Basic facts:

• In the center of the Milky Way there seems to be a black hole, with a mass about
  : M~10⁶M_sun

• In pacsecs (pc = 3.26 lightyear, kpc=1000pc) the central bulge is a few kpc in radius,
  and the disk is at least 15kpc, the Sun is about 8 kpc from the center.

• The density n of stars in the disk is given by a scale length h_R and R
  the distance from the center is given by: n(R)∝ e^-R/h_R h_R is
  in the range of 2-4 kpc

• The thin disk (as opposed to the thick disk, which is a more defuse disk wider then
  the one we normally “see”, contains 95% of the disk stars.

• The Galaxy is rotation, stars in the disk orbit the center at about 200km s^-1,
  it takes the Sun about 250 Myr to orbit the entire galaxy.

• The total luminosity of the disk is about 15-20×10⁹L_sun and
  the mass is around 60 x 10⁹ M_sun. The Bulge L ≈ 5 x 10⁹ L_sun and
  a mass around 20  x 10⁹ M_sun