! Light travels in waves that can add together or cancel out in diffraction patterns.

Our Eyes See Only Some of
the Spectrum of Light

Half of this image was taken with a “visible light” camera, the other half was taken with a “UV camera.”

Bees can see designs on the petals!

This image on the right is a little hard to see on the slide, so look in the textbook to see it more clearly. It shows the electromagnetic spectrum of light, which is the spectrum of different wavelengths of light. At the top of this chart are the smallest of the wavelengths, the gamma rays. These smallest wavelengths also represent the largest energy. Gamma rays are such high energy that they can be deadly (though they are used in radiation therapy for cancer). Next is X-rays. They have larger wavelengths than gamma rays, and therefore less energy. They still can cause damage in large amounts to human tissue, though. Then is ultraviolet (or UV) light, this means “beyond violet.” Some UV light from the Sun makes it through the Earth’s atmosphere, and this is what causes sunburn and some skin cancers. Finally, we get to light that has low enough energy that it is not harmful to humans. In fact, it is the light that our eyes are able to see: visible light. The violet and blue colors have the shortest wavelengths of visible light, while the oranges and reds have the longest wavelengths. Just past what we can see with our eyes is infrared (which means “below red”). Infrared is the light from the Sun that warms the Earth. Anything that has heat gives off infrared wavelengths of light (even people!), which is fun to see with an infrared camera (you can search the web for infrared pictures). We have infrared to thank for our television remotes! The wavelengths longer than infrared are in the radio part of the spectrum (which includes microwaves). This part of the spectrum is used for microwave ovens, radio communication, cell phones, etc. The other image on this slide is very interesting. While humans can only see light in the visible part of the spectrum (without technological help, that is), some animals can see in other parts of the spectrum. The bottom right half of the picture was taken with an ultraviolet camera. It turns out that bees can see in the ultraviolet part of the spectrum, and they can see designs on flower petals. Another example of this is that if you look at a spider web under ultraviolet light, you’ll see a pattern that looks like a flower. Flying bugs that can see in the ultraviolet are attracted to the sight of a flower and get caught in the web. Cool, huh?

As Frequency Increases,
Wavelength Decreases

f is the symbol for frequency
Hertz = 1 wave per second
λ is the symbol for wavelength
λf = c, or
f = c / λ, or
λ = c / f

I mentioned the word frequency before as the number of waves that occur in a given amount of time. The symbol for frequency in equations is a lower-case f (although sometimes you’ll see it as the Greek letter ν, which looks like a v, but is pronounced “nu”). Frequency is measured in the units of Hertz. 1 Hertz equals 1 wave per second. Wavelength is represented by the Greek letter λ (pronounced “lambda”). Now, there’s a very useful relationship between these that involves the speed of light. The relationship can be written in three different ways (just by rearranging the letters algebraically). Basically, wavelength times frequency is equal to the speed of light. So, if you know wavelength, you can calculate frequency and vice versa by rearranging the equation.

Light has properties of both
waves and particles

Shorter wavelength

Higher frequency

Higher energy

More “particle-like”

Longer wavelength

Lower frequency

Lower energy

More “wave-like”

Ok, so now that we’ve established that information about light waves, there’s a weird thing about light. Not only does it behave like a wave, but it also has properties of a particle. This is a fundamental aspect of the field of science known as quantum physics. We won’t be going into it very much in this class, but it is important to know that a particle of light is known as a photon.

Electromagnetic Radiation

A “field disturbance”
Both electric and magnetic properties
A spectrum of waves, varying in wavelength and frequency

But when you drop a rock into a pond, the wave is traveling through water. What is a light wave traveling through? Since light travels through the vacuum of space, it can’t be traveling through any material substance. The answer to this lies in electricity and magnetism. Another term for light is electromagnetic radiation, because it is actually a “field disturbance,” composed of alternating electrical and magnetic fields. So the wave is a disturbance in these two fields, and the waves have different wavelengths (the distance between one wave and the next) and different frequency (the number of waves that pass a point in a certain amount of time).

ConceptCheck

If you cover a white light with a specially designed green plastic gel so that only the green light passes through, which color plastic cover gel do you need to add to the pure green light to make it change to red?
Do light waves move up and down or back and forth as they move through space?
Which form of electromagnetic radiation has a wavelength similar to the diameter of your finger?

Here are the concept checks for section 2-2 of the text.

If you’re unsure about the first one, review the discussion of Newton’s experiment on the nature of light on page 35.

Use the diagram on page 38 to help with the last one.

Pause the audio to think about these, and be sure you understand before moving on.

ConceptCheck

How do the frequencies of the longest wavelengths of light compare to the frequencies of the shortest wavelengths of light?
What is the wavelength of radio waves from your favorite FM radio station?
If a photon’s wavelength is measured to be longer than the wavelength of a green photon, will it have a greater or lower energy than a green photon?

A few more concept checks:

The second item is a calculation check, but you won’t be expected to do calculations on tests in this class. The important thing is to use equations as a guide for thinking. In this case, it’s the equation that relates frequency and wavelength. The equation helps you to understand that as one of these values goes up, the other goes down, because when they’re multiplied together, the result is a constant, the speed of light. Note that FM radio stations are in units of MHz. Remember from the text that “Mega-” is the metric prefix meaning “million” or 106.

Be sure you’re clear on these concept checks before moving on. Remember that the answers are in the back of the chapter of the book, but do your best to figure them out before looking at the answers.

Infrared light can pass through interstellar
clouds that visible light cannot.

If our eyes can only see some parts of the spectrum, there must be things we can’t see.

Infrared light can pass through clouds of dust and gas.

As I mentioned, our eyes can only see some parts of the spectrum, so there must be things we can't see (like the flowers on spider webs that are visible in ultraviolet). There are also things in space that we cannot see in visible light. For example, I mentioned last week that there are stars in our galaxy that we cannot see because they are hidden behind clouds of gas and dust. But they are not hidden in the infrared spectrum. If we use infrared telescopes, we can see right through those clouds of gas and dust to the stars behind. The Spitzer Space Telescope has taken amazing images in this part of the spectrum.

Objects emit specific amounts of light,
revealing their temperatures.

Wien’s Law: The higher the temperature, the more intense the light and the shorter the wavelength….

Remember when I said that all objects that have heat give off infrared light? Well, it turns out that the amount of light in different parts of the spectrum given off by an object reveals that object's temperature. This is called Wien's Law, which states that the higher the temperature of an object, the more intense the light (meaning more light is emitted) and the shorter the wavelength of light. The equation we use for this is seen on the slide, and there is a good explanation of it in Box 2–2 on page 45 of your textbook. This equation relates the wavelength of maximum emission of the object to the temperature of the object (note that the temperature is given in kelvins, so please make sure to review Box 2–1 on page 44 for a discussion of temperatures and temperature scales). There are really only two variables in this equation. The wavelength on the left and the temperature in the denominator on the right. The value in the numerator on the right of the equation is a constant. When the units being used are Kelvin for temperature and meters for wavelength, the constant is 0.0029. The K and the m you see here indicate that the constant includes units so that those units cancel out with the units of wavelength and temperature. Now, what does wavelength of maximum emission mean? An object will actually emit over a range of wavelengths, but there will be a particular wavelength at which an object emits more intensely than the others. Here's an interesting tidbit. If I were to ask you what the wavelength of maximum emission of our Sun is, you would most likely say that it emits most intensely in the yellow part of the visible spectrum. But actually, the wavelength of maximum emission of the Sun is in the green part of the spectrum. So why does the Sun not look green? Because it emits so much light across the visible spectrum that the colors combine to make the white light that we see. What about the yellow color? Well, depending on atmospheric conditions and the time of day, sometimes the sun looks yellow or orange or even red, but it really is white. Interestingly, the color that someone may think the Sun is may have a cultural aspect. If you ask a child in the United States to color the Sun, he or she will likely color it yellow. However, a child in Japan will likely color it red.

How much energy a star emits is determined
by both temperature and surface area.

As temperature increases, the energy released by the object increases.

How much energy a star emits is determined both by its temperature and its surface area (its size). As the temperature goes up, the wavelength of maximum emission goes down (as Wien’s law tells us). And when wavelength goes down, energy goes up (we saw that on the slide of the electromagnetic spectrum, where the small wavelength gamma rays and x-rays had much higher energy than the longer wavelength visible light or radio waves, for example). And what about surface area? Well, for this we need another law, this one called the Stefan-Boltzmann law. This law shows the relationship between temperature and flux, which is the amount of energy per square meter of surface per second. I'm not going to go into detail on that here, but review the discussion in section 2–3 and especially box 2–2, if you would like more information. Suffice it to say, this law actually allows us to calculate the surface area, and therefore the radius of a star.

ConceptCheck

Which form of light is being emitted most intensely by a frozen ice cube at 0° Celsius?
What single piece of information do astronomers need to determine if a star is hotter than our Sun?
Which wavelength of light would our Sun emit most if its temperature were twice its current temperature of 5800 K?
If astronomers observe a red star and a blue star in the sky, how do they distinguish which star is at a higher temperature?

Time for concept checks on this section.

For the first question, remember that the version of Wien’s law in the book requires temperature to be in kelvins.

Pause the audio while you think about these. The calculations are here if you want to explore them, but you will not be tested on the use of the equations.

Identifying Chemical Substances
Using Spectral Lines

The light from a burning chemical makes a special, unique pattern when it passes through a prism.

All right, now we're going to look at how we can figure out just by looking at the light from a star what chemical elements that make up a star. When a chemical is burning (for example if you add a chemical substance to a flame), the light given off makes a special, unique pattern when it is passed through a prism. That pattern is the fingerprint of that chemical. The chemical does not need to be burning for this to happen, it actually just needs to be heated up, and so when we look at the light from stars with special equipment we see these chemical fingerprints.

Electrons Occupy Specific
Orbits within Atoms

Each orbit is a specific energy state.
Electrons “leap” between orbits.

Why is that? Well, it has to do with the structure of atoms. Atoms are composed of a central part called the nucleus, surrounded by electrons. For this discussion, we will use the hydrogen atom, because it is the simplest. The nucleus of a hydrogen atom is composed only of a single, positively charged proton. (Other elements have more than one proton, and usually some number of neutrons that have no charge.) A regular hydrogen atom contains a single, negatively charged electron. That electron can be only in specific energy states, which are sometimes called orbits. But be careful not to think of an electron as orbiting the nucleus like planets orbit the Sun, because it's not quite like that. Suffice it to say, each orbit is a specific energy state (or an amount of energy that the electron can have), and the electron can jump between orbits or energy states. It cannot be in between them. This drawing of an atom is a highly simplified version, called the Bohr model (the physicist Niels Bohr came up with this idea). A useful analogy for this is to think of the electrons as living in an apartment building that has a set number of apartments per floor (because there are only a certain number of electrons that can be in each energy level). The elevator in this apartment building is broken, and the electrons are lazy, so they avoid the stairs when they can. So, if there’s an open apartment in a lower floor, an electron on a higher floor will move downstairs.

Electrons “leap” when they absorb the perfect amount of energy.
Electrons “fall” and emit that same specific amount of energy.

Electrons jump up to a higher energy state only when they absorb the perfect amount of energy in the form of an incoming photon of light. Then, they can fall back down to the lower energy state and emit that same specific amount of energy. Electrons actually don't like to be at higher energy levels if they can help it (remember they’re lazy and the elevator in the apartment building is busted, so falling down to lower ones happens easily). There are specific amounts of energy that it takes to cause an electron to jump to specific energy levels. For example, as you see on this diagram, to go from the second energy level to the third energy level requires a photon of exactly 656.3 nanometers wavelength. If the electron falls down from the third energy level to the second energy level, it will emit a photon of energy with exactly the same wavelength. By observing different elements in the laboratory, scientists have been able to identify all the specific wavelengths of light that can be absorbed and emitted by specific chemical elements and more complex molecules. So detecting specific wavelengths enables us to identify what atoms or molecules we are seeing in distant objects. But how do we detect them?

Kirchhoff’s Laws

Law 1: A hot, opaque body or a hot, dense gas produces a continuous spectrum—a complete rainbow of colors without any spectral lines.

Astronomers use spectrometers attached to telescopes to observe the spectrum of light coming from objects in space. These devices pass the light through something like a prism allowing the light to spread out. When the light is spread out, it is possible to see spectral lines, those fingerprints of different chemical elements. Sometimes, we see a rainbow of colors with dark lines where some colors are missing. Other times, we see bright lines of color against a dark background. Let's see what causes each of these. The scientist Gustav Kirchhoff outlined three laws that are important for astronomers’ analysis of spectral lines. The first law states that a hot opaque body (meaning one that light cannot pass through) or a hot, dense gas produces a continuous spectrum, or a complete rainbow of colors without any spectral lines. The core of stars obey this law.

Kirchhoff’s Laws

Law 2: A hot, transparent gas produces an emission line spectrum—a series of bright spectral lines against a dark background.

Kirchhoff's second law tells us that a hot, transparent gas emits an emission line spectrum, which is a series of bright spectral lines against a dark background. The gas is hot because it is heated by something (most likely a nearby star), and so atoms in it absorb photons, causing their electrons to jump to higher energy levels. But, as I said, electrons don't like to be in higher energy levels, so they pretty quickly will fall back down, emitting photons. Those photons can be emitted in any direction, so observing this cloud of hot gas from any angle will allow you to see those photons being emitted. So, in this image we see the cloud of gas that is being heated by the nearby star, but we are not seeing the starlight passing through it. We are just seeing the photons being emitted by electrons falling to lower energy levels. The emitted photons will correspond to specific chemical elements contained in the cloud of gas, allowing us to identify what those elements are. Again, these bright spectral lines are called emission lines.

Kirchhoff’s Laws

Law 3: A cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum —a series of dark spectral lines among the colors of the continuous spectrum.

Kirchhoff's third law tells us that a cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum, which is a series of dark spectral lines against the colors of a continuous spectrum. So, the continuous spectrum from the core of the star passes through a cloud of gas. Some of the photons will be absorbed by atoms in the cloud of gas, but only those photons that have exactly the right wavelength. As a result, fewer of those photons of those particular wavelengths will make it all the way through the gas cloud to our spectroscope. Therefore, there will be missing wavelengths in the continuous spectrum from the star. Those missing wavelengths will correspond to the specific elements contained in the cloud of gas, so we can identify what those elements are. Again, these dark lines are called absorption lines.

Kirchhoff’s Laws

The wavelengths absorbed by the gas exactly match the wavelengths emitted by the gas.

It is important to note that the wavelengths absorbed by the gas exactly match the wavelength emitted by the gas. So, in our diagram here, if we look at the cloud of gas with the star behind it, we see absorption lines with specific wavelengths. But if we were to be able to see the same cloud of gas from a different angle and see the emission lines, they would be exactly the same wavelengths, because the cloud of gas has the specific elements to create those lines. The fingerprint of those elements is the same regardless of whether we see them in absorption lines or emission lines.

ConceptCheck

What type of spectra would result from a glowing field of hot, dense lava as viewed by an orbiting satellite through Earth’s atmosphere?

Here's a quick concept check. Pause the audio to think about this question.

Spectra Also Reveal Motion

An object’s motion through space is revealed by the precise wavelength positions of its spectrum of light.

The Doppler Effect

Not only can we use the spectral lines to identify the chemical composition of an object, but we can also use them to detect the object's motion through space. Remember that I said that scientists have precisely identified all of the possible spectral lines from atoms and molecules by observing them in a laboratory. When spectral lines are observed from a moving object, those lines turn out to be shifted from the wavelengths at which they appear in the laboratory. They are still identifiable as being from specific atoms and molecules because of their pattern (for example, the number of them and their spacing), but they will be either shifted to longer wavelengths or to shorter wavelengths. This is due to the Doppler effect. You have experienced the Doppler effect when you've heard a moving train whistle or siren, because the Doppler effect applies to sound waves as well as light waves. If the train is moving towards you, then each successive wave is emitted closer in distance to the previous one, and so you perceive a smaller wavelength, and therefore a higher frequency. In sound, a higher frequency means higher pitch. Conversely, if the train is moving away from you, then each wave is emitted further away than the previous one, and so you perceive a larger wavelength and therefore a lower frequency (in sound, a lower pitch). So, if you hear a train approaching you and then moving away from you, you will first hear a high pitch and then a lower pitch. The only way to hear the actual pitch of the train whistle is if it is not moving relative to you.

The same effect occurs with light. If an object is moving towards you, you (or rather your spectroscope) will perceive a smaller wavelength. This is called a blueshift. It does NOT mean that the light will necessarily appear blue, it just means that it is shifted towards a smaller wavelength, and blue light is the smaller wavelength of visible light, so it got that name (much to the chagrin of science students who get confused by it). On the other hand, if an object is moving away from you, your spectroscope will perceive a longer wavelength, and this is called a redshift. Again, it does not mean that the light will necessarily be red.

Exploiting the Doppler Effect

The wavelength

we observe

The velocity of the object, toward or away from us

The wavelength

we “should” observe

The speed of light

But wait, there's more! Not only can we tell whether an object is moving towards us or away from us by observing the Doppler effect, but we can tell how fast it is moving towards us or away from us. All we need to do is set up a proportion, as we see on the slide. The ratio of the observed wavelength to the known laboratory wavelength is equal to the ratio of the velocity of the object to the speed of light. Note that this only measures the velocity towards us or away from us. The object might be moving at an angle, but this method only detects the component of motion toward or away from us.

ConceptCheck

How is the spectrum changed when looking at an emission spectrum from an approaching cloud of interstellar gas as compared to a stationary cloud?
How fast and in what direction is a star moving if it has a line that shifts from 486.2 nm to 486.3 nm?

Time for a concept check. In the first question, note that the fact that it asks about an emission spectrum is not relevant to the explanation. The second question is just to show you how to set up a proportion for the Doppler shift equation, but you won’t be expected to do this on a test. Make sure to pause the audio while you think about this.

Telescopes Gather Light

Light-gathering power is directly related to the size of its objective lens― the gathering area.

Telescopes aren’t primarily used to magnify stars.

Let's talk a bit about telescopes, the primary tools of astronomers. Telescopes are not used to magnify objects (for the most part). Instead, they are used to gather as much light from an object as possible. A telescope's light-gathering power is directly related to the size of its objective lens or mirror. So, if someone tries to sell you a telescope by touting its magnification power, hold onto your wallet and run away.

Refracting Telescopes

Use a lens to concentrate incoming light at a focal point
Magnifies near objects

There are two major types of telescopes. The first to be developed is called a refracting telescope. Recall that we have talked about light refracting or bending. Refracting telescopes use lenses to refract, or bend, incoming light and concentrate it at a focal point. A second lens, the eyepiece, then magnifies that concentrated image for the observer to look at with their eyes. Nearby objects are magnified, but a telescope will not make distant objects like stars appear bigger.

Reflecting Telescopes

Use a curved mirror to concentrate incoming light at a focal point.
More durable, and can be made bigger and less expensive.

The second major type of telescope is called a reflecting telescope. These use a curved mirror to collect light and concentrate it at a focal point. A second mirror (called, rather unimaginatively, the secondary mirror) reflects the light towards the eyepiece. These types of telescopes are less expensive (it is easier and less expensive to grind a curved mirror then it is to create a perfectly shaped lens with no imperfections) and they can also be larger (there is a limit to the size of lenses before the glass can no longer hold its shape against gravity).

Adaptive Optics

Computers compensate for turbulence in the atmosphere.

Both kinds of telescopes are susceptible to issues related to looking through the Earth's atmosphere. As light from distant objects in space travels through the atmosphere, it can be deflected slightly by such things as particles and changing temperature. As a result, objects can appear blurry. Large professional telescopes now have a solution that helps compensate for this. It is called adaptive optics, and it involves shining a laser into the sky. The telescope observes the laser light bouncing around just as a real star would, and computers can calculate how much the turbulence in the atmosphere is moving the light of the laser. The computers can then control actuators that rapidly change the curvature of the mirror to compensate. The image on the left on this slide shows a comparison between a telescopic view with the adaptive optics off on the left and the adaptive optics on on the right. Clearly, no pun intended, a major improvement.

Telescopes in Orbit

Detect light that does not penetrate the atmosphere

Another way to overcome the problems of atmospheric turbulence for telescopes, is to just avoid it altogether by putting the telescopes in orbit. Not only do these space telescopes not suffer from atmospheric turbulence, but they are able to detect wavelengths of light that cannot penetrate the Earth's atmosphere. This is actually the most important thing about space telescopes. Visible light, some ultraviolet, some infrared and some radio waves are able to penetrate the atmosphere, but the rest of the electromagnetic spectrum cannot. This is a good thing, as life would not exist on this planet if gamma rays and x-rays could hit the surface. There is a chart on page 57 in your textbook that shows the wavelengths that can make it through. Note that the infrared wavelengths cannot make it all the way to the surface for the most part, so infrared telescopes on the ground need to be high up on mountain tops (which is actually a good place for telescopes of all kinds to be). The telescopes shown on this slide are called NASA’s Great Observatories. CGRO was the Compton Gamma-Ray Observatory. Chandra is an X-ray observatory. Hubble observes in the visible, ultraviolet, and near infrared. Spitzer observed in the infrared. The four of these observatories together covered most of the electromagnetic spectrum, except for radio.

Looking toward the center of the Milky Way using the best of Earth-based and space telescopes

Here are some examples of what can be seen in different wavelengths using space telescopes. The top image is a visible light image from the ground. The center of the image is looking toward the center of our galaxy, while the rest of the image wraps around behind us. The image on the top right was taken by the IRAS satellite in infrared. The image in the middle is an x-ray image from the ROSAT satellite. The two images on the bottom look sort of similar, but they are very different. The one on the left shows high-energy gamma rays imaged by the Compton Gamma Ray Observatory, while the one on the right shows radio wavelengths imaged from the ground.

Charge-coupled devices record
very fine image details.

Finally, it is important to talk about how the images from satellites are stored. The first method of storing information from a telescope was an astronomer looking through a telescope with his or her eyes and making a drawing. Obviously, this was not a very precise method. When photography was invented, that was a huge step forward for astronomy, as cameras could be put in place of a telescope eyepiece to record permanent images that could be studied and shared. But photographic film only records about 2% of the light hitting it, so the rest of the light is wasted. The real breakthrough came with the invention of the charge-coupled device (CCD), which is an array of light-sensitive squares called pixels (the image on the left shows a CCD). Your digital camera has one of these at its heart. Compare the photographic image in the second image with the CCD image in the third image. It's quite a difference, isn't it? But we don't have to stop there. By passing the light through different color filters, and combining these multiple images using a computer, we can get images like the one on the right.

ConceptCheck

In large sizes, which type of telescope can be made lightest and most inexpensively?
Look at Figure 2-28, which shows the transparency of Earth’s atmosphere. Would astronomers most prefer to have a new ground-based telescope constructed that is most sensitive in the X-ray region, the ultraviolet wavelength region, or in the microwave region?
What is the primary advantage of an orbiting space telescope, compared to a ground-based telescope?
Why can CCDs more efficiently observe faint stars than photographic film or photographic plates?

Here are our final concept checks for chapter 2. Pause the audio to think about these ideas.

And that’s it for chapter 2. It is especially important to review the tutorials in LaunchPad for a lot of the information in this chapter. Next week, in chapter 3, we’ll be looking at the work of some famous astronomers in history: Copernicus, Galileo, Kepler, and Newton.