Jakob Karl Ernst Halm (1866 – 1944)[1][2] was a pioneer ofstellar dynamics and the first person to suggest the existence of amass–luminosity relation for stars.
Halm was born at Bingen am Rhein,Kingdom of Prussia on 30 November 1866.
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 theUniversity Observatory, Strasbourg (1889-1895).First Class Assistant at theRoyal Observatory Edinburgh (1895-1907).Chief Assistant at theRoyal Observatory, Cape of Good Hope (1907-1927).
While at Edinburgh Halm used aheliometer to feed aspectrograph 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.[3] He also gave the first interpretation of what is now called aP Cygni profile[4][5] while discussing the spectrum ofNova Persei 1901.
While at the Royal Observatory, Cape of Good Hope, he was involved in follow-up work on theCape Photographic Durchmusterung (CPD) ofDavid Gill andJacobus 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.[6] This paper went on to say that motion of stars appeared to obey aMaxwellian 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[7] 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.
The paper concluded that there is a relation between spectral type and mass for stars. This was the first announcement of the mass–luminosity relation, later elaborated by many others.[8]His work on determination of magnitudes fromphotographic plates led him to an improved understanding ofreciprocity failure, on which he published a paper,[9] leading to the Kron–Halm catenary equation.
In 1917, Halm was the first person to make an estimate of the total to selectiveextinction of starlight. He determined that the interstellar extinction in magnitudes is a factor of 1.22 times greater inthe blue than in the visible.[10]
Halm believed that many terrestrial features could be explained by an ongoing expansion of theEarth's crust.[11] Though it created considerable interest at the time, since the advent ofplate tectonic theory his expansion hypothesis is no longer considered plausible.
He died 17 July 1944 in South Africa.
| International | |
|---|---|
| Academics | |
| People | |
