Kapteyn Universe

Herschel and later Kapteyn counted stars in different directions to determine the shape of the Milky Way galaxy and our position in the Milky Way. To understand how star counting works, consider

• Because the Milky Way is not spherical and we are not in its center we would expect to see large variations in the number of stars if we look in different directions on the sky.

• Next, since the brightness of a distant declines as
  • Flux ===> Flux = Luminosity / (4 pi distance²)

  we expect that the more distant stars will be fainter (in an average sense since all stars do not have the same intrinsic luminosity.

So, the three-dimensional distribution of stars around us, that is, whether they are distributed in a thin shell or in a spherical distribution or in a disk or a football shaped region or whatever, can also be deduced from star counting and measuring the brightnesses of the stars.

Herschel and Kapteyn performed the star counting and star brightness exercise and found that the Milky Way was a flattened distribution of stars, around 33,000 light years x 6,500 light years and that the Sun was around 2,000 light years from the center of the galaxy. Today, we believe that the Milky Way is at least 80,000 light years x 2,000 light years in size and that the Sun is 25,000 light years from the center of the galaxy!!

Herschel and Kapteyn deduced that the Milky Way was not nearly as flat as we know it to be and that we were very nearly at its center.

What is the meaning of this? Are we near the center of the Milky Way galaxy?

Lecture 1
Lecture 2