Problem set due in class, Apr 23. Show your work, neatly.

1) Show that a central surface brightness of 15 mag arcsec^-2 corresponds to 18,000 solar lums pc^-2. Remember that surface brightness is independent of distance and the abs mag of the Sun is 4.5. If the sky brightness is 22 mag arcsec^-2, what surface brightness level does 1% of sky reach. Compare that to the figures in the lectures.

2) Use the equation of circular velocity to determine the mass of NGC 7331 at 40 kpc (see the figure in the lecture). Use NED to find the apparent magnitude of NGC 7331. With a distance of 11.5 Mpc, what is its abs mag, and determine its M/L in solar units.