11.6 The Structure of the Universe
241
two parts change over time. He also speculated about the role of gravitation
and of an ‘Ur-dust’, out of which nebulae and stars are formed and which
is not affected by gravitation. It is important to note that at this stage he
already wondered what role gravitation plays in the structure and evolution of
systems of stars. It is the beginning of what he would eventually present as the
structure of the Galaxy, in which the distribution of the stars in space, their
mutual attraction and their motions are such that the whole forms a consistent
system that is in equilibrium. What we now call dynamics. But it was not that
far yet.
11.7 The Spatial Distribution of the Stars
In the 1910s Kapteyn paid a lot of attention to how he would eventually, when
sufficient data would be available, solve the problem of the structure of the
Stellar System. The first thing to do was carrying the Plan of Selected Areas
along far enough, but there were also separate investigations that addressed
different aspects of the problem. Some of them were carried out by his PhD
students. Their work has sometimes, but not always been published in the series
Publications of the Astronomical Laboratory at Groningen, but in other cases it has
remained a printed dissertation. One example is the study of proper motions of
more than one hundred stars, on which Etine Imke Smid successfully defended
a PhD thesis in 1914. As mentioned before, she was the first woman in the
Netherlands to obtain a doctorate on an astronomical dissertation. After her
defense, Etine Smid worked for Kamerlingh Onnes in Leiden for some time,
but moved to Deventer after she got married and left science. Another example
is the doctoral research from 1916 by Samuël Cornelis Meijering (1882–1943)
on the motions of the cool K -stars. He found among these motions the same
parameters for the two Star Streams as were known for the other stars. Gerrit
Hendrik ten Bruggen Cate (1883–1963), who later changed his name to ‘ten
Bruggencate’, got his PhD in 1920 on an investigation into the distribution of
stars over spectral types in an area around the North Pole of the sky, using spectra
that Frits Zernike had obtained in Potsdam. Meijering and ten Bruggencate
both became teachers at the HBS.
But in the end it was all about the big picture: determining the distribution of
the stars in space. We have seen that Kapteyn had to make three assumptions.
The first of these was that stars move randomly in space, but that resulted
in the Star Streams. He could re-check this by statistically determining the
average parallax of groups of stars. The second assumption, of the absence of
absorption of starlight by dust in space, had been thoroughly tested by him
241
two parts change over time. He also speculated about the role of gravitation
and of an ‘Ur-dust’, out of which nebulae and stars are formed and which
is not affected by gravitation. It is important to note that at this stage he
already wondered what role gravitation plays in the structure and evolution of
systems of stars. It is the beginning of what he would eventually present as the
structure of the Galaxy, in which the distribution of the stars in space, their
mutual attraction and their motions are such that the whole forms a consistent
system that is in equilibrium. What we now call dynamics. But it was not that
far yet.
11.7 The Spatial Distribution of the Stars
In the 1910s Kapteyn paid a lot of attention to how he would eventually, when
sufficient data would be available, solve the problem of the structure of the
Stellar System. The first thing to do was carrying the Plan of Selected Areas
along far enough, but there were also separate investigations that addressed
different aspects of the problem. Some of them were carried out by his PhD
students. Their work has sometimes, but not always been published in the series
Publications of the Astronomical Laboratory at Groningen, but in other cases it has
remained a printed dissertation. One example is the study of proper motions of
more than one hundred stars, on which Etine Imke Smid successfully defended
a PhD thesis in 1914. As mentioned before, she was the first woman in the
Netherlands to obtain a doctorate on an astronomical dissertation. After her
defense, Etine Smid worked for Kamerlingh Onnes in Leiden for some time,
but moved to Deventer after she got married and left science. Another example
is the doctoral research from 1916 by Samuël Cornelis Meijering (1882–1943)
on the motions of the cool K -stars. He found among these motions the same
parameters for the two Star Streams as were known for the other stars. Gerrit
Hendrik ten Bruggen Cate (1883–1963), who later changed his name to ‘ten
Bruggencate’, got his PhD in 1920 on an investigation into the distribution of
stars over spectral types in an area around the North Pole of the sky, using spectra
that Frits Zernike had obtained in Potsdam. Meijering and ten Bruggencate
both became teachers at the HBS.
But in the end it was all about the big picture: determining the distribution of
the stars in space. We have seen that Kapteyn had to make three assumptions.
The first of these was that stars move randomly in space, but that resulted
in the Star Streams. He could re-check this by statistically determining the
average parallax of groups of stars. The second assumption, of the absence of
absorption of starlight by dust in space, had been thoroughly tested by him