Questions

Review Questions

Question 10.1

Explain the difference between a star’s apparent brightness and its luminosity.

Question 10.2

Why does it take at least six months to make a measurement of a star’s parallax?

Question 10.3

What is the inverse-square law? Use it to explain why an ordinary lightbulb can appear brighter than a star, even though the lightbulb emits far less light energy per second.

Question 10.4

Why is the magnitude scale called a “backward” scale? What is the difference between apparent magnitude and absolute magnitude?

Question 10.5

The star Zubenelgenubi (from the Arabic for “scorpion’s southern claw”) has apparent magnitude 2.75, while the star Sulafat (Arabic for “tortoise”) has apparent magnitude 3.25. Which star appears brighter? From this information alone, what can you conclude about the luminosities of these stars? Explain your answer.

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Question 10.6

Menkalinan (Arabic for “shoulder of the rein-holder”) is an A2 star in the constellation Auriga (the Charioteer). What is its spectral class? What is its spectral type? Which gives a more precise description of the spectrum of Menkalinan?

Question 10.7

If a red star and a blue star both have the same radius and both are the same distance from Earth, which one looks brighter in the night sky? Explain why.

Question 10.8

If a red star and a blue star both appear equally bright and both are the same distance from Earth, which one has the larger radius? Explain why.

Question 10.9

Sketch a Hertzsprung-Russell diagram. Indicate the regions on your diagram occupied by (a) main-sequence stars, (b) red giants, (c) supergiants, (d) white dwarfs, and (e) the Sun.

Question 10.10

What information about stars do astronomers learn from binary systems that cannot be learned in any other way? What measurements do they make of binary systems to garner this information?

Question 10.11

Which is more massive, a red main-sequence star or a blue main-sequence star? Which has the greater radius? Explain your answers.

Web Chat Questions

Question 10.1

In its orbit around Earth, the Hipparcos satellite could measure stellar parallax angles with acceptable accuracy only if the angles were larger than about 0.002 arcsec. Discuss the advantages or disadvantages of making parallax measurements from a satellite in a large solar orbit, say at the distance of Jupiter from the Sun. If this satellite can also measure parallax angles of 0.002 arcsec, what is the distance of the most remote stars that can be accurately determined? How much bigger a volume of space would be covered compared to Earth-based observations? How many more stars would you expect to be contained in that volume?

Question 10.2

As seen from the starship Enterprise in the Star Trek television series and movies, stars appear to move across the sky due to the starship’s motion. How fast would the Enterprise have to move in order for a star 1 pc away to appear to move 1° per second? (Hint: The speed of the star as seen from the Enterprise is the same as the speed of the Enterprise relative to the star.) How does this compare with the speed of light? Do you think the stars appear to move as seen from an orbiting space shuttle, which moves at about 8 km/s?

Collaborative Exercises

Question 10.1

Considering where your group is sitting right now, how many times dimmer would an imaginary, super-deluxe, ultrabright flashlight be if it were located at the front door of the group member who lives farthest away as compared to if it were at the front door of the group member who lives closest. Explain your reasoning.

Question 10.2

As a group, select any two of the stars in a text appendix listing of the sky’s brightest stars and compare the apparent visual magnitudes to determine how many times brighter one is than the other.

Observing Questions

Question 10.1

Use Starry Night™ to examine the 10 brightest stars in Earth’s night sky. Select Favourites > Explorations > Atlas. Use the View > Constellations menu command to display constellation Boundaries, Labels, and Astronomical stick figures. Use the File (Windows) or Starry Night (Mac) menu command to open the Preferences dialog window. Ensure that the Cursor Tracking (HUD) preferences include Apparent Magnitude, Distance from Observer, Luminosity, and Temperature in the Show list. Before closing the Preferences dialog window, it might be helpful to increase the saturation for Star colour under the Brightness/Contrast preferences. Click on the Lists side pane tab, expand the Observing Lists, and click the 10 Brightest Stars option. Then expand the List Viewer layer and select All Targets from the Show dropdown menu to see a list of the 10 brightest stars in Earth’s night sky. Double-click on each of the stars in this list in turn to center the star in the view. Use the HUD to compile a table of these stars that includes each star’s apparent magnitude, distance, luminosity, and temperature. You may also wish to sketch the star’s position within its constellation. Alternatively, you may find it helpful to print out relevant star charts around these stars, using Starry Night™. (a) Which is the brightest star in Earth’s night sky? What features of this star make it so bright in our sky? (b) Which of these brightest stars has the highest temperature? What would you expect to be the color of this star compared to others in the list? (c) Which of these stars is intrinsically the most luminous? (d) Use Starry Night™ to determine which of these stars is visible from your location. Click the Home button, then the Stop button, and finally the Sunset button to show the view from your home location today at sunset. Again, it may be helpful to display the constellation Boundaries, Labels, and Astronomical stick figures in the view. Open the Lists side pane and double-click each entry in the list of the 10 Brightest Stars. If the star is visible in your sky, the program will center it in the view or alternately suggest a Best Time for observing this star. For those stars in the list that are visible from your home location, go outside if possible and observe them in the real sky. See if you can tell which of these stars has the highest temperature on the basis of your conclusion regarding the star’s color and check your estimate against the table you compiled in part a. (Hint: The colors of stars are not very distinct and a dark sky background is needed in order to distinguish differences in stellar colors.)

Question 10.2

Use the Starry Night™ program to investigate the Hertzsprung-Russell (H-R) diagram. Select Favourites > Explorations > Denver. Open the Status pane, expand the H-R Options layer, and choose the following options: Use absolute magnitudes and Labels. In the expanded Labels panel, click On the Gridlines, Regions, Main Sequence, and Spectral class options. Now, expand the Hertzsprung-Russell layer to show the H-R diagram that plots all of the stars that are currently in the main view. This graphical representation shows the absolute magnitudes of stars as a function of their spectral class. The sequence of spectral class, from O, B, A, F, G, K and M, represents the star’s surface temperatures, plotted in an inverse direction, the hottest O-type stars appearing to the left of the diagram. Absolute magnitude is related to the star’s luminosity, the smaller the absolute value of absolute magnitude, the larger the luminosity. (a) Use the hand tool to scroll around the sky. Watch the H-R diagram change as different stars enter and leave the main window. Right-click (Ctrl-click on a Mac) on a blank part of the sky and select Hide Horizon from the contextual menu so that you can survey the entire sky. Does the distribution of stars in the H-R diagram change drastically from one part of the sky to another, or are all types of stars approximately equally represented in all directions from Earth? (b) If you place the cursor over a star, a red dot appears in the H-R diagram at the position for this star. Use this facility to estimate and make a note of the position on the H-R diagram of each of the following stars that are labeled in the main window: Altair, Deneb, Enif, 74 Ophiuchi, and 51 Pegasi. Which of these five stars is most similar to the Sun? (c) What is the name of the region of the H-R diagram occupied by this star?

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