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Unlike most other scientists, astronomers cannot bring their test material into the laboratory for controlled experiments. To learn about our universe, we rely upon our knowledge of the nature of light: how it is produced, how it travels through a vacuum and through media, how it behaves when we "capture" it with a telescope. This lesson gives a survey of what we know about radiation, spectra, and telescopes.
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After completing this lesson, you should be able to
Lesson Four is one of the most important of this course, especially the section on light. All of the information we get from the Universe, with the exception of a few meteorites and the rocks brought back from the Moon, comes from light. We must understand light, electromagnetic radiation—oscillating electric and magnetic fields—to understand the information it brings with it. We must understand how atoms work, how the electrons absorb and emit radiation. We must have confidence that we understand the periodic chart of elements. If we see characteristics of elements billions of light years from us that are identical to the characteristics of elements here on Earth, then those elements must be be present billions of light years away. The chemistry and physics that operates here on Earth operates the same everywhere.
The second part of Lesson Four, telescopes, is covered thoroughly in the text. Although these online notes will cover the types of telescopes, you should read Chapter 5 carefully, especially the sections describing the different kinds of telescopes needed for different regions of the spectrum. Find out why astronomers are so anxious to get observations of objects at all wavelengths, not just the visible light.
Note: the text includes a discussion of the Doppler shift of light, and you should read through this section. We will not be using this concept until later lessons, however.
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To fully understand where light comes from, how it is produced, we must start with a brief summary of what, exactly, atoms are:
This view of the atom is quite different from that given in the text, and is the one we will be using for this course. The classical view of the atom, where electrons are viewed as "mini-planets" orbiting a nucleus "sun" is now over 70 years out of date. Even though we need to stick to a superficial view of the quantum atom, it is not that much harder to understand. Plus, quantum physics is extremely fascinating and tests all that we know about our world.
When describing the orbitals of an electron in an atom, we can state only the probability of where we might "find" it in the atom; we will pick the hydrogen atom in picturing what we mean. The figure depicts the hydrogen atom, nucleus (single proton) at the center, and the electron cloud surrounding the nucleus. As the electron gains energy, either through a collision with another electron or by absorbing a photon, the charged cloud occupies a larger volume. The higher density shows where there is a greater probability of finding the electron. Note that in the more excited state, there is still some probability that the electron will be found closer to the nucleus. (There is even a state where there is a small probability that the electron will be found in the nucleus.) If the electron gains enough energy, it will leave the proton and we say that the atom is ionized. (Atoms with multiple electrons can have multiple ionized states.) Note that the figure is not to scale: the proton would be much too small to see.
For more information on quantum mechanics, and more sketches of some hydrogen atomic orbitals, check out Introduction to Quantum Mechanics provided by the Chemistry Department at the University of Washington.
We can describe the properties of light:
Wavelength, frequency, and the speed of light are related by:
c = wavelength * frequency
The whole electromagnetic spectrum consists of (from highest energy, highest frequency, shortest wavelength . . . to . . . lowest energy, lowest frequency, longest wavelength): gamma rays, x-rays, ultraviolet, visible, infrared, microwave, radio.
The wavelengths range from a picometer (0.000000000001 m) for gamma rays, to 10,000 m for radio. (Note, however, that both ends of the scale are theoretically unbounded.)
| EM Radiation | Frequency (Hz) |
Wavelength (meters) |
Typical Astronomical Sources |
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Lowest frequency Lowest energy |
Longest Wavelengths Greater than 1 cm (0.01 meter) |
Active galaxies Quasars Alien civilizations |
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1 cm-1 mm (approximately) | Cosmic Microwave Background Interstellar molecules | |
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1 mm-700 nm | Interstellar dust | |
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700 nm-400 nm | Stars Interstellar gas | |
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400 nm-10 nm | Hot stars | |
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10 nm-0.1 nm | Black hole accretion disks Pulsars | |
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Highest frequency Highest energy |
Shorter than 0.1 nm (0.0000000001 m) Shortest wavelength |
Supernova explosion Mergers of neutron stars or black holes? |
For a wonderful and informative presentation of light, check out Imagine the Universe: Electromagnetic Radiation. The images in the above table were obtained from that site.
It's important to remember the kinds of electromagnetic radiation and whether they represent high energy (high frequency, short wavelength) or low energy (low frequency, long wavelength), or somewhere in between. Let's take a look at another representation of the full spectrum:
Almost everything we know about our cosmos comes from light: either given off by the object or reflected by the object. We talk about light coming to us from across the Universe. Light is the propagation of electric and magnetic fields. In a vacuum, it travels at an astounding 300,000 kilometers per second (or, more precisely: 299,792.458 km/sec). The changing electric field generates a magnetic field, and the changing magnetic field generates an electrical field—nature's most perfect dynamo-electromagnet.
Here's the strangest thing about light: it can behave either as a wave or as a particle. How it behaves depends on what it is traveling through, or how you are trying to measure it, among other things. For example, light enters the lens of an eye as a wave, and is refracted and focused on the retina. There, the light acts as particles, transmitting their energy to the rods and cones, stimulating them to send the nerve impulses to our brain. Light travels through a vacuum and through our atmosphere as a wave. When we hold our solar-powered calculator to the light, it then behaves as a particle, generating an electrical current when it strikes the solar energy cell. Light behaves as a wave when refracted through raindrops forming a rainbow.
As a wave, the characteristics of light are described by its wavelength, velocity (speed), and frequency. These three properties are related by:
(from Imagine
the Universe)
As a particle, light behaves as tiny packets of energy called "photons" (do not confuse with "protons"). Photons at radio frequencies have the lowest energy—that's why we can live with radio waves passing by us and through us continuously. Photons at gamma ray frequencies have the highest energy—that's why you do not want to meet up with an atomic bomb or a nuclear meltdown. Gamma ray photons destroy life. The energy of a photon is related to its frequency by:
E = hv
E is the energy of the photon; h is another fundamental constant of the Universe called Planck's constant (of order 7 × 10-34 in units of energy * time!); and v is the Greek letter nu representing frequency. One can see by this relationship that the higher the frequency of the light, the higher the energy contained in its photons. Light as a wave and light as a particle are intimately related, and one cannot think of light without considering both of these characteristics.
(Thought questions: if infrared, microwave, and radio waves are all forms of electromagnetic radiation, why can't we operate our calculators by holding them up to a stove or our cell phones or towards some nearby radio towers? In a pinch during a nuclear war, would gamma rays work to run our calculators?)
A continuous spectrum is what we normally think of as all of the colors or the rainbow. Any object having a temperature above absolute zero, emits thermal radiation. Humans, with a temperature of about 310 Kelvin, radiate at infrared wavelengths. If the thermal radiator is a perfect radiator, we call it a blackbody, and it will emit radiation over a range of wavelengths and form a continuous spectrum.
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A University of Oregon
Web page does a terrific job of explaining what blackbody radiation,
star color, and stellar temperature are all about. (It takes a few seconds
for the program to load, but it is worth it. Don't worry about the formula—just
enjoy the interaction with the charts. An Angstrom is an outdated unit
for wavelength that some astronomers are continuing to use. One nanometer,
a billionth of a meter, is equal to 10 Angstroms.) Under "Surfing
the Web," your textbook includes the link |
The text covers blackbody radiation at a comprehensive level. The Self-Review at the end of these notes covers the important points you should glean from the text.
What process do astronomers use in order to tell the temperature of a star just from its color?
Take a look at the blackbody curves shown in the figure that follows. Now consider being able to filter out all colors except for blue and yellow, or blue and red, or yellow and red, etc. Compare the amount of light that the telescope would receive from a star with a temperature of 7500 K through a blue filter as compared to a red filter. Do the same for a star of 6000 K and 4500 K. Can you think of how it works? We know the mathematical shapes of the blackbody curves exactly, and can compute a curve that matches the observations of the star's color.
Light is emitted or absorbed when electrons of an atom change their energy states. We think of electrons as jumping from one energy state to another. A photon is emitted when an electron jumps from a high energy state to a lower energy state. The photon must somehow get rid of that energy and it does so by emitting energy in the form of electromagnetic radiation. (Think of gravitational potential energy: if you are 3 meters up on a ladder, you have more potential energy than when you are standing on the ground. If you were to jump down to the floor, you would lose that potential energy.) In order for an electron to jump to a higher energy state, it needs to somehow acquire enough energy to do so. It can get that energy at the expense of another electron by bumping into it (not on purpose, of course), or it can absorb the energy contained in a photon. (In our ladder analogy, you had to take in energy in order to be able to climb the ladder.) The more energy an electron absorbs, the higher the energy state.
A curious thing about atoms and their electrons: the electrons exist only in discrete energy states, and these energy states are different for the atoms of each element in the periodic chart. Each element of the periodic chart has a unique spectrum. Study and compare the emission lines of the various elements shown below: hydrogen, helium, neon, mercury. How do the patterns of the emission lines differ? How do you think astronomers use this knowledge of the patterns and colors to determine if certain elements are present in stars? The different spectra of stars are often compared to individuals having different fingerprints. Why is this a good analogy? Because every element's signature spectrum is unique. Not only that, but the spectra of that same element ionized one or more times are different. Isotopes of an element have different spectra. As part of the lab for this lesson, you will be closely examining the spectra of a few different elements.
| Element | Spectrum |
|---|---|
| Argon |  |
| Helium |  |
| Mercury |  |
| Neon |  |
| Sodium |  |
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Now let's apply those principles to observations of an astronomical object, the Ring Nebula found in the constellation Lyra. Let's say we are at our telescope, using a spectrograph to record the spectra of the various regions in and around the Ring Nebula. What would we see?
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Since most of the gas expelled was hydrogen, we see that familiar pattern. The Ring Nebula is an example of a planetary nebula. The electrons of the hydrogen atoms in the gas are being energized by the photons coming from the hot white dwarf. Planetary nebulae will be studied in greater detail when we get to Chapter 13; you may want to look ahead to that chapter now. |
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Cycle of the different spectra "recorded" at each location. |
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(Note that the side view sketch of the geometry of the Ring Nebula does not show how the Ring must actually look from the side; it certainly would not be a perfect rectangle. Compare this image to Fig. 4.18 of the text. Does it make sense?) |
Let's take some of the concepts introduced above and work with them so we get a better understanding. We will just be dealing with the hydrogen atom. It's complicated enough, and the helium atom—with just two electrons—is so much more complicated that it would be impossible to describe here. We can use the Bohr model of the atom only for hydrogen. The above link will take you to a more detailed mathematical explanation. It is important that you try to follow the logic.
If you take a look back at the deep-sky images that were included in Lesson One, in particular the eta Carinae, horsehead, and Orion nebulae, you will see red, glowing gas. That gas is hydrogen, and it looks read because there are electrons jumping from energy level 3 to energy level 2 and emitting a photon having a wavelength of 656.3 nm. Now, the electrons are also jumping from higher energy levels to energy level 2, they all have multiple energy levels available to them, remember, but the red wavelengths dominate. Take a look also at the image of the Eagle Nebula that opens Chapter 12 (p. 262) of your text. Hydrogen gas here is getting energy from the nearby stars that is precisely the right amount to excite electrons from energy level 2 to higher energies. Then, the electrons jump back to energy level 2, and the energy is emitted as photons. Where we are observing the thin cloud of hydrogen gas without a continuous source behind it (that is, a star) we will see an emission spectrum containing all of the Balmer lines. Sincere there are places where we must look through the thin cloud of gas in order to observe a star, we will see an absorption spectrum.
A word of caution must be introduced here when it comes to interpreting the colors seen in the images in the text, online, and in these notes: colors can be enhanced and misrepresented for clarity (and confusion, unfortunately). For example, you just took a look at the Eagle Nebula on page 262. Now, take a look at Fig. 12.3 (b) that shows the region around the constellation of Orion. "Look at all of that glowing hydrogen!" you may think. But, take a look at the caption: "Heated dust clouds dominate in this false color [infrared] image, and . . . ." Dust, not hydrogen. What's important here is to read the accompanying text completely and find out in what part of the spectrum the object was being observed to fully understand what is being presented.
One more area of confusion is related to the energies of the electrons in atoms and the emission or absorption lines the energy jumps produce, and the amount of energy and maximum wavelengths we measure as a result of the blackbody temperature of the star. A physical process can produce lines in the high-energy part of the spectrum (ultraviolet, x-ray, and gamma ray). The corona of the Sun has a temperature of over 1 million degrees and so we will see emission lines of hydrogen that are from the Lyman series mentioned above because at that temperature there is a lot of high-energy photons that can make an electron jump up from energy level 1. But, the corona of the Sun is definitely not a blackbody. We would have to do spectroscopy with a telescope, though, that can detect ultraviolet radiation, and also one that needs to orbit above the Earth's atmosphere. On the other hand, a very hot star—one with a blackbody temperature of 30,000 Kelvin—radiates most of its energy at ultraviolet wavelengths, around 100 nm. These stars also need a telescope above the atmosphere to detect most of their light, but the detectors or the purpose of the research may be different.
Now, let's apply our newly found knowledge about light to the most important tools for astronomy: telescopes. The primary purpose of a department-store telescope is to gather the light from a nearby object and magnify it. The primary purpose of astronomical telescopes is to simply gather as much light as possible and guide it to a detector for recording the signal, and then to storing the information on a computer. The text goes into detail about the different kinds of telescopes, where telescopes are located, the names of telescopes, etc. Read through those sections noting what objects in the Universe will be observed at what wavelengths with what kinds of telescopes. Refer to the table above for additional information. Note the regions of the spectrum shere we need to have a telescope above the atmosphere, versus on the ground. For our purposes in these notes, it is enough to understand the differences between a refracting and a reflecting telescope, to understand how we calculate the magnification of a telescope, and to know the terminology used when talking about telescopes.
Refracting telescopes work on the principle that light (electromagnetic radiation) bends or refracts when it passes from one medium into another. We are all familiar with a magnifying glass (and, hopefully, not through the number of ants we toasted) and how it can focus sunlight into a small spot. Refracting telescopes use the refractive properties of lenses to focus light from a distant object onto a detector. An objective lens focuses this light to its focal point, and then an eyepiece lens passes the light onto a detector.
The ratio of the focal length of the objective lens to the eyepiece lens gives the magnification of the telescope:
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Refracting telescopes have serious drawbacks:
Reflecting telescopes, which overcome all of these problems, are the primary type of telescope used by astronomers today.
Find a vanity mirror that enlarges the image of your face—it uses the same principle that reflecting telescopes use. While a flat mirror (hopefully, the main mirror in your bathroom) bounces light directly back at you giving an image the same size as your face, a curved mirror causes an image (your face, in this case) to be distorted. We can shape a mirror such that it will reflect incoming light and focus it at one point, and combine this primary mirror with additional mirrors to magnify objects.
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The aperture of a telescope, its light gathering power, and its angular resolution are intimately related. The aperture of a telescope is the diameter of the primary mirror (talking about a reflecting telescope here). The light gathering power of the telescope is proportional to the diameter of the telescope squared; thus, if you double the diameter of your primary mirror, you quadruple the amount of light-gathering power it has. The angular resolution of a telescope describes how much detail it "sees": the larger the telescope the better the resolution.
There's a caveat to this simplified relationship between aperture and resolution—the resolution for a given telescope also depends on the wavelength at which the observations are being made. As the wavelength of radiation doubles, resolution is halved. This is one of the reasons we need extremely large radio dishes to resolve radio sources, but only a small satellite to resolve ultraviolet sources.
Reflecting telescopes have a number of positive characteristics:
Q: Where in an atom would you find electrons? Protons? Neutrons? Summarize the modern view of an atom, using hydrogen as an example.
We will be talking about different wavelengths and different regions of the electromagnetic spectrum throughout the remainder of this course. It is advantageous for you to be able to recognize what energies we are considering.
Q: Which of the following options orders light from the longest wavelengths to the shortest correctly? (Try not to refer to your notes.)
Q: After you have checked the answer, have we ordered the light from the lowest energies to the highest energies, or from highest to lowest?
Q: What do we mean when we talk about light being a particle and light being a wave?
Q: Given these examples, is light going to behave as a particle or as a wave?
Q: The spectrum
for the hydrogen atom is shown here in emission and in absorption. There are
no differences in the wavelengths of the lines between the emission spectrum
and the absorption spectrum. Why not? Should there be?
Q: What is a blackbody? How does the energy emitted by a blackbody depend on its temperature? How does the "peak" wavelength—where the maximum of the blackbody curve occurs—depend on the temperature?
Q: Assume the Sun radiates as a blackbody with a temperature of 5800 degrees Kelvin. Calculate the wavelength at which it radiates most of its energy. What part of the spectrum does this wavelength lie? Does radiation at these wavelengths reach the surface of the Earth? How does this wavelength correlate to the range of wavelengths that our eyes are capable of seeing?
Q: Here is a deep-sky object taken from Lesson One, the Horsehead Nebula, with different objects or lines of sight marked. Also shown is a sketch of how this region might look if seen from the side, with the arrows marking where our telescopes are pointing—Earth would be about 1500 light years to the right of this sketch. Arrow A points to a star, acting as a blackbody, observed through the thinner and cooler gas cloud of hydrogen. Arrow B points to the cloud of gas, with just empty space seen behind it. Arrow C points to the dust pillar, dust that is heated to a temperature of 300 degrees Kelvin. We'll assume that the dust is radiating as a blackbody. Along which arrow will we see an absorption spectrum? How about an emission spectrum? A continuous spectrum?
Q: State the most important function of research telescopes and why aperture is important.
Q: Contrast a reflecting telescope with a refracting telescope in design and usage. Which is most advantageous overall?
Q: Go back up in these notes to the table containing the various kinds of electromagnetic radiation, the frequencies, wavelengths, and typical astronomical sources. Relate the types of telescopes and the corresponding wavelengths to the sorts of objects or physical conditions that would be observed with them, and whether or not these telescopes can be ground-based or must be above the atmosphere. A nicely organized table should do.
Q: Why are astronomers concerned with observing objects at all wavelengths?
