Ejnar Hertzsprung (;

Copenhagen Copenhagen ( or .; da, København ) is the capital and most populous city of Denmark, with a proper population of around 815.000 in the last quarter of 2022; and some 1.370,000 in the urban area; and the wider Copenhagen metropolitan a ...

, 8 October 1873 – 21 October 1967,

Roskilde Roskilde ( , ) is a city west of Copenhagen on the Danish island of Zealand. With a population of 51,916 (), the city is a business and educational centre for the region and the 10th largest city in Denmark. It is governed by the administrative ...

) was a Danish chemist and astronomer.

Career

Hertzsprung was born in

Frederiksberg Frederiksberg () is a part of the Capital Region of Denmark. It is formally an independent municipality, Frederiksberg Municipality, separate from Copenhagen Municipality, but both are a part of the City of Copenhagen. It occupies an area of ...

, Denmark, the son of Severin and Henriette. He studied chemical engineering at Copenhagen Polytechnic Institute, graduating in 1898. After spending two years working as a chemist in St. Petersburg, in 1901 he studied

photochemistry Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet ( wavelength from 100 to 400 nm), visible light (400� ...

Leipzig University Leipzig University (german: Universität Leipzig), in Leipzig in Saxony, Germany, is one of the world's oldest universities and the second-oldest university (by consecutive years of existence) in Germany. The university was founded on 2 December ...

for a year. His father was an amateur astronomy, which led to Ejnar's interest in the subject. He began making astronomical observations in Fredericksberg in 1902, and within a few years had noticed that stars with similar

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

could have widely different

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it ...

s. In 1909, he took a position at the Göttingen Observatory under director

Karl Schwarzschild Karl Schwarzschild (; 9 October 1873 – 11 May 1916) was a German physicist and astronomer. Schwarzschild provided the first exact solution to the Einstein field equations of general relativity, for the limited case of a single spherical non-r ...

. In 1911 Hertzsprung developed the

Hertzsprung–Russell diagram The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their stellar classifications or effective te ...

, independently developed in 1913 by Henry Norris Russell. In 1913 Hertzsprung determined the distances to several

Cepheid A Cepheid variable () is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid varia ...

variable stars by

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby object ...

, and was thus able to calibrate the relationship, discovered by Henrietta Leavitt, between Cepheid period and

luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...

. In this determination he made a mistake, possibly a slip of the pen, putting the stars 10 times too close. He used this relationship to estimate the distance to the

Small Magellanic Cloud The Small Magellanic Cloud (SMC), or Nubecula Minor, is a dwarf galaxy near the Milky Way. Classified as a dwarf irregular galaxy, the SMC has a D25 isophotal diameter of about , and contains several hundred million stars. It has a total mass of ...

. From 1919 to 1946, Hertzsprung worked at

Leiden Observatory Leiden Observatory ( nl, Sterrewacht Leiden) is an astronomical institute of Leiden University, in the Netherlands. Established in 1633 to house the quadrant of Rudolph Snellius, it is the oldest operating university observatory in the world, wit ...

in the Netherlands, from 1937 as director. Among his graduate students at Leiden was Gerard Kuiper. Perhaps his greatest contribution to astronomy was the development of a classification system for stars to divide them by spectral type, stage in their development, and luminosity. He used the earlier classification system developed by Antonia Maury in his work.Hoffleit, D. "Reminiscences on Antonia Maury and the c-Characteristic." ''The MK Process at 50 Years: A Powerful Tool for Astrophysical Insight''. Vol. 60. 1994. The so-called "Hertzsprung–Russell Diagram" has been used ever since as a classification system to explain stellar types and

stellar evolution Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...

. He also discovered two

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

s, one of which is 1627 Ivar, an

Amor asteroid The Amor asteroids are a group of near-Earth asteroids named after the archetype object 1221 Amor . The orbital perihelion of these objects is close to, but greater than, the orbital aphelion of Earth (i.e., the objects do not cross Earth's orbi ...

. His wife Henrietta (1881–1956) was a daughter of the Dutch astronomer

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 f ...

. Hertzsprung died in

in 1967. The asteroid 1693 Hertzsprung was named in his honour.

Asteroids discovered

* 1627 Ivar (25 September 1929) * 1702 Kalahari (7 July 1924)

Honors

; Awards *

Gold Medal of the Royal Astronomical Society The Gold Medal of the Royal Astronomical Society is the highest award given by the Royal Astronomical Society (RAS). The RAS Council have "complete freedom as to the grounds on which it is awarded" and it can be awarded for any reason. Past awar ...

in 1929 *

Bruce Medal The Catherine Wolfe Bruce Gold Medal is awarded every year by the Astronomical Society of the Pacific for outstanding lifetime contributions to astronomy. It is named after Catherine Wolfe Bruce, an American patroness of astronomy, and was fi ...

in 1937 ; Named after him * lunar impact crater '' Hertzsprung'' * main-belt asteroid 1693 Hertzsprung

Sources

* ''Sky & Telescope'', January, 1968, Sky Publishing Corporation,

Cambridge Cambridge ( ) is a university city and the county town in Cambridgeshire, England. It is located on the River Cam approximately north of London. As of the 2021 United Kingdom census, the population of Cambridge was 145,700. Cambridge bec ...

References

External links

*
Bruce Medal page

{{DEFAULTSORT:Hertzsprung, Ejnar 1873 births 1967 deaths 20th-century Danish astronomers People from Copenhagen Danish chemists Discoverers of asteroids Recipients of the Gold Medal of the Royal Astronomical Society