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11 The Kapteyn Universe
would he have had to go on before he would have come up with the idea
of the Star Streams? His approach was looking for regular patterns in the
motions across the sky, and from these he could immediately recognize—‘il
n’y avait qu’un pas’, (it was just one step)—that there were two Streams. Hale
admitted that induction played an important role in Kapteyn’s discovery of the
Star Streams. But he defended the view that ‘a combination of deduction and
induction offers more chance of success’. He clarified this through the process
of making hypotheses, all of which have consequences and of which you can
quickly eliminate several. The exchange of letters went on for a long time and
the letters on this subject were detailed. Kapteyn rather stubbornly stuck to
his view that the way of working had to be completely inductive. Hale insisted
that he admitted that deduction certainly played a role as well. He suggested
that Kapteyn should see Lorentz. Kapteyn first talked to his friend Heymans,
philosopher and psychologist, as well as to Leiden theoretical physicist Paul
Ehrenfest, Lorentz’ successor. They both emphatically opted for Hale’s more
subtle and flexible point of view and not for Kapteyn’s somewhat inflexible
approach. But Kapteyn admitted no more to Hale than that between their
points of view there could be no more than just a gradual difference.
11.4 Helium Stars
An important subject of Kapteyn’s research concerned the so-called helium
stars. He was already working on this in 1910, because he reported on the
progress of this research during the congress of the Solar Union on Mount
Wilson that year, when he gave a special presentation about the helium stars.
Helium stars are stars with particularly strong helium lines in their spectrum
and correspond to the current spectral type B in the sequence on the horizontal
axis of the Hertzsprung–Russell diagram. They are bright, heavy and shortlived stars. Such stars are also hot; the heaviest stars (spectral type O) are so
hot that their surface the helium is ionized. Helium is the second element in
the Periodic System and thus has two protons in its nucleus. In a neutral state,
a helium atom has two electrons, but in the hottest O-stars, one has been
detached. In the slightly less heavy and therefore slightly less hot B-stars that
Kapteyn studied, helium atoms still have both electrons.
Of these stars, Kapteyn had found that their proper motions showed a high
degree of what he called ‘parallelism’, at least unless you consider very large parts
of the sky. This indicated that these stars would generally have a low velocity
of their own in space relative to each other. This opened up a new possibility,
because their proper motion would then be largely due to the velocity of the
Sun and distances could be determined more accurately in the secular parallax
approach.
11 The Kapteyn Universe
would he have had to go on before he would have come up with the idea
of the Star Streams? His approach was looking for regular patterns in the
motions across the sky, and from these he could immediately recognize—‘il
n’y avait qu’un pas’, (it was just one step)—that there were two Streams. Hale
admitted that induction played an important role in Kapteyn’s discovery of the
Star Streams. But he defended the view that ‘a combination of deduction and
induction offers more chance of success’. He clarified this through the process
of making hypotheses, all of which have consequences and of which you can
quickly eliminate several. The exchange of letters went on for a long time and
the letters on this subject were detailed. Kapteyn rather stubbornly stuck to
his view that the way of working had to be completely inductive. Hale insisted
that he admitted that deduction certainly played a role as well. He suggested
that Kapteyn should see Lorentz. Kapteyn first talked to his friend Heymans,
philosopher and psychologist, as well as to Leiden theoretical physicist Paul
Ehrenfest, Lorentz’ successor. They both emphatically opted for Hale’s more
subtle and flexible point of view and not for Kapteyn’s somewhat inflexible
approach. But Kapteyn admitted no more to Hale than that between their
points of view there could be no more than just a gradual difference.
11.4 Helium Stars
An important subject of Kapteyn’s research concerned the so-called helium
stars. He was already working on this in 1910, because he reported on the
progress of this research during the congress of the Solar Union on Mount
Wilson that year, when he gave a special presentation about the helium stars.
Helium stars are stars with particularly strong helium lines in their spectrum
and correspond to the current spectral type B in the sequence on the horizontal
axis of the Hertzsprung–Russell diagram. They are bright, heavy and shortlived stars. Such stars are also hot; the heaviest stars (spectral type O) are so
hot that their surface the helium is ionized. Helium is the second element in
the Periodic System and thus has two protons in its nucleus. In a neutral state,
a helium atom has two electrons, but in the hottest O-stars, one has been
detached. In the slightly less heavy and therefore slightly less hot B-stars that
Kapteyn studied, helium atoms still have both electrons.
Of these stars, Kapteyn had found that their proper motions showed a high
degree of what he called ‘parallelism’, at least unless you consider very large parts
of the sky. This indicated that these stars would generally have a low velocity
of their own in space relative to each other. This opened up a new possibility,
because their proper motion would then be largely due to the velocity of the
Sun and distances could be determined more accurately in the secular parallax
approach.