BUCKNELL UNIVERSITY

Doppler formula:                speed of emitter     change in frequency
                              ------------------- = --------------------
                                 speed of wave         rest frequency

alpha/57.3^o = w/R

Multiple Choice Questions:

Click on the highlighted letter of the correct answer to get an  explanation.

1) When stars like the Sun run out of hydrogen to fuse in their cores, they "puff up," becoming many times larger in radius, and their outer layers cool off a little bit (from, say 5800 K to about 4000 K). How would the position on the H-R Diagram of such a star change as it transforms from its Main Sequence (a.k.a. H-fusing) stage to this post-Main Sequence stage?

• a) It would move downward and to the left.
• b) It would move upward and to the left, along the Main Sequence.
• c) It would move upward and to the right.
• d) It would move downward and to the right, along the Main Sequence.

2) 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.

Consider the point labeled P on the second screen. It is on the symmetry axis of the problem; that is, P is at the point where a straight line from the light bulb and passing directly in between the two slits in the first screen meets the second screen. Based on the geometry of the situation, what would you expect to see on the second screen at point P?

• a) a bright "fringe," signifying constructive interference of the waves from the two slits.
• b) a dark "fringe," signifying destructive interference of the waves from the two slits.
• c) there isn't enough information to determine whether it will be bright or dark.

3) Name the constellation depicted in the diagram below:

• a) Cassiopeia
• b) Cepheus
• c) Camelopardis
• d) Cancer

4) Choose the warmest of the following zones of the Sun

• a) the spicule zone
• b) the chromosphere
• c) the photosphere
• d) the corona

5) From a distance of 100 meters, you watch your friend is hammer a nail into a block of wood. What is the time interval between the time you see him hit the nail, and the time you hear the hammer blow?

• a) 3.4 seconds.
• b) 0.29 seconds
• c) 3.3 x 10^-7 seconds
• d) 0 seconds

6) Sirius has an apparent magnitude of -1.5, while Adhara (one of the stars in the Big Dipper) has an apparent magnitude of 1.5. What is the ratio of the fluxes from these two stars?

• a) 2.512
• b) 3.0
• c) 7.536
• d) 15.85
• e) 72.46

7) Two Main Sequence stars are located at the same distance from Earth. One has a surface temperature of 3000 K, while the other has a surface temperature of 6000 K. Which star will appear brighter to the naked eye?

• a) the 3000 K star
• b) the 6000 K star
• c) they will have equal flux
• d) you can't tell without knowing the size of each star

8) A hypothetical atom has the following energy structure:

ground state: energy = 0 Joules

first excited state = 2.1 x 10^-22 Joules

second excited state = 3.2 x 10^-22 Joules

third excited state = 4.9 x 10^-22 Joules

• a) 3.2 x 10^-22 Joules
• b) 1.1 x 10^-22 Joules
• c) 1.7 x 10^-22 Joules
• d) 2.8 x 10^-22 Joules
• e) both a) and b)

9) A star with a surface temperature of 3000 K will emit a blackbody spectrum which peaks at what wavelength?

• a) 1 x 10^-6 m
• b) 9 x 10^-7 nm
• c) 9 nm
• d) 100 nm
• e) 1000 m

10) A Main Sequence M star has a radius of about 7 x 10⁷ m and a surface temperature of about 3000 K. An M supergiant star has the same surface temperature, but has a radius 1000 times bigger. How much more luminous is the M supergiant than the Main Sequence M star?

• a) they have the same luminosity
• b) the M supergiant is 1000 times more luminous
• c) the M supergiant is a million times more luminous
• d) you can't tell without knowing the distances to the two stars

11) The diagram above shows a collimated telescope whose objective and eyepiece are separated by 1.5 meters. If the eyepiece has a focal length of 25mm, what is the focal length of the objective?

• a) 2.5 x 10^-2 m
• b) 0.6 m
• c) 1.475 m
• d) 6 m
• e) 15.95 m

12 and 13) (i.e., this one is worth 6 points) Briefly explain how astronomers conclude that all stars are made of mostly hydrogen even though some stars don't show strong hydrogen absorption lines.

Answer

Problems:

ANSWER

Problem 2) A Main Sequence G star has a surface temperature of 5800 K while a Main Sequence M star has a surface temperature of 3500 K. The G star has a radius ten times bigger than the M star. Calculate the ratio of their luminosities.

ANSWER

Problem 3) A giant cloud of excited hydrogen gas is moving toward the Earth at a speed of 200,000 m/s. Several of the hydrogen atoms in the cloud make a transition from the N=77 energy state to the N=76 energy state, releasing photons of energy 9.74 x 10^-24 J. What is the frequency of the radiation from this cloud that observers on the Earth would measure?

ANSWER