Astronomy 122&, Fall 2009

Homework D Answers

Part I: Multiple choice. One point each.

1. The Sun is how many times more massive than Jupiter?
c) 1000

2. The Sun is made mostly of
a) gaseous hydrogen and helium

3. The diameter of the Sun is about
e) all of the above: 860,000 miles; 1.5 million km; 109 Earth diameters; 0.01 AU

4. The temperature of the solar photosphere (the optically visible "surface") is about
b) 6000 K

5. What causes the solar granulation?
b) convection (up and down mass motions)

6. We know the chemical composition of the Sun from its
a) spectrum

7. The surface of the Sun is
a) an opaque hydrogen-helium gas

8. The temperature of the Sun's corona is about
c) 2 million K

9. The third most abundant element in the Sun is
c) oxygen (Hydrogen and helium are 1 and 2.)

10. If you took all the hydrogen and helium away from the Sun, the resulting mixture would have a chemical composition rather similar to
a) the Earth's crust

11. The Sun rotates
a) every 25 days at the equator in the direction of the planetary orbits (though it takes longer toward the poles)

12. Sunspots are produced by
c) magnetic fields that block convection

13. The solar corona is heated by
d) magnetism

14. What event on the Sun causes aurorae on Earth?
a) coronal mass ejections

15. The Sun's magnetic field is produced by its
d) rotation and convection

16. The lifetime of a particular sunspot is closest to
b) a week

17. The length of the solar sunspot cycle (ignoring the direction of magnetic fields) is
c) 11 years

18. The solar wind
b) is a flow of particles from the Sun

19. When the sunspots (magnetic activity) disappeared around the year 1700,
e) North America and Europe became colder (the event called the "Maunder Minimum")

20. Where does solar hydrogen fusion take place?
e) the deep core (inner quarter of the radius)

21. In the center of the Sun, two protons fuse directly to
a) deuterium

22. About how much hydrogen-burning time does the Sun have left to it, in billions of years?
b) 5 (The solar life time is 5 billion years, and we are now 5 billion years old.

23. What particles immediately escape the solar core following fusion?
a) neutrinos

24. What makes the Sun hot in its center (hot enough to run fusion)?
e) gravitational compression

25. Why does the Sun not explode as a hydrogen bomb?
c) The first reaction of the proton-proton chain is too slow

Part II. Show all your work.

1. (10 points) The luminosity of the Sun (L) is 3.8 X 10**26 watts = joules per second, so in one second the Sun produces 3.8 X 10**26 joules, which equals Mc**2. Mass (M) converted per second = L/c**2 = 3.8 X 10**26/(3 X 10**8)**2 = 4.2 X 10**9 (4.2 billion) kilograms.

2. (10 points) The (optically visible) hydrogen lines are not the strongest in the Sun because the electrons that produce them jump upward from the second level. The vast majority are always in the ground state, the first orbit. At the solar temperature, very few electrons are pre-excited to level 2 by collisions, that is, very few hydrogen atoms are capable of absorbing photons. Other atoms and ions, however, can be produced by electrons in the ground state, so that nearly all of them are capable of producing the lines.

3. (5 points) If the Sun stopped rotating, the magnetic field would shut down, and there would be little if any corona, no coronal loops, no sunspots, and no coronal mass ejections. The solar wind would be severely depleted. There would be little if any auroral activity on Earth, but neither would there be any heating by the solar wind and solar ultraviolet and X-rays. Spaceflight would be safer and orbiting spacecraft less subject to failure. On the other hand, there is evidence (from the Maunder minimum) that some of the heating of the Earth comes from solar magnetic activity. (As an interesting aside, the solar rotation must be slowing down. The solar wind drags the magnetic field of the Sun outward, and the field, anchored to the Sun, supplies a slow brake. Solar activity will thus slowly diminish. We cannot see this actually happening, but it is statistically evident in studies of other stars.)

Note: Express your answer to question 1 in kilograms. A joule is the unit of energy; one joule/second = 1 watt. The speed of light must be expressed in meters/second to obtain energy in joules.