Edward Purcell (1979) - The Interstellar Medium

Edward Purcell (1979)

The Interstellar Medium

Edward Purcell (1979)

The Interstellar Medium

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While Edward Purcell also attended the 4th Lindau physics meeting in 1962, the present lecture is the only one he ever gave there. The work that brought him the Nobel Prize in Physics 1952 concerned magnetic properties of solid matter in the laboratory. But Purcell also had other interests and it was he and his collaborators who found the sharp 21 cm radio astronomy line emanating from clouds of hydrogen in space. In 1979, Purcell had for several years worked on a problem connected with interstellar matter in our galaxy, i.e. matter between the stars or their planets. At that time it was thought that only 10% of the matter could be found between the stars and it was also unclear if other stars had planets. Today the estimate is that most matter is located in the interstellar and intergalactic medium. We also believe that most stars have planets and the research into what is today named dark matter and dark energy is a very hot topic, as is the search for new planets. But for Purcell, space was mainly empty, only containing a very dilute mixture of atoms, molecules and dust grains. Although scarce, the grains play an important role in the chemistry going on in space by, e.g., acting as surface catalysts for chemical reactions such as the formation of hydrogen molecules, H2, from free hydrogen atoms H. The grains also interact with the radiation from the stars and this was Purcell’s main interest. It was known that there are large-scale magnetic fields in space and his idea was that these magnetic fields align the dust grains and thus act as a polarizer of the radiation from the stars. In a sense this astrophysical problem made contact with Purcell’s laboratory experiments, where he had studied such phenomena as relaxation effects in ensembles of magnetic atoms and nuclei.

Anders Bárány

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