Prof Jacobus Cornelius Kapteyn FRS FRSE LLD (19 January 1851 – 18 June 1922) was a Dutch astronomer. He carried out extensive studies of the Milky Way and was the discoverer of evidence for galactic rotation. Kapteyn was also among the first to suggest the existence of dark matter using stellar velocities as early as 1922.
Jacobus Cornelius Kapteyn
Jacobus Kapteyn. Painting by Jan Veth (1921).
|Died||18 June 1922 (aged 71)|
|Alma mater||University of Utrecht|
|Known for||discovery of evidence for galactic rotation|
|Awards||Bruce Medal 1913|
Kapteyn was born in Barneveld to Gerrit J. and Elisabeth C. (née Koomans) Kapteyn, He got into university at the age of 16 and went to the University of Utrecht to study mathematics and physics in 1868. He did very well in his studies and when he graduated he was magna cum laude. This later laid the foundation for his later career. In 1875, after having finished his thesis, he worked for three years at the Leiden Observatory. In 1878 Kapteyn was inducted as the first professor of astronomy and theoretical mechanics at the University of Groningen. Also at Groningen, he founded the Astronomical Laboratory in 1896 and consequently became the director of the facility. Kapteyn fulfilled both occupational roles as professor and director at Groningen until his retirement in 1921. In 1888 he became a member of the Royal Netherlands Academy of Arts and Sciences.
Between 1896 and 1900, lacking an observatory, he volunteered to measure photographic plates taken by David Gill, who was conducting a photographic survey of southern hemisphere stars at the Royal Observatory at the Cape of Good Hope. To measure these plates he used a special parallacticic instrument that Kapteyn himself had constructed. The results of this collaboration was the publication of Cape Photographic Durchmusterung, a catalog listing positions and magnitudes for 454,875 stars in the Southern Hemisphere.
In 1904, studying the proper motions of stars, Kapteyn reported that these were not random, as it was believed in that time; stars could be divided into two streams, moving in nearly opposite directions. It was later realized that Kapteyn's data had been the first evidence of the rotation of our Galaxy, which ultimately led to the finding of galactic rotation by Bertil Lindblad and Jan Oort.
In 1906, 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. This enormous project was the first coordinated statistical analysis in astronomy and involved the cooperation of over forty different observatories.
He was awarded the James Craig Watson Medal in 1913. Kapteyn later retired in 1921 at the age of seventy, but 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.
His life's 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, now known as the Kapteyn Universe. In his model the Galaxy was thought to be 40,000 light years in size, the sun being relatively close (2,000 light years) to its center. The model was valid at high galactic latitudes but failed in the galactic plane because of the lack of knowledge of interstellar absorption.
It was only after Kapteyn's death, in Amsterdam, that Robert Trumpler determined that the amount of interstellar reddening 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 the galactic center.
The astronomy institute of the University of Groningen is named after Kapteyn. A street in the city of Groningen is also named after Kapteyn: the J.C. Kapteynlaan. And the Isaac Newton Group of Telescopes on La Palma in the Canary Islands named the Jacobus Kapteyn Telescope (JKT) after him.
His daughter Henrietta (1881-1956) married astronomer Ejnar Hertzsprung.
Jacobus Kapteyn was a very doting father during the earlier years in his career and took an interactive role which during the 1880s was not done much by fathers. When Kapteyn’s children reached schooling age both his daughters, Jacoba Cornelia and Henrietta, and his son, Gerrit, went to a boys school. His daughter later made significant strides for women by going to university for law and medicine.
The Kapteyn UniverseEdit
The structure and size of the galaxy was of great interest to many astronomers during Kapteyn's time. Around 1900, Hugo von Seegler made an attempt at the structure of the galaxy by making counts of stars between successive magnitudes. Ultimately, von Seegler was able to determine the rates at which the galaxy was diminishing in multiple different areas of the sky. In 1901, Kapteyn employed the proper motions technique and derived a statistical approach that allowed him to estimate the average distance to stars between successive magnitudes, effectively providing a scale for von Seegler's discoveries. Conclusive results from von Seegler and Kapteyn estimated the galaxy to be an oblate star system approximately 10 kpc in width and 2 kpc in thickness. Using the method of star counts, Kapteyn was able to confirm his results and dubbed his findings as his first model of the galaxy.
Despite having made great strides and providing a model for the galaxy, there was one major issue. Kapteyn's work was based off an unproven presumption, that is, that there is no light absorption in the space. If there were light absorption in space, then stars would look more faint, thus, seeming further away than they actually are. Light absorption would also cause the galaxy to disperse more quickly than it actually does. If light absorption proved to be a considerable factor, then Kapteyn's work, or at least his conclusions, would more or less be obsolete. To circumvent this dilemma, Kapteyn spent years studying and measuring the amount of absorption. By 1918, he was convinced light absorption was of negligible amounts and confidently stood behind his first model for the structure of the galaxy.
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- Gold Medal of the Royal Astronomical Society (1902)
- James Craig Watson Medal (1913)
- Bruce Medal (1913)
Named after him
- Kapteyn, Jacobus Cornelius (1922). "First attempt at a theory of the arrangement and motion of the sidereal system". Astrophysical Journal. 55: 302–327. Bibcode:1922ApJ....55..302K. doi:10.1086/142670. It is incidentally suggested that when the theory is perfected it may be possible to determine the amount of dark matter from its gravitational effect. (emphasis in original)
- Hockey, Thomas (2009). The Biographical Encyclopedia of Astronomers. Springer Science+Business Media. ISBN 978-0-387-31022-0. Retrieved August 22, 2012.
- van der Kruit, Pieter (2015). Jacobus Cornelius Kapteyn, Born Investigator of the Heavens. Astrophysics and Space Science Library. 416. Springer Science+Business Media. doi:10.1007/978-3-319-10876-6. ISBN 978-3-319-10876-6.
- Thomas A Hockey; Virginia Trimble; Katherine Bracher, eds. (2007). The biographical encyclopedia of astronomers. Springer. ISBN 9780387310220. OCLC 65764986.
- "Jacob Cornelius Kapteyn (1851 - 1922)". Royal Netherlands Academy of Arts and Sciences. Retrieved 22 July 2015.
- Belkora, Leila (2003). Minding the Heavens The story of Our Discovery of the Milky Way. Bristol: Institute of physics Publishing. pp. 206–244. ISBN 0-7503-0730-7.
- Berendzen, Richard. (1984). Man discovers the galaxies. Hart, Richard (Richard Cullen), Seeley, Daniel. ([Morningside ed.] ed.). New York: Columbia University Press. ISBN 0231058268. OCLC 10375021.