9.3 Kapteyn Cottage
183
according to my wishes. That is the ‘maladie du doute’ [disease of doubt], deep
in my character.
It is clear from correspondence that George Hale had a completely different
opinion about whether Kapteyn did enough. I’ll come back to this later when
I describe how Kapteyn wanted to end the contract when he no longer could
visit Mount Wilson. The position of Hale and the Carnegie Institution was
that Kapteyn worked for them and that it did not matter where he did that.
9.4 Dust
We have seen above that Kapteyn, in his attempts to determine the distribution
of stars in space, had to make three assumptions. That of the lack of a preferred
direction in the velocity distribution of the stars had already been proved
wrong by his discovery of the Star Streams. But he was able to take this into
account in the analysis. He never had to give up the assumption that the
intrinsic luminosity (the absolute magnitude) of stars was distributed the same
everywhere in space; especially for the stars that Kapteyn used, which are
generally not that far from the Sun, that assumption is not bad at all. But what
worried him for years, and which also explains why his model turned out to
be partly incorrect, was the assumption of the absence of scattering of light
by particles of dust in space (usually summarized as ‘absorption’, although it
is that only partially). It is especially this particular issue on which he spent
much time in the early years of his visits to Mount Wilson.
It was not the first time he had made a study on the subject. Earlier, he
had addressed the issue also in response to an article by Edward Pickering in
1903 about the distribution of the stars. Pickering thought that if you looked at
fainter stars, the number of stars across the whole sky grew at a slower rate than
what was expected ‘theoretically’. This theoretical value is then the increase in
the simplest case that all stars are intrinsically equally bright and uniformly
distributed throughout space, i.e. their density—the number of stars per unit
volume such as cubic parsec—does not change with distance from the Sun.
The theoretical derivation is explained with some inevitable algebra in Appendix A.6. Pickering stated that this deviation from the theoretical value was
probably the result of absorption (or scattering) of starlight in space. But this
had to be considerable in order to explain the numbers observed. A similar
suggestion was made in 1904 by George Comstock of the Washburn Observatory. He had used Kapteyn’s method of estimating secular parallax distances
for various groups of stars, and had come to the conclusion that stars that are
faint in the sky are only partly that because they are further away, but for a
183
according to my wishes. That is the ‘maladie du doute’ [disease of doubt], deep
in my character.
It is clear from correspondence that George Hale had a completely different
opinion about whether Kapteyn did enough. I’ll come back to this later when
I describe how Kapteyn wanted to end the contract when he no longer could
visit Mount Wilson. The position of Hale and the Carnegie Institution was
that Kapteyn worked for them and that it did not matter where he did that.
9.4 Dust
We have seen above that Kapteyn, in his attempts to determine the distribution
of stars in space, had to make three assumptions. That of the lack of a preferred
direction in the velocity distribution of the stars had already been proved
wrong by his discovery of the Star Streams. But he was able to take this into
account in the analysis. He never had to give up the assumption that the
intrinsic luminosity (the absolute magnitude) of stars was distributed the same
everywhere in space; especially for the stars that Kapteyn used, which are
generally not that far from the Sun, that assumption is not bad at all. But what
worried him for years, and which also explains why his model turned out to
be partly incorrect, was the assumption of the absence of scattering of light
by particles of dust in space (usually summarized as ‘absorption’, although it
is that only partially). It is especially this particular issue on which he spent
much time in the early years of his visits to Mount Wilson.
It was not the first time he had made a study on the subject. Earlier, he
had addressed the issue also in response to an article by Edward Pickering in
1903 about the distribution of the stars. Pickering thought that if you looked at
fainter stars, the number of stars across the whole sky grew at a slower rate than
what was expected ‘theoretically’. This theoretical value is then the increase in
the simplest case that all stars are intrinsically equally bright and uniformly
distributed throughout space, i.e. their density—the number of stars per unit
volume such as cubic parsec—does not change with distance from the Sun.
The theoretical derivation is explained with some inevitable algebra in Appendix A.6. Pickering stated that this deviation from the theoretical value was
probably the result of absorption (or scattering) of starlight in space. But this
had to be considerable in order to explain the numbers observed. A similar
suggestion was made in 1904 by George Comstock of the Washburn Observatory. He had used Kapteyn’s method of estimating secular parallax distances
for various groups of stars, and had come to the conclusion that stars that are
faint in the sky are only partly that because they are further away, but for a