Herschel'2 48 inch telescope

Herschel, Kapteyn, and the Milky Way

Kapteyn

Herschel (1785) and later Kapteyn (1922) 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. This does not seem to be the case (to even a casual observer of the night sky).

• Also, since the brightness of a distant star gets smaller as

  • ===> Flux ("brightness") = Luminosity / (4 pi distance2)

  more distant stars will be fainter (in an average sense but because all stars do not have the same intrinsic luminosity, this is not true strictly speaking). 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.

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  Hershel's Star Gauging Herschel using his 48 inch telescope (above) mapped the sky and performed a star counting exercise by looking along 683 lines of sight to find a model for the Milky Way. He assumed that all stars had the same luminosity to get their relative distances. Herschel deduced that the Milky Way was flattened and that the Sun was nearly at the center of the distribution of stars!!

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  Kapteyn (1920s) used photographic star counts and estimated distances using parallaxes and examining the proper motion of nearby stars. Kaptyen 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?

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