228
11 The Kapteyn Universe
step, but for that the spectral type had to be known. But much more important
for Kapteyn’s program was that for a good understanding of the motions of
the stars in space, the radial velocity had to be known. But one then had to be
able to measure relatively small shifts of the absorption lines in the spectrum
of a star, and to do that for many stars a large telescope was necessary. We have
already seen that the Sun moves at 20 km/s relative to the nearby stars, so it is
necessary to measure radial velocities with a accuracy of about 1 km/s.
The light of a star has to be dispersed over a larger area and this can only
be done for the brighter stars to have sufficient signal on each part of the
photographic plate that is used to record the spectrum. This kind of studies
had already been done by Edwin Frost with the 40 in. refractor at Yerkes
Observatory, the largest refracting telescope which had been built by George
Hale before he started Mount Wilson, and by William Campbell with the 36
in. telescope at Lick Observatory. Especially the latter yielded good results, but
only for the brightest stars, which meant in practice those visible to the naked
eye. No wonder that the 60 in. telescope was extensively used for this type
of measurement as soon as it was available. The person who did most of that
work at Mount Wilson was Walter Adams.
Obtaining such radial velocities was part of Kapteyn’s Plan of Selected Areas,
but because it is only feasible for relatively bright stars this was not limited to
the Areas themselves, but all stars of sufficient brightness were selected. In 1913,
Campbell published radial velocities for 900 stars that were all brighter than
magnitude 5, and in 1915, Adams came up with 500 stars between magnitude
5 and 8.
This has yielded important new results, particularly regarding the link
between proper motion, radial velocity and spectral type. An important
achievement was a publication by Kapteyn and Frost in 1910, building on
earlier work by Frost and Adams, when Adams was still working under Frost
at Yerkes. For a collection of 210 stars, in fact a sample of everything available
in the literature about them, it appeared that if the velocities of these stars were
corrected for that of the Sun towards the Apex, their average velocity depended
strongly on the spectral type. This is, as we now understand, a consequence of
their age. Stars are born of gas at low velocities relative to each other, but due
to the gravitational force of the large complexes of gas from which the stars
emerge, their average random velocity increases over time. It turns out that
hot, bright stars are heavy and short-lived. So, these are generally very young,
which is why they generally have lower random motions through space.
Kapteyn and Adams published a more precise analysis in 1915; undoubtedly
they had worked together on this already before Kapteyn was prevented from
visiting Mount Wilson. It was an analysis based on the velocities of Adams
11 The Kapteyn Universe
step, but for that the spectral type had to be known. But much more important
for Kapteyn’s program was that for a good understanding of the motions of
the stars in space, the radial velocity had to be known. But one then had to be
able to measure relatively small shifts of the absorption lines in the spectrum
of a star, and to do that for many stars a large telescope was necessary. We have
already seen that the Sun moves at 20 km/s relative to the nearby stars, so it is
necessary to measure radial velocities with a accuracy of about 1 km/s.
The light of a star has to be dispersed over a larger area and this can only
be done for the brighter stars to have sufficient signal on each part of the
photographic plate that is used to record the spectrum. This kind of studies
had already been done by Edwin Frost with the 40 in. refractor at Yerkes
Observatory, the largest refracting telescope which had been built by George
Hale before he started Mount Wilson, and by William Campbell with the 36
in. telescope at Lick Observatory. Especially the latter yielded good results, but
only for the brightest stars, which meant in practice those visible to the naked
eye. No wonder that the 60 in. telescope was extensively used for this type
of measurement as soon as it was available. The person who did most of that
work at Mount Wilson was Walter Adams.
Obtaining such radial velocities was part of Kapteyn’s Plan of Selected Areas,
but because it is only feasible for relatively bright stars this was not limited to
the Areas themselves, but all stars of sufficient brightness were selected. In 1913,
Campbell published radial velocities for 900 stars that were all brighter than
magnitude 5, and in 1915, Adams came up with 500 stars between magnitude
5 and 8.
This has yielded important new results, particularly regarding the link
between proper motion, radial velocity and spectral type. An important
achievement was a publication by Kapteyn and Frost in 1910, building on
earlier work by Frost and Adams, when Adams was still working under Frost
at Yerkes. For a collection of 210 stars, in fact a sample of everything available
in the literature about them, it appeared that if the velocities of these stars were
corrected for that of the Sun towards the Apex, their average velocity depended
strongly on the spectral type. This is, as we now understand, a consequence of
their age. Stars are born of gas at low velocities relative to each other, but due
to the gravitational force of the large complexes of gas from which the stars
emerge, their average random velocity increases over time. It turns out that
hot, bright stars are heavy and short-lived. So, these are generally very young,
which is why they generally have lower random motions through space.
Kapteyn and Adams published a more precise analysis in 1915; undoubtedly
they had worked together on this already before Kapteyn was prevented from
visiting Mount Wilson. It was an analysis based on the velocities of Adams