Q: What is the average value of the surface density of HI between radii of 2 and 20 kiloparsecs? average "sigma" = approx 7 solar masses per square pc Q: Compute the area of a circular disk of radius 20 kpc. area A = (pi) * (20,000 pc)^2 9 = 1.3 x 10 square pc Q: What is the total mass of the HI gas within 20 kpc of the galactic center? mass M = "sigma" * A 9 = ( 7 solar mass / pc^2) * (1.3 x 10 pc^2) 9 = 9 x 10 solar masses in HI