Astronomy 122&, Fall 2009

Homework C Answers

Part I: Multiple choice. One point each.

1. Which particle carries the negative electric charge?
a) only the electron

2. Which particles have nearly the same mass?
b) proton and neutron

3. Which force holds protons and neutrons together in an atomic nucleus?
b) strong

4. Which force holds the electron to the nucleus?
d) electromagnetic

5. The kind of chemical element is determined by the number of
a) protons in the nucleus

6. The second lightest of all elements is
b) helium

7. Isotopes of the same chemical element have different numbers of
a) neutrons

8. The isotope of uranium U-238 has 238
d) protons plus neutrons

9. An ion is an atom with missing or extra
c) electrons

10. A nucleus of the carbon isotope carbon-14 has 6 protons. How many neutrons does it have?
d) 8 (14 total particles minus 6 protons)

11. The last (heaviest) stable chemical element is
d) bismuth

12. Which is the correct order from shortest-wave (highest-energy) radiation to longest-wave (lowest-energy) radiation.
b) gamma-ray, X-ray, blue light, red light, radio

13. Which color light refracts the least?
c) red

14. A blackbody
c) absorbs all radiation that falls upon it

15. If you double the temperature of a heated solid body, it will radiate how many times more energy?
d) 16 (Stefan-Boltzmann law: flux of radiation proportional to T**4)

16. Of those listed below, a body at 5 degrees Kelvin can produce only
a) radio radiation

17. Absorption and emission lines are caused by energy changes in atomic and molecular
a) electrons

18. The H-Beta Balmer line is formed by a transition between levels
d) 2 and 4

19. The age of the Sun, Earth, and Solar System are calculated to be
d) 4.5 billion years

20. We know the ages of the meteorites (asteroids that hit the Earth), hence the age of the Earth and the Solar System, from
b) the decays of radioactive elements

Part II

1. (10) From page 126, the wavelength at which a body produces its maximum radiation is:
lambda (max) meters = 2.898**-3/T, lambda (max) Angstroms = 2.898**7/T (where T is degrees Kelvin).

a) T = 10 K: lambda (max) = 0.0002898 meters = 0.02898 cm (radio or far infrared)
b) T = 2000 K: lambda (max) = 14,490 Angstroms (infrared)
c) T = 5780 K: 5014 A (visual or optical)
d) T = 50,000 K: 580 A (ultraviolet)
e) T = 1,000,000 K: 29 A (X-ray)

2. (5) The Sun's surface temperature is 5780 K. The radiative flux from a body in energy flow per unit area per second is proportional to temperature to the fourth power, T**4. At T = 12,000 K, the flux is (12,000/5780)**4 = 18.6 times that from the Sun.

3. (10) All reasonable attempts at recording spectra are acceptable.

4. (5) a) The light gathering power of a telescope goes as the square of the diameter. A 100-inch telescope therefore gathers (100/20)**2 = 5*2 = 25 times more light than a 20-inch telescope.
b) Single-dish radio telescopes have poorer resolving power than do optical telescopes because radio waves are so much longer than optical waves. (To increase the resolving power of radio telescopes, astronomers link two or more single-dish radio telescopes together as interferometers.)