Kapteyn, Jacobus Cornelius (1851–1922)
Dutch astronomer who discovered star
streaming, now known to be the observed effect of the rotation of the
Milky Way Galaxy, and compiled the Cape
Photographic Durchmusterung of almost half a million stars from
photographs taken in South Africa by the Scottish astronomer David Gill.
Kapteyn studied at Utrecht University and served as professor of astronomy
at the University of Groningen from 1878 to 1921. After his work on the
Durchmusterung he was involved with a program of photographically
determining the parallaxes of 10,000 stars,
during which he discovered the high-parallax star now known as Kapteyn's
Star and the fact that there are two preferred streams of stellar motion.
In 1906 he inaugurated a plan to measure the positions, magnitudes,
spectral types, and proper
motions of stars in selected areas of the sky with a view to determining
the shape and structure of the Galaxy (then thought to be the entire universe).
Kapteyn's selected areas, as they became known, consisted
of 206 areas, each about 1° × 1°, uniformly spaced at about
15° intervals over the whole sky, plus 46 others in regions of special
importance such as around the galactic poles. From an analysis of the plates,
Kapteyn and fellow Dutchman Pieter van Rhijn (1886–1960) came up with
a model, known as Kapteyn's Universe, which was correct
in some details. It described a lens-shaped stellar system whose density
decreased away from the center. However, its estimate of the size of the
Galaxy (40,000 light-years across) and, in particular, its placement of
the Sun only 2,000 light-years from the center, were incorrect because of
the lack of knowledge of interstellar
absorption.
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• ASTRONOMERS
AND ASTROPHYSICISTS
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