Kapteyn Universe
Hershcel and later Kapteyn counted stars in various directions to determine the
shape of the Milky Way 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. This does not seem to be the case (to even a casual observer of
the night
sky).
- Also, since the brightness of a distant declines as
- Flux ===> Flux = Luminosity / (4 pi distance**2)
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,
whether the stars are distributed in a thin shell around us or in a spherical
distribution or a disk or a football shaped object or whatever,
can also be deduced from star counting.
Kapteyn (1920's) found that the Milky Way was a flattened distribution of
stars round 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. We know 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!!
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?