Ejnar Hertzsprung was born Oct 8, 1873 in Copenhagen, Denmark. Hertzsprung was the Danish astronomer who pioneered the study of the birth and death of stars. He proved the existence of giant and dwarf stars as well as co-authored the Hertzsprung-Russel Diagram.

He studied chemical engineering in Copenhagen, and became a specialist in photochemistry. He worked as a chemist in St. Petersburg before returning to Denmark to become an independent astronomer. In 1902 he was invited to Guttingen to work with Karl Schwarzschild. He accompanied Schwarzschild to the Potsdam Astrophysical Observatory in 1909 where he stayed in Potsdam until moving to join De-Sitter in Leiden. From 1919-44 he worked at the Leiden Observatory in the Netherlands. During his last 9 years there he was its director. He then retired to Denmark but continued to work into his 90's.

It is surprising to note that most of Hertzsprung's work was done as desk work. He did not do the field research that many astronomers did but rather he specifically worked with information from other scientists looking for data that had previously been overlooked . He his first major paper on the existence of giant and dwarf stars while working at a private observatory. He also worked out the general relationships between star spectral types, and the temperature and brightness of stars.

Between 1913 and 1917, Hertzsprung claimed from his research that the color of a star has everything to do with how hot it is. He theorized that blue stars are the hottest and largest, and that red dwarf stars are the smallest and coolest. He proposed that a star began its life as a hot blue star and degraded to a red dwarf.

Karl Schwarzschild (left) & Ejnar Hertzsprung

In 1912, working independently, Ejnar Hertzsprung from Denmark and Henry Norris Russell from the United States of America made the discovery that the luminosity of a star is related to its surface temperature. His work combined with that of Russell, resulted in the Hertzsprung-Russell diagram of types of stars. The Hertzsprung-Russell diagram is a graph which has the absolute magnitudes of stars plotted against their spectral classification. When the luminosity of a star is plotted against its temperature, thousands of stars fall into 3 well-defined groups. The first of the 3 groups is called the Main Sequence. The Sun is an example of a main sequence star as are most stars. There are two other groups; one contains the rare red giants that are very luminous, and the other contains white dwarfs, which are not very luminous but are hot stars.

Hertzsprung also made many other scientific discoveries. In 1913 he developed the method of distance determination using Cepheid variables, which are basically a class of pulsating stars with very regular light variations. In 1914 he used this method to measure the distance to a large magellanic cloud. He was also the first astronomer to advance the use of the Absolute Magnitude. During his time at Potsdam he developed a technique for observing double stars, using a great refractor which eliminated errors and improved results to the point where they were ten times more accurate than before. By taking a large number of exposures on one plate and measuring them carefully he obtained more accurate results than ever before.

Ejnar Hertzsprung died October 21st, 1967 at the age of 94.