Quiz 1

Student Last Name____________First Name________________________9/23/2014
1. Sound, light, and gamma rays are all forms of electromagnetic radiation.
2. In a vacuum, electromagnetic waves all travel at the same speed.
3. A blackbody emits all its radiation at a single wavelength or frequency.
4. For an emission spectrum produced by a container of hydrogen gas, changing the amount of hydrogen in the container will change the color of the lines in the spectrum.
5. The wavelengths of the emission lines produced by an element are different from the wavelengths of the absorption lines produced by the same element.
6. The energy of a photon is inversely proportional to the wavelength of the radiation.
7. An electron can have any energy within an atom, so long as that energy is above the ground-state energy.
8. The _____ of a wave is the distance between any two adjacent wave crests.
9. When a charged particle moves, information about this motion is transmitted through space via the particle’s changing _____ and _____ fields.
10. Earth’s atmosphere has low opacity for three forms of electromagnetic radiation. They are _____, _____, and _____.
11. The peak of an object’s emitted radiation occurs at a frequency or wavelength determined by the object’s _____.
12. Two identical objects have temperatures of 1000 K and 1200 K. It is observed that one of the objects emits roughly twice as much radiation as the other. Which one is it? _____.
13. Fraunhofer discovered absorption lines in the spectrum of _____.
14. When moving to a lower energy level in an atom, an electron _____ a photon of a specific energy.
15. The radiation received from a source moving away from the observer will be shifted toward _____ frequencies by the _____ effect.
16. An x- wave moving through the medium has a frequency of 200 Hz and a wavelength of 6 m. What is the speed of  x- wave  in the medium?
17. What is the wavelength of a 10-MHz (“FM 10”) radio signal?
18. What would be the frequency of an electromagnetic wave having a wavelength equal to Earth’s diameter? In what part of the electromagnetic spectrum would such a wave lie?
19. The blackbody emission spectrum of object A peaks in the ultraviolet region of the electromagnetic spectrum, at a wavelength of 200 nm. That of object B peaks in the red region, at 650 nm. According to Wien’s law, how many times hotter than B is A? According to Stefan’s law, how many times more energy per unit area does A radiate per second?
20. Normal human body temperature is about 37°C. What is this temperature in kelvins? What is the peak wavelength emitted by a person with this temperature? In what part of the spectrum does this lie?
21. According to the Stefan-Boltzmann law, how much energy is radiated into space per unit time by each square meter of the Sun’s surface (see More Precisely 2-2)? If the Sun’s radius is 696,000 km, what is the total power output of the Sun?
22. By what factor does the energy of a 1000 MHz photon exceed that of a 10 MHz radio photon?
23. How many different photons (that is, photons of different frequencies) can be emitted as a hydrogen atom in the second excited state falls back, directly or indirectly, to the ground state? What are their wavelengths? What about a hydrogen atom in the third excited state?
24. The line (Sec. 2.6) of a certain star is received on Earth at a wavelength of 656 nm. What is the star’s radial velocity with respect to Earth?
25. Imagine you are observing a spacecraft moving in a circular orbit of radius 100,000 km around a distant planet. You happen to be located in the plane of the spacecraft’s orbit. You find that the spacecraft’s radio signal varies periodically in wavelength between 2.99964 m and 3.00036 m. Assuming that the radio is broadcasting normally, at a constant wavelength, what is the mass of the planet?

2.4 Thermal Radiation All macroscopic objects—fires, ice cubes, people, stars—emit radiation at all times. They radiate because the microscopic charged particles in them are in constant random motion, and whenever charges change their state of motion, electromagnetic radiation is emitted. The temperature of an object is a direct measure of the amount of microscopic motion within it (see More Precisely 2-1). The hotter the object, the higher its temperature, the faster its constituent particles move and the more energy they radiate. The Blackbody Spectrum Intensity is a term often used to specify the amount or strength of radiation at any point in space. Like frequency and wavelength, intensity is a basic property of radiation. No natural object emits all of its radiation at just one frequency. Instead, the energy is often spread out over a range of frequencies. By studying the way in which the intensity of this radiation is distributed across the electromagnetic spectrum, we can learn much about the object’s properties. Figure 2.9 illustrates schematically the distribution of radiation emitted by any object. Note that the curve peaks at a single, well-defined frequency and falls off to lesser values above and below that frequency. However, the curve is not symmetrical about the peak—the falloff is more rapid on the high-frequency side of the peak than it is toward lower frequencies. This overall shape is characteristic of the radiation emitted by any object, regardless of its size, shape, composition, or temperature. Figure 2.9 Ideal Blackbody Curve The blackbody curve represents the spread of the intensity of the radiation emitted by any object. The curve drawn in Figure 2.9 refers to a mathematical idealization known as a blackbody—an object that absorbs all radiation falling upon it. In a steady state a blackbody must reemit the same amount of energy as it absorbs. Theblackbody curve shown in the figure describes the distribution of that reemitted radiation. No real object absorbs or radiates as a perfect blackbody. However, in many cases the blackbody curve is a very good approximation to reality, and the properties of blackbodies provide important insights into the behavior of real objects, including stars.The Radiation Laws The blackbody curve shifts toward higher frequencies (shorter wavelengths) and greater intensities as an object’s temperature increases. Even so, the shape of the curve remains the same (see Figure 2.10). This shifting of radiation’s peak frequency with temperature is familiar to us all. Very hot glowing objects, such as lightbulb filaments or stars, emit visible light because their blackbody curves peak in or near the visible range. Cooler objects, such as warm rocks or household radiators, produce invisible radiation—they are warm to the touch but are not glowing hot to the eye. These latter objects emit most of their radiation in the lower-frequency infrared portion of the electromagnetic spectrum. Figure 2.10 Blackbody Curves Comparison of blackbody curves for four cosmic objects. (a) A cool, invisible galactic gas cloud called Rho Ophiuchi. At a temperature of 60 K, it emits mostly low-frequency radio radiation. (b) A dim, young star (shown red in the inset photograph) near the center of the Orion Nebula. The star’s atmosphere, at 600 K, radiates primarily in the infrared. (c) The Sun’s surface, at approximately 6000 K, is brightest in the visible region of the electromagnetic spectrum. (d) Some very bright stars in a cluster called Omega Centauri, as observed by a telescope aboard the space shuttle. At a temperature of 60,000 K, these stars radiate strongly in the ultraviolet. (Harvard-Smithsonian Center for Astrophysics; J. Moran; AURA; NASA)  The Planck Spectrum This animation shows the growth of the black-body curve and the corresponding shift of peak wavelength with temperature for the spectrum of a heated object. The visible region of the spectrum is indicated. As temperature rises, the "red end" of the spectrum fills up first, so the object would begin to glow red, then orange-red, then yellowish-red. It eventually becomes "white hot" when the entire visible portion of the spectrum is filled in a roughly uniform way. At extremely high temperatures, such as the surface temperatures of very massive young stars, the peak of the spectrum moves beyond the visible range toward higher frequencies, and the resulting tilt in the portion of the spectrum that remains in the visible range causes the object to appear slightly more blue than white. There is a very simple connection between the frequency or wavelength at which most radiation is emitted and the absolute temperature (that is, temperature measured in kelvins—see More Precisely 2-1) of the emitting object: The peak frequency is directly proportional to the temperature. This relationship is more conventionally stated in terms of wavelength and is known as Wien’s law (after Wilhelm Wien, the German scientist who formulated it in 1897). In essence, it tells us that the hotter the object, the bluer its radiation. For example, an object with a temperature of 6000 K (Figure 2.10) emits most of its energy in the visible part of the spectrum, with a peak wavelength of 480 nm. At 600 K, the object’s emission would peak at 4800 nm, well into the infrared. At a temperature of 60,000 K, the peak would move all the way through the visible range to a wavelength of 48 nm, in the ultraviolet. It is also a matter of everyday experience that as the temperature of an object increases, the totalamount of energy it radiates (summed over all frequencies) increases rapidly. For example, the heat given off by an electric heater increases sharply as the heater warms up and begins to emit visible light. In fact, the total amount of energy radiated per unit time is proportional to the fourth power of an object’s temperature: This relationship is called Stefan’s law, after the nineteenth-century Austrian physicist Josef Stefan. It implies that the energy emitted by a body rises dramatically as the body’s temperature increases. Doubling the temperature, for example, causes the total energy radiated to increase by a factor of 16. The radiation laws are presented in more detail in More Precisely 2-2. Astronomical Applications Astronomers use blackbody curves as thermometers to determine the temperatures of distant objects. For example, study of the solar spectrum makes it possible to measure the temperature of the Sun’s surface. Observations of the radiation from the Sun at many frequencies yield a curve shaped somewhat like that shown in Figure 2.9. The Sun’s curve peaks in the visible part of the electromagnetic spectrum. The Sun also emits a lot of infrared and a little ultraviolet radiation. Applying Wien’s law to the blackbody curve that best fits the solar spectrum, we find that the temperature of the Sun’s surface is approximately 6000 K. (A more precise measurement yields a temperature of 5800 K.) Other cosmic objects have surfaces very much cooler or hotter than the Sun’s, emitting most of their radiation in invisible parts of the spectrum (Figure 2.10). For example, the relatively cool surface of a very young star might measure 600 K and emit mostly infrared radiation. Cooler still is the interstellar gas cloud from which the star formed. At a temperature of 60 K, such a cloud would emit mainly long-wavelength radiation in the radio and infrared parts of the spectrum. The brightest stars, by contrast, have surface temperatures as high as 60,000 K and hence emit mostly ultraviolet radiation. CONCEPT CHECK Describe, in terms of the radiation laws, how the appearance of an incandescent lightbulb changes as you turn a dimmer switch to increase its brightness from “off” to “maximum.”

2.3 The Electromagnetic Spectrum White light is a mixture of colors, which we conventionally divide into six major hues—red, orange, yellow, green, blue, and violet. As shown in Figure 2.7, we can separate a beam of white light into a rainbow of these basic colors—called a spectrum (plural: spectra)—by passing it through a prism. This experiment was first reported by Isaac Newton more than 300 years ago. In principle, the original beam of white light could be recovered by passing the spectrum through a second prism to recombine the colored beams. Figure 2.7 Visible Spectrum When passed through a prism, white light splits into its component colors, spanning red to violet in the visible part of the electromagnetic spectrum. The slit narrows the beam of radiation. The image on the screen is just a series of different-colored images of the slit. Human eyes are insensitive to radiation of wavelength shorter than 400 nm or longer than 700 nm. The Components of Visible Light What determines the color of a beam of light? The answer is its frequency (or, equivalently, its wavelength)—we see different colors because our eyes react differently to electromagnetic waves of different frequencies. Red light has a frequency of roughly corresponding to a wavelength of about  Violet light, at the other end of the visible range, has nearly double the frequency——and (since the speed of light is the same in either case) just over half the wavelength— The other colors we see have frequencies and wavelengths intermediate between these two extremes. Scientists often use a unit called the nanometer (nm) when describing the wavelength of light (see Appendix 2). There are  nanometers in one meter. An older unit called the angstrom  is also widely used by many astronomers and atomic physicists, although the nanometer is now preferred. Thus, the visible spectrum covers the wavelength range from 400 to 700 nm (4000 to 7000 Å). The radiation to which our eyes are most sensitive has a wavelength near the middle of this range, at about 550 nm (5500 Å), in the yellow-green region of the spectrum. The Full Range of Radiation Figure 2.8 plots the entire range of electromagnetic radiation. To the low-frequency, long-wavelength side of visible light lies radio and infrared radiation. Radio frequencies include radar, microwave radiation, and the familiar AM, FM, and TV bands. We perceive infrared radiation as heat. To the high-frequency, short-wavelength side of visible light lies ultraviolet, X-ray, and gamma-ray radiation. Ultraviolet radiation, lying just beyond the violet end of the visible spectrum, is responsible for suntans and sunburns. X rays are perhaps best known for their ability to penetrate human tissue and reveal the state of our insides without our resorting to surgery. Gamma rays are the shortest-wavelength radiation. They are often associated with radioactivity and are invariably damaging to any living cells they encounter. Figure 2.8 Electromagnetic Spectrum The entire electromagnetic spectrum. All these spectral regions, including the visible, collectively make up the electromagnetic spectrum. Remember that despite their greatly differing wavelengths and the very different roles they play in everyday life on Earth, all types of electromagnetic radiation are basically the same and all move at the same speed—the speed of light c. Figure 2.8 is worth studying carefully, as it contains a great deal of information. Note that wave frequency (in hertz) increases from left to right, and wavelength (in meters) increases from right to left. Scientists often disagree on the “correct” way to display wavelengths and frequencies in diagrams of this type. This book consistently adheres to the convention that frequency increases toward the right. Notice that the wavelength and frequency scales in Figure 2.8 do not increase by equal increments of 10. Instead, successive values marked on the horizontal axis differ by factors of 10—each successive value is 10 times greater than its neighbor. This type of scale (called alogarithmic scale) is often used in science in order to condense a very large range of some quantity into a manageable size. Throughout the text we will often find it convenient to use such a scale in order to compress a wide range of some quantity onto a single easy-to-view plot. Figure 2.8 shows wavelengths extending from the height of mountains for radio radiation to the diameter of an atomic nucleus for gamma-ray radiation. The box at the upper right emphasizes how small the visible portion of the electromagnetic spectrum is. Most objects in the universe emit large amounts of invisible radiation. Indeed, many objects emit only a tiny fraction of their total energy in the visible range. A wealth of extra knowledge can be gained by studying the invisible regions of the electromagnetic spectrum. To remind you of this important fact and to identify the region of the electromagnetic spectrum in which a particular observation was made, we have attached a spectrum icon—an idealized version of the wavelength scale in Figure 2.8—to every astronomical image presented in this text. Only a small fraction of the radiation arriving at our planet actually reaches Earth’s surface because of the opacity of Earth’s atmosphere. Opacity is the extent to which radiation is blocked by the material through which it is passing—in this case, air. The more opaque an object is, the less radiation gets through. Earth’s atmospheric opacity is plotted along the wavelength and frequency scales at the bottom of Figure 2.8. Where the shading is greatest, no radiation can get in or out—the energy is completely absorbed by atmospheric gases. Where there is no shading at all, our atmosphere is almost totally transparent. Note that there are just a few windows, at well-defined locations in the electromagnetic spectrum, where Earth’s atmosphere is transparent. In much of the radio and in the visible portions of the spectrum, the opacity is low, and we can study the universe at those wavelengths from ground level. In parts of the infrared range, the atmosphere is partially transparent, so we can make certain infrared observations from the ground. Over the rest of the spectrum, however, the atmosphere is opaque. As a result, ultraviolet, X-ray, and gamma-ray observations can be made only from above the atmosphere, from orbiting satellites. CONCEPT CHECK In what sense are radio waves, visible light, and X rays one and the same phenomenon?

2.2 Waves in What?

Waves of radiation differ in one fundamental respect from water waves, sound waves, or any other waves that travel through a material medium—radiation needs no such medium. When light travels from a distant cosmic object, it moves through the virtual vacuum of space. Sound waves, by contrast, cannot do this, despite what you have probably heard in almost every sci-fi movie ever made! If we were to remove all the air from a room, conversation would be impossible (even with suitable breathing apparatus to keep our test subjects alive) because sound waves cannot exist without air or some other physical medium to support them. Communication by flashlight or radio, however, would be entirely feasible.

The ability of light to travel through empty space was once a great mystery. The idea that light, or any other kind of radiation, could move as a wave through nothing at all seemed to violate common sense, yet it is now a cornerstone of modern physics.

Interactions Between Charged Particles

Figure 2.4 Charged Particles (a) Particles carrying like electrical charges repel one another; particles with unlike charges attract. (b) A charged particle is surrounded by an electric field, which determines the particle’s influence on other charged particles. We represent the field by a series of field lines. (c) If a charged particle begins to vibrate, its electric field changes. The resulting disturbance travels through space as a wave.

To understand more about the nature of light, consider anelectrically charged particle, such as an electron or aproton. Electrons and protons are elementary particles—fundamental components of matter—that carry the basic unit of charge. Electrons are said to carry a negative charge, while protons carry an equal and opposite positive charge. Just as a massive object exerts a gravitational force on any other massive object, an electrically charged particle exerts an electrical force on every other charged particle in the universe. (Sec. 1.4) Unlike gravity, however, which is always attractive, electrical forces can be either attractive or repulsive. Particles having like charges (both negative or both positive) repel one another; particles having unlike charges attract (Figure 2.4a).Extending outward in all directions from our charged particle is an electric field, which determines the electric force exerted by the particle on other charged particles (Figure 2.4b). The strength of the electric field, like that of the gravitational field, decreases with increasing distance from the source according to an inverse-square law. By means of the electric field, the particle’s presence is “felt” by other charged particles, near and far.

Now suppose our particle begins to vibrate, perhaps because it becomes heated or collides with some other particle. Its changing position causes its associated electric field to change, and this changing field in turn causes the electrical force exerted on other charges to vary (Figure 2.4c). If we measure the changes in the forces on these other charges, we learn about our original particle. Thus, information about our particle’s motion is transmitted through space via a changing electric field. This disturbance in the particle’s electric field travels through space as a wave.

Electromagnetic Waves

	Figure 2.5 Magnetism Earth’s magnetic field interacts with a magnetic compass needle, causing the needle to become aligned with the field—that is, to point toward Earth’s north (magnetic) pole.

The laws of physics tell us that a magnetic field must accompany every changing electric field. Magnetic fields govern the influence of magnetized objects on one another, much as electric fields govern interactions between charged particles. The fact that a compass needle always points to magnetic north is the result of the interaction between the magnetized needle and Earth’s magnetic field (Figure 2.5). Magnetic fields also exert forces on moving electric charges (that is, electric currents)—electric meters and motors rely on this basic fact. Conversely, moving charges create magnetic fields (electromagnets are a familiar example).

Electric and magnetic fields are inextricably linked to one another. A change in either onenecessarily creates the other. For this reason, the disturbance produced by our moving charge actually consists of oscillating electric and magnetic fields, always oriented perpendicular to one another and moving together through space (Figure 2.6). These fields do not exist as independent entities. Rather, they are different aspects of a single physical phenomenon: electromagnetism. Together they constitute an electromagnetic wave that carries energy and information from one part of the universe to another.

Electromagnetic Waves

Now consider a distant cosmic object—a star. It is made up of charged particles, mainly protons and electrons, in constant motion. As these charged contents move around, their electric fields change, and electromagnetic waves are produced. These waves travel outward into space, and eventually some reach Earth. Other charged particles, either in our eyes or in our experimental apparatus, respond to the electromagnetic field changes by vibrating in tune with the received radiation. This response is how we “see” the radiation—with our eyes or with our detectors.

Figure 2.6 Electromagnetic Wave Electric and magnetic fields vibrate perpendicular to each other. Together they form an electromagnetic wave that moves through space at the speed of light.

All electromagnetic waves move at a very specific speed—the speed of light (always denoted by the letter c). Its value is 299,792.458 km/s in a vacuum (and somewhat less in material substances, such as air or water). In this text, we round this value off to  This is an extremely high speed. In the time needed to snap a finger—about a tenth of a second—light can travel three quarters of the way around our planet! According to the Theory of Relativity (see More Precisely 13-1), the speed of light is the fastest speed possible.

CONCEPT CHECK

What is light? List some similarities and differences between light waves and waves on water or in air.

2.1 Information from the Skies

Figure 2.1 shows our nearest large galactic neighbor, which lies in the constellation Andromeda. On a dark, clear night, far from cities or other sources of light, the Andromeda Galaxy, as it is generally called, can be seen with the naked eye as a faint, fuzzy patch on the sky, comparable in diameter to the full Moon. Yet the fact that it is visible from Earth belies this galaxy's enormous distance from us. It lies roughly 2.5 million light-years away. An object at such a distance is truly inaccessible in any realistic human sense. Even if a space probe could miraculously travel at the speed of light, it would need two and a half million years to reach this galaxy and two and a half million more to return with its findings. Considering that civilization has existed on Earth for fewer than 10,000 years (and its prospects for the next 10,000 are far from certain), even this unattainable technological feat would not provide us with a practical means of exploring other galaxies—or even the farthest reaches of our own galaxy, several tens of thousands of light-years away.

	Figure 2.1 Andromeda Galaxy The pancake-shaped Andromeda Galaxy is about 2.5 million light-years away and contains a few hundred billion stars. (T. Hallas)

Light and RadiationHow do astronomers know anything about objects far from Earth? How can we obtain detailed information about any planet, star, or galaxy too distant for a personal visit or any kind of controlled experiment? The answer is that we use the laws of physics, as we know them here on Earth, to interpret the electromagnetic radiation emitted by these objects.Radiation is any way in which energy is transmitted through space from one point to another without the need for any physical connection between those two locations. The term electromagnetic means that the energy is carried in the form of rapidly fluctuating electric and magnetic fields (see Section 2.2). Virtually all we know about the universe beyond Earth's atmosphere has been gleaned from analysis of electromagnetic radiation received from afar.
Visible light is the particular type of electromagnetic radiation to which the human eye happens to be sensitive. But there is also invisible electromagnetic radiation, which goes completely undetected by our eyes. Radio, infrared, and ultraviolet waves, as well as X rays and gamma rays, all fall into this category. You should recognize that, despite the different names, the words light, rays, electromagnetic radiation, and waves really all refer to the same thing. The names are just historical accidents, reflecting the fact that it took many years for scientists to realize that these apparently very different types of radiation are in reality one and the same physical phenomenon. Throughout this text, we will use the general terms “light” and “electromagnetic radiation” more or less interchangeably.
Wave MotionAll types of electromagnetic radiation travel through space in the form of waves. To understand the behavior of light, then, we must know a little about this kind of motion. Simply stated, a wave is a way in which energy is transferred from place to place without physical movement of material from one location to another. In wave motion, the energy is carried by a disturbance of some sort that occurs in a distinctive, repeating pattern. Ripples on the surface of a pond, sound waves in air, and electromagnetic waves in space, despite their many obvious differences, all share this basic defining property.

Figure 2.2 Water Wave The passage of a wave across a pond causes the surface of the water to bob up and down, but there is no movement of water from one part of the pond to another. Here waves ripple out from the point where a pebble hit the water to the point where a twig is floating. The inset shows a series of “snapshots” of the pond surface as the wave passes by. The points numbered 1 through 5 represent surface locations that bob up and down with the passage of the wave.

As a familiar example, imagine a twig floating in a pond (Figure 2.2). A pebble thrown into the pond at some distance from the twig disturbs the surface of the water, setting it into up-and-down motion. This disturbance propagates outward from the point of impact in the form of waves. When the waves reach the twig, some of the pebble's energy is imparted to it, causing the twig to bob up and down. In this way, both energy and information—the fact that the pebble entered the water—are transferred from the place where the pebble landed to the location of the twig. We could tell just by observing the twig that a pebble (or some small object) had entered the water. With a little additional physics, we could even estimate the pebble's energy.A wave is not a physical object. No water traveled from the point of impact of the pebble to the twig—at any location on the surface, the water surface simply moved up and down as the wave passed. What, then, does move across the pond surface? The answer is that the wave is thepattern of up-and-down motion, and it is this pattern that is transmitted from one point to the next as the disturbance moves across the water.
Figure 2.3 shows how wave properties are quantified. The wave period is the number of seconds needed for the wave to repeat itself at some point in space. The wavelength is the number of meters needed for the wave to repeat itself at a given moment in time. It can be measured as the distance between two adjacent wave crests, two adjacent wave troughs, or any other two similar points on adjacent wave cycles (for example, the points marked “X” in Figure 2.3). The maximum departure of the wave from the undisturbed state—still air, say, or a flat pond surface—is called itsamplitude.

	Figure 2.3 Wave Properties Representation of a typical wave, showing its direction of motion, wavelength, and amplitude. In one wave period the entire pattern shown moves one wavelength to the right.

The number of wave crests passing any given point per unit time is called the wave'sfrequency. If a wave of a given wavelength moves at high speed, then many crests pass by per second and the frequency is high. Conversely, if the same wave moves slowly, then its frequency is low. The frequency of a wave is just one divided by the wave's period:

Frequency is expressed in units of inverse time (cycles per second), called hertz (Hz) in honor of the nineteenth-century German scientist Heinrich Hertz, who studied the properties of radio waves. A wave with a period of five seconds has a frequency of (1/5) 

Basic Wave Properties

A wave moves a distance equal to one wavelength in one wave period. The product of wavelength and frequency therefore equals the wave velocity:

Thus, if the wave in our earlier example had a wavelength of 0.5 m, its velocity is Wavelength and wave frequency are inversely related—doubling one halves the other.

CONCEPT CHECK
What is a wave? What four basic properties describe a wave, and what relationships, if any, exist among them?

LEARNING GOALS

	LEARNING GOALS Studying this chapter will enable you to: Discuss the nature of electromagnetic radiation and tell how that radiation transfers energy and information through interstellar space. Describe the major regions of the electromagnetic spectrum. Explain how we can determine an object's temperature by observing the radiation it emits. Describe the characteristics of continuous, emission, and absorption spectra, and the conditions under which each is produced. Specify the basic components of the atom and describe our modern conception of its structure. Explain how electron transitions within atoms produce unique emission and absorption spectra. Describe how the relative motion of a source of radiation and an observer can change the perceived wavelength of the radiation, and explain the importance of this phenomenon to astronomy. Visit astro.prenhall.com/chaisson for additional annotated images, animations and links to related sites for this chapter.
The Ring Nebula in the constellation Lyra is one of the most magnificent objects in our galaxy. Seen here glowing in the light of its own fading radiation, the nebula is actually the expanding atmosphere of a nearly dead star. The dying dwarf star can be seen at the center of the ring of hydrogen-rich gas, shown here in true color. Owing to the nebula's distance of some 5000 light-years, its apparent size is just 1/100th that of the full Moon. It is too dim to see with the naked eye. (STScl)
Astronomical objects are more than just things of beauty in the night sky. Planets, stars, and galaxies are of vital significance if we are to understand fully the big picture—the grand design of the universe. Every object is a source of information about the universe—its temperature, its chemical composition, its state of motion, its past history. The starlight we see tonight began its journey to Earth decades, centuries—even millennia—ago. The faint rays from the most distant galaxies have taken billions of years to reach us. The stars and galaxies in the night sky show us not just the far away but also the long ago. In this chapter, we begin our study of how astronomers extract information from the light emitted by astronomical objects. The observational and theoretical techniques that enable researchers to determine the nature of distant atoms by the way they emit and absorb light are the indispensable foundation of modern astronomy.