# Homework Assignment #9

Name:

Section number

Date Due: Nov. 17

1. The blue supergiant star Spica has a parallax of 0.012 arc-seconds. (a) What is its distance in parsecs? (b) Suppose the parallax measurement is in error by + or - 0.003 arc-seconds. What lower and upper limits can be set on the distance to Spica?

2. Star A and Star B have the same surface temperature. Star A is three times the diameter of star B, yet it is four times farther away than star B. What is the apparent brightness ratio between stars A and B?

3. The Sun's temperature is about 5,800 K, and the temperature of Sirius is about 10,000 K. The radius of Sirius is about 1.8 times that of the sun. Calculate the luminosity of Sirius, relative to that of the Sun. Use ratios and proportions, rather than absolute units, to find the solution.

4. Two stars in a binary system have an orbital period of 2 years, and an average orbital separation of 4 AU. What is the combined mass of the pair?

5. Examine the H-R diagram (Fig. 12.16) in your text.

(a) Compare the main-sequence star Sirius and the blue supergiant Deneb, which have roughly the same spectral class (A) and temperature (9,000 K). Which of the two stars is the most luminous? By what factor (over the other)?

(b) Next, compare the stars Deneb and the red supergiant Betelgeuse, which have roughly the same luminosity (> 105 · Sun's). Which of the two stars is the coolest? What is its approximate temperature?

6. Why is it that astronomers never see a massive protostar directly (optically), until it reaches the main-sequence stage?