Jakob Karl Ernst Halm (1866 – 1944) was a pioneer of

stellar dynamics Stellar dynamics is the branch of astrophysics which describes in a statistical way the collective motions of stars subject to their mutual gravity. The essential difference from celestial mechanics is that the number of body N \gg 10. Typica ...

and the first person to suggest the existence of a

mass–luminosity relation In astrophysics, the mass–luminosity relation is an equation giving the relationship between a star's mass and its luminosity, first noted by Jakob Karl Ernst Halm. The relationship is represented by the equation: :\frac = \left(\frac\right)^a whe ...

for stars.

Early life

Halm was born at Bingen am Rhein,

Kingdom of Prussia The Kingdom of Prussia (german: Königreich Preußen, ) was a German kingdom that constituted the state of Prussia between 1701 and 1918.Marriott, J. A. R., and Charles Grant Robertson. ''The Evolution of Prussia, the Making of an Empire''. Re ...

on 30 November 1866.

Education

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.

Career

JKE_Halm_making_solar_observations_in_Edinburgh_ca_1905

Assistant at the University Observatory, Strasbourg (1889-1895). First Class Assistant at the

Royal Observatory Edinburgh The Royal Observatory, Edinburgh (ROE) is an astronomical institution located on Blackford Hill in Edinburgh. The site is owned by the Science and Technology Facilities Council (STFC). The ROE comprises the UK Astronomy Technology Centre (UK&nbs ...

(1895-1907). Chief Assistant at the

Royal Observatory, Cape of Good Hope The Royal Observatory, Cape of Good Hope, is the oldest continuously existing scientific institution in South Africa. Founded by the British Board of Longitude in 1820, it now forms the headquarters building of the South African Astronomical Obs ...

(1907-1927).

Scientific contributions

While at Edinburgh Halm used a

heliometer A heliometer (from Greek ἥλιος ''hḗlios'' "sun" and ''measure'') is an instrument originally designed for measuring the variation of the sun's diameter at different seasons of the year, but applied now to the modern form of the instrument ...

to feed a

spectrograph An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify mate ...

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 P Cygni (34 Cygni) is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in ''Uranometria'' as a nova. Located about 5,300 light-years (1,560 parsecs) from Earth, it is a hypergiant ...

profile while discussing the spectrum of Nova Persei 1901. Halm

While at the Royal Observatory, Cape of Good Hope, he was involved in follow-up work on the

Cape Photographic Durchmusterung In astronomy, Durchmusterung or Bonner Durchmusterung (BD) is an astrometric star catalogue of the whole sky, compiled by the Bonn Observatory in Germany from 1859 to 1903. The name comes from ('run-through examination'), a German word used for ...

(CPD) of David Gill and

Jacobus Kapteyn Prof Jacobus Cornelius Kapteyn FRS FRSE LLD (19 January 1851 – 18 June 1922) was a Dutch astronomer. He carried out extensive studies of the Milky Way and was the discoverer of evidence for galactic rotation. Kapteyn was also among the fi ...

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 * Maxwell–Boltzmann distribution * The Maxwellians ''The Maxwellians'' is a book by Bruce J. Hunt, published in 1991 by Cornell University Press; a paperback edition appeared in 1994, and the book was reissued in 2005. It chronicles the developm ...

, 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. 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. His work on determination of magnitudes from

photographic plate Photographic plates preceded photographic film as a capture medium in photography, and were still used in some communities up until the late 20th century. The light-sensitive emulsion of silver salts was coated on a glass plate, typically thinn ...

s led him to an improved understanding of

reciprocity Reciprocity may refer to: Law and trade * Reciprocity (Canadian politics), free trade with the United States of America ** Reciprocal trade agreement, entered into in order to reduce (or eliminate) tariffs, quotas and other trade restrictions on ...

failure, on which he published a paper, leading to the Kron–Halm catenary equation. In 1917, Halm was the first person to make an estimate of the total to selective

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

of starlight. He determined that the interstellar extinction in magnitudes is a factor of 1.22 times greater in the blue than in the visible. Halm believed that many terrestrial features could be explained by an ongoing expansion of the earth's crust. Though it created considerable interest at the time, since the advent of plate tectonic theory his expansion hypothesis is no longer considered plausible.

Death

He died 17 July 1944 in South Africa.

References

{{DEFAULTSORT:Halm, Jakob Karl Ernst 1866 births 1944 deaths South African astronomers Academic staff of the University of Strasbourg Emigrants from the German Empire Immigrants to the Cape Colony