Jakob Karl Ernst Halm

Jakob Karl Ernst Halm (born Bingen am Rhein: 30 November 1866 – South Africa 17 July 1944) was a pioneer of stellar dynamics and the first person to suggest the existence of a mass-luminosity relation for stars.

Halm went to school in Bingen and studied later at Giessen, Berlin and Kiel. He obtained his PhD at Kiel in 1890 for work on linear differential equations.

Assistant at the University Observatory, Strasbourg (1889-1895). First Class Assistant at the Royal Observatory Edinburgh (1895-1907). Chief Assistant at the Royal Observatory, Cape of Good Hope (1907-1927).

While at Edinburgh Halm used a heliometer to feed a spectrograph in order to study the differential rotation of the Sun at different latitudes. He discovered that absorption lines near the edge of the Solar disc are displaced towards the red, compared with their positions at the centre. This was not due to obvious effects such as rotation. He also gave the first interpretation of what is now called a P Cygni profile while discussing the spectrum of Nova Persei 1901.

While at the Royal Observatory, Cape of Good Hope, he was involved in follow-up work on the Cape Photographic Durchmusterung (CPD) of David Gill and Jacobus Kapteyn using radial velocities and proper motions. Repeated observations had already led to the discovery of two star streams by Kapteyn and Halm was able to identify a third (Halm 1911) associated with what he called "Orion-type" stars. This paper went on to say that stars appeared to obey a Maxwellian distribution, implying equipartition of energy, i.e., the less massive ones moved more rapidly than the massive ones. His conclusion was based on stellar masses derived from a number of well-studied binaries. Eddington showed, however, that stellar interactions, owing to their rarity, could not produce this result. However Halm's work was important in stimulating research on the subject.

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