Homework 4 : Galaxies

(due at the start of class on the 8^th May 2003)

1. [5 points] Describe two properties possessed by Cepheid variable stars that make them very important for measuring the distance to nearby galaxies.

2. [5 points] State Hubble’s law. Suppose we observe a galaxy to be receding from us at a speed of 13,000km/s. If Hubble’s constant has a value of 65 km/s/Mpc, how far away is this galaxy?

3. This question concerns the mass of galaxies:

1. [5 points] Suppose a star orbits around the center of its galaxy at a distance of 10,000pc and with a speed of 250km/s. How much mass does the region of the galaxy within this radius possess? Show your working. [Hint: you will need to know that 1pc=3´10¹⁶m, and that Newton’s constant of Gravitation is G=6.67´10^-11 N m² kg^-2. To check that your answer makes sense, you might want to compare it with the mass of the Sun.]

2. [5 points] What measurements lead us to believe that galaxies contain large quantities of dark matter?

4. [5 points] A non-scientist friend of yours is skeptical about the existence of supermassive black holes at the centers of galaxies. Using language that they can understand, write a brief and concise summary of the current evidence for supermassive black holes (you might want to make your case by focusing on two or three specific cases).