11.5 Walter Adams
237
kinetic energy in motion. If everything would move in circular orbits, then all
kinetic energy would be in the collective velocity of the rotation, but if there
are also random motions, part of the kinetic energy is contained in that, so
less kinetic energy is needed in rotation. How much less depends then on how
large these random motions are: the larger they are, the slower the systematic
rotation is allowed to be.
If you now look at a group of stars that have average high velocities relative
to each other, then they will as a collective move slower around the center
of the Galaxy than a group in which stars move only little relative to each
other. That first group will then be lagging behind in the rotation around the
center of the Galaxy compared to the second group. This systematically slower
motion through space of stars with large random velocities is called Strömberg
asymmetric drift. This manifests itself, for example, in the case of young stars,
such as Kapteyn’s helium stars, which have small random velocities relative to
each other. At the time of Kapteyn this was not yet understood; the asymmetric
drift was only discovered and published in 1924 and 1925.
11.6 The Structure of the Universe
In the period after 1910, when the Plan of Selected Areas was well underway,
Kapteyn published few papers for a while. Part of this was due to his work in
the field of helium stars and statistics, which yielded few publications in the
short term. It is also due to the fact that in Groningen he had to catch up with
the lectures that he could not give during his visits to Mount Wilson. He still
presented contributions to the Royal Academy and he also gave a number of
general lectures for large audiences, which took a lot of time in preparation and
execution. I will take a closer look at some of those lectures to follow Kapteyn’s
evolving insight into the structure of the Universe.
I have already mentioned his lecture on Mount Wilson during the great
congress of 1910, and that to the American Academy of Arts and Sciences in
Washington in 1913, for which he especially had to cross the ocean. Another
important lecture took place when the organization Dutch Natural and Medical Sciences Congress (NNGC) held its meeting in Groningen in 1911. This
NNGC, which was founded in 1887 and of which Kapteyn had become a
member at an early age, had the aim of bringing together scientists from various disciplines; to this end it regularly organized congresses lasting several
days. The NNGC still exists and now organizes a symposium twice a year on
a specific subject. At the meeting in Groningen in 1911 Kapteyn was one of
the keynote speakers.
237
kinetic energy in motion. If everything would move in circular orbits, then all
kinetic energy would be in the collective velocity of the rotation, but if there
are also random motions, part of the kinetic energy is contained in that, so
less kinetic energy is needed in rotation. How much less depends then on how
large these random motions are: the larger they are, the slower the systematic
rotation is allowed to be.
If you now look at a group of stars that have average high velocities relative
to each other, then they will as a collective move slower around the center
of the Galaxy than a group in which stars move only little relative to each
other. That first group will then be lagging behind in the rotation around the
center of the Galaxy compared to the second group. This systematically slower
motion through space of stars with large random velocities is called Strömberg
asymmetric drift. This manifests itself, for example, in the case of young stars,
such as Kapteyn’s helium stars, which have small random velocities relative to
each other. At the time of Kapteyn this was not yet understood; the asymmetric
drift was only discovered and published in 1924 and 1925.
11.6 The Structure of the Universe
In the period after 1910, when the Plan of Selected Areas was well underway,
Kapteyn published few papers for a while. Part of this was due to his work in
the field of helium stars and statistics, which yielded few publications in the
short term. It is also due to the fact that in Groningen he had to catch up with
the lectures that he could not give during his visits to Mount Wilson. He still
presented contributions to the Royal Academy and he also gave a number of
general lectures for large audiences, which took a lot of time in preparation and
execution. I will take a closer look at some of those lectures to follow Kapteyn’s
evolving insight into the structure of the Universe.
I have already mentioned his lecture on Mount Wilson during the great
congress of 1910, and that to the American Academy of Arts and Sciences in
Washington in 1913, for which he especially had to cross the ocean. Another
important lecture took place when the organization Dutch Natural and Medical Sciences Congress (NNGC) held its meeting in Groningen in 1911. This
NNGC, which was founded in 1887 and of which Kapteyn had become a
member at an early age, had the aim of bringing together scientists from various disciplines; to this end it regularly organized congresses lasting several
days. The NNGC still exists and now organizes a symposium twice a year on
a specific subject. At the meeting in Groningen in 1911 Kapteyn was one of
the keynote speakers.