240
11 The Kapteyn Universe
these spiral nebulae fit into the organization of the Universe or what place they
occupied in it.
But then he picked up the subject of the Star Streams. For this he used
Fig. 11.5 as an illustration. The three panels are more or less the three different
stages of evolution of stars as he discussed it. Here we see the velocities of stars
in projection on the plane of the Milky Way. Stars in the first Star Stream move
to the left and it is clear that there is a big difference between these ‘stages’. By
the way, this kind of observations was reason for Kapteyn (and also for Arthur
Eddington) to reject the interpretation by Karl Schwarzschild, that the streams
in reality are no more than a reflection of asymmetry in the spatial velocities
of the stars. After all, there should be no differences in the composition in
terms of type of star. The inequality that Kapteyn stressed later disappeared
with better observations, and Schwarzschild was eventually proven right.
Kapteyn thought a galaxy was formed from two moving parts, as shown in
Fig. 11.6. It is not yet very clear to him exactly how this all worked in detail. The
figure is schematic and he assumed that the directions of the velocities of the
Fig. 11.6 The way Kapteyn thought about the two Star Streams. The ‘z’ in the overlap
area indicates the position of the Sun. From Kapteyn’s NNGC lecture published in 1912.
Kapteyn Astronomical Institute, University of Groningen
11 The Kapteyn Universe
these spiral nebulae fit into the organization of the Universe or what place they
occupied in it.
But then he picked up the subject of the Star Streams. For this he used
Fig. 11.5 as an illustration. The three panels are more or less the three different
stages of evolution of stars as he discussed it. Here we see the velocities of stars
in projection on the plane of the Milky Way. Stars in the first Star Stream move
to the left and it is clear that there is a big difference between these ‘stages’. By
the way, this kind of observations was reason for Kapteyn (and also for Arthur
Eddington) to reject the interpretation by Karl Schwarzschild, that the streams
in reality are no more than a reflection of asymmetry in the spatial velocities
of the stars. After all, there should be no differences in the composition in
terms of type of star. The inequality that Kapteyn stressed later disappeared
with better observations, and Schwarzschild was eventually proven right.
Kapteyn thought a galaxy was formed from two moving parts, as shown in
Fig. 11.6. It is not yet very clear to him exactly how this all worked in detail. The
figure is schematic and he assumed that the directions of the velocities of the
Fig. 11.6 The way Kapteyn thought about the two Star Streams. The ‘z’ in the overlap
area indicates the position of the Sun. From Kapteyn’s NNGC lecture published in 1912.
Kapteyn Astronomical Institute, University of Groningen