BUCKNELL UNIVERSITY

Astronomy 102

First Hour Exam 

February 16, 2000
This Exam will be scored on a 100 point scale and has two parts: a multiple choice/short answer section of 10 questions each worth 5 points and a problem section with 3 problems, each worth 16 points. You get 2 points for writing your name and seat number.

Name & Seat# ______________________


Here are some quantities and relationships that you might find useful. Remember that you may use your notes and/or the textbooks for this exam.
  •     for a wave: speed = wavelength x frequency
  •     the speed sound in air: cs = 340 meter/sec,           the speed of light: c = 3 x 108 meter/sec
  •     for light waves: energy = h x frequency = h x c/wavelength
  •     Planck's constant: h = 6.626 x 10-34 Joule x sec
  •     Doppler formula:           speed of emitter         change in frequency

    •                                           ------------------- = --------------------
                                               speed of wave           rest frequency
  •     wavelength of spectrum peak (in nanometers) =(3 x 106 nm K)/temperature
  •     1 nanometer = 10-9 meters
  •     for blackbody emitters: intensity = sigma x (temperature)4
  •     sigma = 5.67 x 10-8 W/(m2 K4)
  •     luminosity = intensity x surface area, or: Intensity = luminosity/area.
  •     for a sphere: surface area = 4 x p x radius
  •     magnitude scale: difference of one magnitude corresponds to a factor of ~2.5 in intensity
  •     distances: 1 Astronomical Unit (1 earth-sun distance)  = 1.495 x 1011 meter
  •     luminosity (power!) of the sun: 1 Lo = 3.8 x 1026 Watt
  •     1eV = 1.609 x 10-19 Joule

    • Part I: Multiple Choice Questions:

    1) A slinky is wiggled 10 times in 5 seconds. The distance between waves crests in the slinky is found to be 0.5 meters. The speed of the wave in the slinky is therefore 2) Lines of decreased brightness in the rainbow spectrum of a G2 star are probably due to 3) Light from a light bulb passes through a screen with two small openings and because light is a wave, produces an interference pattern of light and dark fringes on the second screen. What would you see on the second screen?

    4)  If you move at a very high speed away from a red traffic light you may see it (in your rear view mirror) as a 5) Satellite dishes absorb 6) Alpha Centauri, a G2 star, is 4.3 Light Years from earth. It has an apparent magnitude of 0.0. The G star 104-Tori in the constellation Taurus has an apparent magnitude 5.0. That second star is 7) Two spheres with 100 Watt power are located at the same distance from an observer. The first has a surface temperature of 2000 K, while the second has a surface temperature of 3000 K. We can therefore conclude that: 8) Helium atoms have the following energy levels:
    Third excited state      ___________________________ 51 eV
    Second excited state  ____________________________ 48.4 eV
     
     
     

    First excited state      ____________________________ 40.8 eV
     
     
     
     
     

    Ground state            _____________________________0.0 eV
    Given that these atoms are in the second excited state, they can absorb a photon of which of the following energies?

    9) Of the two stars of type G2 and G9 which one has the hotter surface? 10) Two stars are of types O2 and K9. The O2 star has ____ than the K9 star.

     

    Part II: Problems:

    (Show your work!! I will be very generous with partial credit!!!)
    Problem 1) Two stars, both in the Hyades cluster, have the same surface temperature of 2500 degrees Kelvin.
    a. (6 points) What is the color of these stars?

    Answer

    b. (6 points) If the first star has an overall surface area that is roughly two and a half times that of the second star, how much brighter will it be?

    Answer

    c. (3 points) If the magnitude of the first star is +12.3, what will be the magnitude of the second star? (3 sig. figs here please!)

    Answer

    Problem 2)
    a. (10 points) How would you find how many microwave photons (wavelength = 106 nm) it take to equal the energy of one X-ray photon (wavelength = 1 nm)?  Explain in ~ two or three short sentences.

    Answer

    b.(6 points) If the space shuttle moves at 105 meters/second towards a microwave emitting communication center (wave length of emitted microwaves = 106 nm), what will be the frequency received by the space shuttle?
     

    Answer

    Problem 3) A giant tenuous cloud of excited neon, hydrogen and oxygen gasses has just been observed in the remnants of a blown-up star by the Chandra X-ray satellite and other telescopes (American Astronomical Society press release, January 15, 2000).
    a. (6 points) In lab  and in class we noticed that neon and hydrogen have different spectra. Which gas has more lines in the visible part of the spectrum?

    Answer
     

    b. (6 points) Which of these gasses, neon and hydrogen, would have more lines in the X-ray part of the spectrum? (Hint: O stars don't have many hydrogen lines. Also look at question 10 in part I.)

    Answer

    c. (3 points) The emission lines from neon in the X-ray part of the spectrum are fainter than the lines of oxygen in X-rays . What information can you get from this fact?

    Answer