11.2 Radial Velocities
229
from Mount Wilson and by Campbell from Lick. This analysis confirmed the
earlier result that the average radial velocity (in the line of sight) was smaller for
stars with a smaller proper motion. Because small proper motion statistically
indicates greater distance and thus intrinsically bright stars, this seemed to mean
that the radial velocity and the absolute brightness (the intrinsic luminosity)
of a star correlated with each other. That would say something about the
properties of stars, but, much more importantly, in principle opened up the
possibility of getting an idea of the distance of a star based on its radial velocity.
Adams investigated this further and found that for stars from which the distance could be determined independently with statistical methods, the radial
velocity indeed was small if the distance was large and the star therefore intrinsically relatively bright. These were developments in which Kapteyn was very
much interested. We will see later that this work would lead to serious friction
between Kapteyn and Adams.
11.3 Correspondence with Hale
During the Great War, Kapteyn and George Hale maintained an extensive
correspondence. Two themes stand out. The first is Kapteyn’s appointment as
paid Research Associate at the Carnegie Institution. Kapteyn felt, as we saw
before, that he could contribute too little to justify this, especially now that he
was no longer able to visit Mount Wilson due to the war. In a letter dated March
26, 1916, for example, Kapteyn stated that he hoped to return to California
that year. This message crossed a letter from Hale dated March 9 (so mail did
not travel quickly in those years, despite the neutrality of the Netherlands),
in which he urged him not to come because of the risks. Kapteyn planned to
come in mid-July, as in March his daughter Henriette was staying in Groningen
because she was pregnant. And he remarked that the ship companies more and
more often did not dare to offer tickets, so finding a ship for the crossing
might even have been impossible. But then he concluded that if it would not
be possible or desirable to come, he would resign as Associate of the Carnegie
Institution. In fact, he also stated that in the five years that remained until his
retirement, he had better concentrate anyhow on his work for his laboratory so
as not to have to leave unfinished work behind. He was dissatisfied with what
he has been able to do for Mount Wilson, and continuing the link with the
Carnegie Institution was therefore undesirable. But in view of what George
Hale has done for him, he wanted to consult him first before handing in his
resignation.
229
from Mount Wilson and by Campbell from Lick. This analysis confirmed the
earlier result that the average radial velocity (in the line of sight) was smaller for
stars with a smaller proper motion. Because small proper motion statistically
indicates greater distance and thus intrinsically bright stars, this seemed to mean
that the radial velocity and the absolute brightness (the intrinsic luminosity)
of a star correlated with each other. That would say something about the
properties of stars, but, much more importantly, in principle opened up the
possibility of getting an idea of the distance of a star based on its radial velocity.
Adams investigated this further and found that for stars from which the distance could be determined independently with statistical methods, the radial
velocity indeed was small if the distance was large and the star therefore intrinsically relatively bright. These were developments in which Kapteyn was very
much interested. We will see later that this work would lead to serious friction
between Kapteyn and Adams.
11.3 Correspondence with Hale
During the Great War, Kapteyn and George Hale maintained an extensive
correspondence. Two themes stand out. The first is Kapteyn’s appointment as
paid Research Associate at the Carnegie Institution. Kapteyn felt, as we saw
before, that he could contribute too little to justify this, especially now that he
was no longer able to visit Mount Wilson due to the war. In a letter dated March
26, 1916, for example, Kapteyn stated that he hoped to return to California
that year. This message crossed a letter from Hale dated March 9 (so mail did
not travel quickly in those years, despite the neutrality of the Netherlands),
in which he urged him not to come because of the risks. Kapteyn planned to
come in mid-July, as in March his daughter Henriette was staying in Groningen
because she was pregnant. And he remarked that the ship companies more and
more often did not dare to offer tickets, so finding a ship for the crossing
might even have been impossible. But then he concluded that if it would not
be possible or desirable to come, he would resign as Associate of the Carnegie
Institution. In fact, he also stated that in the five years that remained until his
retirement, he had better concentrate anyhow on his work for his laboratory so
as not to have to leave unfinished work behind. He was dissatisfied with what
he has been able to do for Mount Wilson, and continuing the link with the
Carnegie Institution was therefore undesirable. But in view of what George
Hale has done for him, he wanted to consult him first before handing in his
resignation.