Physics 20083 - Introductory Astronomy - Spring 2002
Exam #1b

Answer any 7 of the following 8 questions. All relevant equations have been given on the cover page (not visible in WWW version). Each is worth 14 points.

1) Astronomers make careful observations of star Alpha and star Beta to determine their properties. Both stars are determined to have the same spectral line widths in their continuous spectrum, but Star Alphašs continuous spectrum peaks at a much longer wavelength than star Beta.

a) (2 pts) Which of these two stars has a higher absolute luminosity (or are they probably about the same)?

b) (12 pts) Justify your answer to part (a). As part of your answer, explain why line width relates to stellar size.


2) A certain spectral absorption feature in Star Alpha occurs at a wavelength of precisely 553 nanometers (nm) when the source of the absorption line is at rest relative to the Earth. Star Alpha is 500 light years away from Earth and is not moving with respect to us. No explanations are needed for this problem.

a) (3 pts) Star Beta is 1000 light years away from Earth (twice as far as Star Alpha) and is moving toward us with a speed of 1000 meters/sec. Will its absorption line be observed to be smaller, larger or the same wavelength as Star Alphašs?

b) (3 pts) Star Gamma is 1000 light years away from Earth and is moving toward us with a speed of 2000 meters/sec. Will its absorption line be observed to be smaller, larger or the same wavelength as Star Betašs?

c) (3 pts) Star Delta is 500 light years away from Earth (same distance as Star Alpha) and is moving toward us with a speed of 1000 meters/sec. Will its absorption line be observed to be smaller, larger or the same wavelength as Star Betašs?

d) (3 pts) Star Epsilon is 1000 light years away from Earth and is not moving at all relative to us. Will its absorption line be observed to be smaller, larger or the same wavelength as Star Alphašs?

e) (2 pts) Star Omega is 2000 light years away from Earth and is not moving at all relative to us. Will its absorption line be observed to be smaller, larger or the same wavelength as Star Epsilonšs?


3) Currently the number of stars for which Astronomers can measure parallax angles is only a very small fraction of the total number of stars in the galaxy. Suppose we set up an Astronomical parallax observatory on the surface of Mercury, which orbits at an average distance of 36 million miles from the Sun (as opposed to Earthšs 93 million miles). Would the number of stars with measurable parallax angles by larger or smaller compared with what we can see from Earth? As part of your explanation for this answer, explain the relationship between parallax angle and the changed variable in the parallax equation.


4) Suppose the Sunšs photosphere had its coolest part near the base, deep beneath the outer surface of the photosphere. Thus, the hottest part of the photosphere would be the layer closest to the outer surface.

a) (2 pts) Would the limb of the Sun appear brighter, darker or about the same compared to the center of the visible disk?

b) (12 pts) Explain your answer to part (a), including a disucssion of why we see different layers along different lines of sight (a diagram may help here but is not required).


5) High temperature is an important ingredient in many astronomical situations, like fusion, ionization, excitation, emission, absorption, motion and so on.

a) (7 pts) Explain why high temperatures are needed in order for gases to be ionized.

b) (7 pts) Explain why high temperatures are needed in order for gases to undergo fusion.


6) Recently, some scientists claimed to have discovered a process by which they could force the fusion of Hydrogen at room temperature.

a) (7 pts) Explain how the "cold fusion" process worked to achieve fusion at low temperatures.

b) (7 pts) Briefly state (one sentence) one reason why cold fusion claims are not accepted by the scientific community. ALSO explain why the theory of nuclear fusion in the Sunšs core is still accepted by the scientific community despite problems that had been raised with it.


7) An Astronomer would like to find the distance to the star Capella using the standard candle technique; however, no star with spectral features similar to Capella can be found in her catalog. Fortunately, Star Kappa has just had its parallax angle determined, and Kappašs spectrum is identical to that of Capella. How can this information be used to find the distance to Capella?


8) You should show your work on both parts of this problem off to the side of the diagram.

a) (7 pts) Draw in the approximate location of a star (mark as ŗA˛) that has a temperature of one-half the Sunšs and four times the size (radius).

b) (7 pts) Draw in the approximate location of a star (mark as ŗB˛) that has a temperature twice that of the Sun and a size one-tenth that of the Sun.