History of Astronomy

father had obtained a degree in astronomy in the local university. Although awarded

a gold medal for some of his work in astronomy, his father gave up science due to

the poor financial returns involved and eventually became director of the Denmarks

national life insurance company.

In 1893, when Ejnar was 20 years old, his father died. After this, his fathers books

were sold as there was nobody in the family, including Ejnar, who had an interest in

astronomy. His father never wanted his son to study astronomy or insurance or

mathematics. As a result, Ejnar himself went to university and graduated with a

degree in chemical engineering and never studied astronomy.

Between 1899 and 1901, Ejnar was employed as an engineer at St. Petersburg and

studied photochemistry at Leipzig. In 1902, Hertzsprung returned to Denmark after

the death of his mother and started working as an amateur astronomer at the

Copenhagen University Observatory and the Urania Observatory in Frederiksberg,

which belonged to a well-known amateur astronomer Victor Neilsen. Victor is quoted

as saying to a fellow amateur; Hertzsprung is a clever young man who intends to

measure star colours photographically. Little did they know that this was to start a

life-long interest in stellar measurement and lead to ground-breaking research in

stellar physics.

In 1905 Hertzsprung published Zur Strahlung der Sterne (Radiation of Stars) in the

periodical "Zeitschrift fr Wissenschaftliche Photographie" (Magazine for Scientific

Photography). What was unusual about this was that he did not publish them in an

astronomical journal. This paper made the following conclusions:

• Stars in the spectral-classes G, K and M are divided into two series with different luminosity.

• Luminous red stars must be massive.

• The small number of red giants indicates that that these stars are in a stage of fast evolution.

In 1907 Hertzsprung published "Zur Bestimmung der Photographischen

Sterngrssen" (Determination of the Photographic Star Sizes) in the same journal

with his follow-on results.

Karl Schwartzchild was the leading astronomer in Germany at the time and was the

director of Gttingen Observatory. Upon reading Hertzsprungs papers he secured a

post for him firstly at Gttingen Observatory and later took him to Potsdam, where

Schwartzchild also moved. Within seven months, Hertzsprung become a senior staff

member at one of Europes premier observatories.

Meanwhile, across the Atlantic and one year later, Henry Norris Russell came to

conclusions very similar to those determined by Hertzsprung but was completely

unaware of the Danish mans work, partly due to the unusual nature of the

publication of Hertzsprungs work.

The graphical form of both Hertzsprungs and Russells work wasnt published until

1911 by Hertzsprung in the paper "Publikationen des Astrophysikalischen

Observatorium zu Potsdam" (Astrophysical Publications of Potsdam Observatory).

Figure 1 Charts from Hertzsprungs Paper of 1911

 

Figure 2 Hertzsprung-Russell Diagram

 

Both astronomers became attributed to this common piece of work. However, there

was no animosity between the two gentlemen. Russell, when asked about the

diagram, was quick to point out that it was Hertzsprung who started work on

developing it. Hertzsprung, for his part, is quoted as saying It is Russells merit that

he presented this complex of problems in a spirited, easily understood form in

current journals.

In 1919, Hertzsprung became Adjunct Director of the University Observatory of

Leiden, Holland and was promoted to rank of associate professor in 1920.

In 1922, Hertzsprung published his catalogue of data representing 734 stars, all of

which were brighter than 5th magnitude. At the time, accurate information was not

available on star parallax so he used data from stars proper motion to calculate

absolute magnitude and thence construct a diagram showing a relationship between

colour and luminosity.

However, Hertzsprung also undertook research in others areas of stellar

measurement. In 1911 he developed a light curve for Polaris. Shortly after, he

determined the distance of the Small Magellanic Cloud using Cepheid variable stars.

In 1926, he undertook further work on Cepheid variable stars and developed a

relationship between period length and light curve shape based on photographs he

took at an observatory in South Africa during an 18-month trip there in 1923-1925.

However, he undertook extensive work in the measurement of double stars. He

developed techniques which would allow him to measure relative positions between

stars to an accuracy of a few thousandths of an arc second.

During his work, he also discovered two asteroids; 1627 Ivar in the year 1929 and

the asteroid 1702 Kalahari 5 years earlier.

During his time, Hertzsprung received many awards, including doctorates from the

universities of Utrecht (1923), Copenhagen (1946) and Paris (1949). He also

received the Gold Medal from the Royal Astronomical Society in 1929 and the Bruce

Medal in 1937 from the Astronomical Society of the Pacific. In 1959, the city of

Copenhagen gave him its Ole Roemer Medal.

Hertzsprung continued to take measurements long after his retirement and only

slowed down 2 to 3 years before his death. He is quoted as saying I cannot

compete with Dr. Strand any longer!; Dr. Strand being a former student of his who

was Director of the US Naval Observatory.

Hertzsprung died in 1967 in Roskilde, Denmark, at the age of 94 and his papers

(12,153 pages with original measurements) were given to the USNO Library.

Michael OConnell 2006