29:50 MODERN ASTRONOMY
Third Homework Set...October 1, 1999
Note: Be sure to consult the Appendices in your book.
Having problems? Don't go around confused and despondent! Ask for help! The purpose
of these problem sets is to help you learn something.
(1) Use number from the Appendices to calculate the speed at which the space shuttle
moves in a circular orbit around the Earth. You can assume that the radius of the orbit
is the radius of the Earth.
7.90 kilometers/sec.
(2) Using the Earth as a small, orbiting mass, calculate the mass of the Sun. Does it
agree with the number I gave you in class? It had better!
kilograms.
(3) Calculate the Main Sequence lifetime of a star with a mass of 5 solar masses.
0.0178 times the MS lifetime of the Earth, or 179 million years.
(4) Calculate the wavelength at which blackbody radiation from a molecular cloud
would be brightest.
meters
(5) Explain in words (using physical concepts discussed in class) why a spectral class
B star would be more effective at photodissociating and photoionizing gas than a star
like the Sun.
B star is hotter, so it emits more ultraviolet photons (those with enough
energy to photoionize hydrogen) per unit surface area than the Sun, and it is more
luminous, so there are still more. Since more UV photons are emitted, it can keep a larger
volume photoionized.
(6) Calculate the speed at which a spaceship would orbit just above the surface of a
white dwarf star. When you have the number, think about something remarkable about it
(how do you like that for an open-ended question?).
Assume a mass of 1 solar mass and a radius of 1 Earth radius (=6378 kilometers;
check p457 of your textbook). Then 
meters/sec = 4570 kilometers/sec.
This is greater than one percent of the speed of light.