Jacobus Cornelius Kapteyn (1851 - 1922)
A Short Biography
Jacobus Cornelius Kapteyn was born in the village of
Barneveld, the Netherlands
on 19th January 1851 as one of many children of the village's schoolmaster.
Although his father had rather seen him as a teacher, he went to the
University of Utrecht
to study mathematics and physics in 1868. In 1875 Kapteyn finished his thesis,
which was about the vibration of a membrane. He then decided to apply for the
job of observer at
Leiden Observatory,
where he was appointed the same year. Only three years later, as a result of
the new Law on High Education, he became the
first Professor of Astronomy and Theoretical Mechanics at the
University of Groningen.
The young professor married
Catharina Elizabeth Kalshoven in 1879. In the
subsequent years, two daughters and a son
were born. Meanwhile, as there was
no money for a telescope or observatory, Kapteyn devoted himself to theoretical
and mathematical studies with his brother and professor in mathematics Willem Kapteyn.
THE CAPE PHOTOGRAPHIC DURCHMUSTERUNG
As Kapteyn's repeated requests for an observatory did not have any effect,
he decided to turn this lack into an advantage. At the time
David Gill was
conducting a photographic survey of Southern Hemisphere stars at the
Cape Town Observatory
in South Africa. He desperately needed some help in measuring the photographic
plates, as he couldn't do all the work alone in a reasonable
amount of time. Kapteyn, seeing here a possibility for an astronomy professor
without an observatory, offered Gill his assistance, which was gladly accepted.
The plates sent to Kapteyn were measured in two small rooms in the basement of
the Groningen Physiology Laboratory, which were offered as a temporary housing
by Kapteyn's
friend prof. D. Huizinga. This was Kapteyn's first `Astronomical Laboratory'.
It would take until 1913 for him to get a laboratory of his own.
Between 1896 and 1900 the survey resulted in the publication of the
Cape Photographic Durchmusterung, listing positions and magnitudes
for 454,875 stars on the Southern Hemisphere. It was received well
by the astronomical community: Kapteyn was awarded the Gold Medal
of the Royal Astronomical Society.
THE THEORY OF TWO STAR STREAMS
Kapteyn's subsequent work, published in 1904, concerned the
proper motions
of stars. Kapteyn found evidence that these proper motions
were not random, as was generally accepted in that time. Surprisingly,
the stars could be divided into two streams, apparently moving in
opposite directions. This discovery ultimately led to the finding
of galactic rotation by
Lindblad and
Oort.
George Ellery Hale,
whom Kapteyn met at the St. Louis Conference in 1904,
invited him to become a `research associate' at the
Mount Wilson Observatory
in Pasadena, California. There at the time was the largest reflecting
telescope of the world, carrying a 60-inch mirror. From 1908 to the Great
War in 1914, Kapteyn and his wife were welcomed in Pasadena every summer.
As on Mount
Wilson there was no proper accommodation for married couples, the observatory
staff built a small guesthouse for the Kapteyns, which is still known
today as the
`Kapteyn Cottage'.
THE PLAN OF SELECTED AREAS
Aided by his rising fame, Kapteyn launched a plan for a major study of the
distribution of stars in the
Galaxy, using counts of stars in different directions. The plan involved
measuring the apparent
magnitude,
spectral type,
radial velocity and proper motion
of stars in 206 zones. It was received with enthusiasm. Starting in
1906, it became the first major international collaborative project in
astronomy, involving over 40 observatories.
BACK TO LEIDEN
Kapteyn retired in 1921 at the age of 70. On the request of his former
student and director of Leiden Observatory
Willem de Sitter, Kapteyn went back
to Leiden to assist in upgrading the observatory to contemporary astronomical
standards.
After a lifetime of measurement, Kapteyn finally found the time to
do some theoretical astronomy. His life-work
"First attempt at a theory of the arrangement and motion of the sidereal system"
was published in 1922, and described a lens-shaped `island universe' of which
the density decreased away from the center. This galaxy was thought to be
40,000 light years in size, the sun being relatively close (2,000 light years)
to its center.
Unfortunately, Kapteyn and his contemporaries underestimated the influence
of interstellar
extinction. Only after Kapteyn's death R.J. Trumpler determined that
the amount of extinction was actually much greater than had been assumed.
This discovery increased the estimate of the galaxy's size to 100,000 light
years, with the sun replaced to a distance of 30,000 light years from its
center.
Kapteyn died in Amsterdam on 18th June 1922.
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Petra van der Heijden