The Universe is not a random collection of stars and planets. You are currently on a planet (the Earth) that orbits around a star (the Sun). This star is in turn part of a larger constellation of billions of stars (the Milky Way). Leiden astronomer Jan Oort showed how large this galaxy is and how it rotates.
How can we prove that we live in a galaxy? Leiden astronomer Jan Hendrik Oort managed to do so by analysing a vast collection of photographs of the movement of stars. These photographs showed that some stars are part of a common structure that slowly rotates. This structure has a core in which the stars are relatively close together and an outer layer in which they form a kind of spiral structure.
Much larger
With these measurements Oort had already made a name for himself as early as 1927 – even before he was appointed professor in Leiden. It was at this time that he proved that the Milky Way was much larger than assumed by astronomers. In his later work he continued to focus on understanding the Milky Way better, if only on paper.
The story goes that it was only as a middle-aged man, on a trip to the Southern Hemisphere, that Oort was able for the first time to see with his own eyes the beauty of the Milky Way in the night sky. The sight amazed him. For a long time he lay flat on his back, staring up into the sky and nobody could persuade him to stop looking.
More theoretical
And yet observation was not Oort’s strong suit. He was born in Franeker in 1900 and went on to study astronomy in Groningen. Immediately after his graduation in 1922, he was invited to Yale University in the US. But this work, which required him to make his own measurements, did not appeal to Oort and he gratefully accepted Willem de Sitter’s offer to come to Leiden to do more theoretical work.
On his thirtieth birthday Oort was offered a Chair at Harvard University. Two years later Columbia University also tried to tempt him. Oort turned down both of these prestigious American universities and in 1935 he was appointed professor in Leiden, where he stayed for the remainder of his career. He continued to work even after his retirement in 1970. Oort died in Leiden in 1992, aged 92. The building currently housing the Leiden astronomers is named after him. All the scientific and personal documents that belonged to Jan Oort are now housed in the Leiden University Library.
