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In the early 1950’s Russian astronomers suggested that the
radiation from the Crab Nebula might be synchrotron radiation.
Oort had long been interested in the Crab Nebula. Some ten years
earlier he had collaborated with the sinologist J.J.L. Duyvendak
in an attempt to identify the event that had given rise to the
Crab Nebula. They concluded that it must have been the ‘guest star’
of 1054, and that it must have been a supernova rather than a nova.
At the time Oort heard of a synchrotron suggestion, he was working
with T. Walraven on an instrument to determine possible expansion of
the nebula. The instrument was easily adapted for measuring polarization,
the footprint of synchrotron radiation, and they succeeded in showing
that the Crab Nebula did indeed show strong polarization, confirming its
synchrotron character and allowing conclusions to be drawn about the
energy source.
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 29. J.J.L. Duyvendak, Professor of Chinese
in Leiden University, about the identification of the Crab Nebula with
the supernova of 1054. Letter by Duyvendak , Oegstgeest, July 30, 1940,
to J.H. Oort. Oort added in pencil: ‘Must write an airmail letter about
this to Mayall and Baade, as soon as I am back in Leiden’. Duyvendak’s
letter begins as follows: ‘Amice, I have succeeded in finding another
place where your Nova is mentioned. There exists an extensive work, of
which a facsimile edition was published only a few years ago (and which
could not have been known to earlier researchers), treating the institutions
of the Sung dynasty, which includes the year 1054. The name is Sung Hui Yao.
In vol. 54 of this work,....’.
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 30. Correspondence between J.H. Oort and W. Baade
about the Crab Nebula (1955). Carbon copy of one of Oort’s letters to Baade
(April 26, 1955), and Baade’s handwritten answer to Oort (Pasadena, April 30,
1955).
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 31. ‘Further data bearing on the identification of
the Crab Nebula with the supernova of 1054 A.D.’ Part I. J.J.L. Duyvendak,
‘The ancient oriental chronicles’ Part II. N.U. Mayall and J.H. Oort, ‘The
astronomical aspects’ (in Publications of the Astronomical Society of the
Pacific 54 (1942), pp. 91-94 and 95-104).
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