Download Homework Problems for Quiz 1 – AY 5 – Spring 2013

Homework Problems for Quiz 1 – AY 5 – Spring 2013 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1. If the Earth’s spin axis was not tilted with respect to the Earth’s orbital plane (around the Sun),
which of the following would be true, which false?:
T The number of daylight hours in a day would not change through the year.
F The length of a day would increase from 24 hours to 365 days
F The change of seasons would become more pronounced in the Northern Hemisphere.
F There would be no change in the pattern of the seasons compared to the real situation
2. Suppose you identified in a distant galaxy the Hydrogen emission line corresponding to an
electron dropping from the second excited level to the first excited level. The “rest” wavelength
of this photon is = 6365Å yet in the galaxy spectrum you measure it at 5365Å.
(a) Is this galaxy moving toward
or away
from the Earth?
The emission line is blueshifted so the galaxy is moving toward the Earth
(b) What is the relative speed between the Earth and the Galaxy?
The wavelength shift is 1000 Å so the speed is: s = c x (1000/6365) = 4.7 × 104 km/sec
3. What are the differences (name 2) and similarities (name 2) between red light and x-rays.
Similarities: both electromagnetic radiation, both travel at ”c” in vacuum. Differences: frequency and
wavelength of the EM waves
4. What time does the new moon rise? Draw a picture to demonstrate your answer.
New moon rises at dawn with the Sun.
5. Given the speed of light c = 186, 000 miles/second, how many miles is a “light-minute”?
186,000 miles/second X 60 seconds = 11,160,000 miles
Homework Problems for Quiz 1 – AY 5 – Spring 2013 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
6. Suppose you lived on the Moon where there is essentially no atmosphere. Label the following
statements as (T)rue or (F)alse:
F The Sun would appear redder (compared to as seen from the Earth) during the day.
T The color of the sky (looking away from the Sun during the day) would be black.
F At sunset, the Sun would appear redder than it does at noon.
F The “green flash” would be a “red flash”
7. You measure the spectrum of a solid green sphere of aluminum and the highest intensity is at a
wavelength of 2 × 10−4 cm. What is the temperature of this sphere?
λmax = 0.29/T(K) so, T(K) = 0.29/2 x 10−4 =1450K
8. What color does a red apple appear to be when illuminated with white light?
Red
9. What is the frequency of light with a wavelength of 20cm?
f=c/λ = (3 × 105 km/sec)/(20 × 10−5 km) = 1.5 × 109 hz
10. What is 103 × 105 × 10−6 ?
102 = 100
11. Star A has twice the trigonometric parallax angle and twice the luminosity of Star B. (Assume no
dust toward either star)
a) What are the relative distances of the two stars?
Star A has twice the parallax angle so is at 1/2 the distance of Star B
b) what are the relative brightnesses of the two stars?
Based on their relative luminosities, Star A would be twice as bright as Star B at the same distance. But, Star A
is 1/2 the distance which is a factor of 4 in brightness (inverse square law). In combination, the two effects add
up to Star A being (2 x 4) = 8 times the brightness of Star B.
Homework Problems for Quiz 1 – AY 5 – Spring 2013 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
12. If a red star and a blue star both have the same radius and both are the same distance from the
Earth, which one is brighter in the sky?
The blue star produces more energy per unit surface area than the red star based on Stephan’s Law. If the two
stars have the same radius, they have the same surface area and the blue star must emit more total area (it has a
larger luminosity). If the two stars are at the same distance then the blue star must also be brighter.
13. Which of the following can be used to estimate the temperature of a star? For clarity, label each
of these T(rue) or F(alse)
T Its color
F Its proper motion
T The wavelength at which it emits the most electromagnetic radiation
F Its radial velocity
14. Label the following T(rue) or (False)
T The nearest star to the Sun is about 1 parsec distant.
T The largest stars have a radius about 103 the radius of the Sun.
T The surface temperature of the hottest stars is about 50000K
T The least-massive stars are about 1/10th the mass of the Sun.