For each of the following wavelengths, state whether it is in the radio, microwave, infrared, visible, ultraviolet, X-ray, or gamma-ray portion of the electromagnetic spectrum and explain your reasoning:
A cellular phone is actually a radio transmitter and receiver. You receive an incoming call in the form of a wave of frequency 880.65 MHz. What is the wavelength (in meters) of this wave?
Using Wien’s law and the Stefan-Boltzmann law, explain the color and intensity changes that are observed as the temperature of a hot, glowing object increases.
If you double the Kelvin temperature of a hot piece of steel, how much more energy will it radiate per second?
The bright star Bellatrix in the constellation Orion has a surface temperature of 21,500 K. What is its wavelength of maximum emission in nanometers? What color is this star?
The bright star Antares in the constellation Scorpius (the Scorpion) emits the greatest intensity of light at a wavelength of 853 nm. What is the surface temperature of Antares? What color is this star?
Explain how Bohr’s model of the atom accounts for spectra.
Why do different elements display different patterns of lines in their spectra?
What is the Doppler effect? Why is it important to astronomers?
If you see a blue star, what does its color tell you about how the star is moving through space? Explain your answer.
With the aid of a diagram, describe a refracting telescope.
With the aid of a diagram, describe a reflecting telescope.
Which dimensions of the telescope determine its light-gathering power?
What is the purpose of a telescope eyepiece?
Quite often advertisements appear for telescopes that extol their magnifying power. Is this a good criterion for evaluating telescopes? Explain your answer.
Explain some of the disadvantages of refracting telescopes compared to reflecting telescopes.
What is the angular resolution of a telescope?
What is adaptive optics?
What is a charge-coupled device (CCD)? Why have CCDs replaced photographic film for recording astronomical images?
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Why can radio astronomers make observations at any time during the day, whereas optical astronomers are mostly limited to observing at night? (Hint: Does your radio work any better or worse in the daytime than at night?)
Why must astronomers use satellites and Earth-orbiting observatories to study the heavens at X-ray and gamma-ray wavelengths?
The human eye is most sensitive over the same wavelength range at which the Sun emits the greatest intensity of light. Suppose creatures were to evolve on a planet orbiting a star somewhat hotter than the Sun. To what wavelengths would their vision most likely be sensitive?
Why do you suppose that ultraviolet light can cause skin cancer but ordinary visible light does not?
If you were in charge of selecting a site for a new observatory, what factors would you consider important?
Discuss the advantages and disadvantages of using a small telescope in Earth’s orbit versus a large telescope on a mountaintop.
The Doppler effect describes how relative motion impacts wavelength. With a classmate, stand up and demonstrate each of the following: (a) a blueshifted source for a stationary observer; (b) a stationary source and an observer detecting a redshift; and (c) a source and an observer both moving in the same direction, but the observer is detecting a redshift. Create simple sketches to illustrate what you and your classmate did.
Stand up and have everyone in your group join hands, making as large a circle as possible. If a telescope mirror were built as big as your circle, what would be its diameter?
Are there enough students in your class to stand and join hands and make two large circles that recreate the sizes of the two Keck telescopes? Explain how you determined your answer.
You can use the Starry Night™ program to measure the speed of light by observing a particular event, in this case one of Jupiter’s moons emerging from the planet’s shadow, from two locations separated by a known distance. These two locations are at the north poles of Earth and the planet Mercury respectively. Open Favourites > Explorations > Io from Earth. The view shows Jupiter as seen from the north pole of Earth at 9:12:00 p.m. Standard Time on September 25, 2010. You will see the label for Io to the left of the planet. Keep the field of view about 11 arcmin wide. Click the Play button and observe Io suddenly brighten as it emerges from Jupiter’s shadow. Depending upon your computer monitor, you may need to Zoom out slightly so that Io’s transition from being invisible to visible occurs instantaneously (at high zoom levels, Io will brighten gradually). Next, use the time controls to Step time backward and forward in 1-s intervals to determine the time to the nearest second at which Io brightens. Open the Status pane and expand the Time layer. Record the Universal Time for this event. Then open the Info pane and be sure that Io Info appears at the top of the Info pane. Expand the Position in Space layer and record the value given for Distance from Observer. Now select Favourites > Explorations > Io from Mercury.
This view once again shows Io labeled to the left of Jupiter but in this instance you are viewing the scene from the north pole of the planet Mercury. With the field of view set to 11 arcmin wide, click the Play button. Stop time flow as soon as you see Io suddenly brighten as it emerges from eclipse. Again, it may be necessary to adjust the Zoom level to make this event appear instantaneous rather than gradual. Use the time controls to find the time to the nearest second at which Io brightens. Open the Status pane and record the Universal Time for this event as seen from Mercury. Then open the contextual menu for Io and select Show Info. Record the Distance from Observer of Io from the Position in Space layer. Use your observations to calculate the speed of light. First, calculate the difference in the time between the two observations in seconds. Then calculate the difference in the Distance from Observer in AU for each of the locations. Divide the difference in the distance by the difference in time to calculate the speed of light in AU per second. Finally, convert this value to kilometers per second by multiplying the result by 1.496 × 108 (the number of kilometers in one AU). How does your result compare to the accepted value of the speed of light of 2.9979 × 105 km/s? Explain the difference between your calculated value from these observations and the accepted value for the speed of light.
Use Starry Night™ to examine the temperatures of several relatively nearby stars. 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.
Then answer the following questions:
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?
Use Starry Night™ to examine the celestial objects listed below. Select Favourites > Explorations > Atlas to show the whole sky as would be seen from 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 objects listed below, 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. The objects to observe are: