Question 4.1

A blackbody glowing with which of the following colors is hottest?

• a. yellow
• b. red
• c. orange
• d. violet
• e. blue

Question 4.2

Of the following photons, which has the lowest energy?

• a. infrared
• b. gamma ray
• c. visible light
• d. ultraviolet
• e. X-ray

Question 4.3

The spectrum of which of the following objects will show a blueshift?

• a. an object moving just eastward on the celestial sphere
• b. an object moving just northward on the celestial sphere
• c. an object moving directly toward Earth
• d. an object moving directly away from Earth
• e. an object that is not moving relative to Earth

Question 4.4

What is a blackbody? What does it mean to say that a star appears almost like a blackbody? If stars appear to be like blackbodies, why are they not black?

Question 4.5

What is Wien’s law? How could you use it to determine the temperature of a star’s surface?

Question 4.6

What is the Stefan-Boltzmann law? How do astronomers use it?

Question 4.7

Using Wien’s law and the Stefan-Boltzmann law, state the changes in color and intensity that are observed as the temperature of a hot, glowing object increases.

Question 4.8

What color will an interstellar gas cloud composed of hydrogen glow, and why?

Question 4.9

What is an element? List the names of five different elements, and briefly explain what makes them different from each other.

Question 4.10

How are the three isotopes of hydrogen different from each other?

Question 4.11

Explain how the spectrum of hydrogen is related to the structure of the hydrogen atom.

Question 4.12

Why do different elements have different patterns of lines in their spectra?

Question 4.13

What is the Doppler shift, and why is it important to astronomers?

Question 4.14

Explain why the Doppler shift tells us only about the motion directly along the line of sight between a light source and an observer, but not about motion across the celestial sphere.

Question 4.15

*Approximately how many times around the world could a beam of light travel in 1 s?

Question 4.16

*The bright star Regulus in the constellation of Leo (the Lion) has a surface temperature of 12,200 K. Approximately what is the dominant wavelength (λ_max) of the light that it emits?

Question 4.17

*The bright star Procyon in the constellation of Canis Minor (the Little Dog) emits the greatest intensity of radiation at a wavelength λ_max = 445 nm. Approximately what is the surface temperature of the star in kelvins?

Question 4.18

*As observed from Earth, the wavelength of H_α in the spectrum of the star Megrez in the Big Dipper is 486.112 nm. Laboratory measurements demonstrate that the normal wavelength of this spectral line is 486.133 nm. Is the star coming toward us or moving away from us? At what speed?

Question 4.19

*In the spectrum of the bright star Rigel, H_α is observed to have a wavelength of 656.331 nm. Is the star coming toward us or moving away from us? How fast?

Question 4.20

*Imagine driving down a street toward a traffic light. How fast would you have to go so that the red light (700 nm) would appear green (500 nm)? What fraction of the speed of light c (c = 300,000 km/s) is this velocity?

Question 4.21

Compare the technique of identifying chemicals by their spectral line patterns with that of identifying people by their fingerprints.

Question 4.22

Suppose you look up at the night sky and observe some of the brightest stars with your naked eye or binoculars. Is there any way of telling which stars are hotter and which are cooler? Explain your answer.

Question 4.23

How could we exclude Earth’s atmosphere as the source of the iron absorption lines in Figure 4-6?

Question 4.24

*The Sun were twice its actual diameter but still had the same surface temperature? At what wavelength would that new Sun emit its radiation most intensely? How many times brighter would the new Sun be than the present Sun? How might things on Earth be different under the bigger Sun?

Question 4.25

All of the nearby stars were observed to have redshifted spectra? What conclusions could we draw? What other measurement would be useful to have for each star, and why?

Question 4.26

No stars had any Doppler shift? What would that say about stellar motions relative to the solar system? Is that possible? Hint: Consider Newton’s law of gravitation from Chapter 3.

Question 4.27

The spectrum of the Sun (Figure 4-4) had several absorption lines missing when taken from above Earth’s atmosphere compared to its spectrum taken at Earth’s surface? What would this indicate about the chemistries of the Sun and of Earth’s atmosphere?

Question 4.28

*Blackbody Peak Colors. Search the Web for an interactive blackbody calculator that uses Wien’s equation to give the peak wavelength as a function of temperature (try the keywords blackbody spectrum calculator). Noting that the visible spectrum runs from 400 nm (violet) to 700 nm (red), determine whether any of the following stars have a peak wavelength in the visible spectrum: Rigel, T = 14,000 K; Deneb, T = 9500 K; Arcturus, T = 4500 K; Vega, T = 11,500 K; Betelgeuse, T = 3100 K. Hint: You may have to convert length scales. Recall that 1 nm = 10⁻⁹ m.

Question 4.29

What color does the Sun emit most intensely?

Question 4.30

A star of which of the following colors is coolest?

• a. blue
• b. red
• c. orange
• d. yellow
• e. violet

Question 4.31

If a yellow star cools off, what color will it become next?

Question 4.32

Do we see all the colors from each star? Why or why not?

Question 4.33

Use the Starry Night™ program to compare the brightness of two similarly sized stars in the constellation Auriga. Select Favourites > Explorations > Auriga. The two stars Capella and Delta Aurigae are labeled in this view. Select Preferences from the File menu (Windows) or Starry Night menu (Mac) and set Cursor Tracking/HUD options so that Temperature and Radius are shown in the HUD display. You will notice that these two stars have the same radius but differ in temperature. From these data, which of these stars is intrinsically brighter and by what proportion?

Question 4.34

Use Starry Night™ to examine the temperatures of several relatively nearby stars, listed below. Select Favourites > Explorations > Atlas. Use the File menu (Starry Night menu on a Mac) and select Preferences… to open the Preferences dialog window. Click the box in the top left of this dialog window and choose Cursor Tracking (HUD). Scroll through the Show list and click the checkbox next to Temperature to turn this option on. Then close the Preferences dialog window. Next, open the Find pane, click the magnifying glass icon in the edit box at the top of this pane, and select Star from the dropdown menu. To locate each of the following stars: (i) Altair; (ii) Procyon; (iii) Epsilon Indi; (iv) Tau Ceti; (v) Epsilon Eridani; (vi) Lalande 21185, type the name of the star in the edit box and then press the Enter (Return) key. Use the HUD to find and record the star’s temperature. (a) Which of the stars have a longer wavelength of maximum emission λ_max than the Sun? (b) Which of the stars have a shorter λ_max than the Sun? (c) Which of the stars will have a reddish color?

126

Question 4.35

Use Starry Night™ to examine the light from the following celestial objects: (a) The Lagoon Nebula in Sagittarius. (In a field of view of about 6° × 4°, you can compare and contrast the appearance of the Lagoon Nebula with the Trifid Nebula, just to the north of it.) (b) M31, the great galaxy in the constellation Andromeda. (Hint: The light coming from this galaxy is the combined light of hundreds of billions of individual stars.) (c) The Moon. (Hint: Moonlight is simply reflected sunlight.)

Select Favourites > Explorations > Atlas to show the whole sky from the viewpoint of the center of a transparent Earth. Ensure that deep space objects are displayed by selecting View > Deep Space > Messier Objects and View > Deep Space > Bright NGC Objects from the menu. Also, select View > Deep Space > Hubble Images and ensure that this option is turned off. To display each of the above objects, open the Find pane and then type the name of the selected object in the edit box followed by the Enter (Return) key. This object will be centered in the view. Use the zoom controls to adjust your view until you can see the object in detail. For each object, decide whether you think it will have a continuous spectrum, an absorption line spectrum, or an emission line spectrum, and explain your reasoning.