Ejnar Hertzsprung introduced the concept of absolute magnitude, the intrinsic brightness of a star. He worked out the relationship between a star's brightness and its color (which indicates its surface temperature) at different stages in its evolution. The American astronomer Henry Russell (1877-1957) independently arrived at this relationship a few years later, and was the first to publish it in the form of a diagram. Hertzsprung-Russell diagrams, as they came to be called, are still among the standard tools of astronomy. They are used to provide insights into the changes in individual stars over time, as well as to characterize populations of stars.
Hertzsprung was born in Frederiksberg, Denmark, near Copenhagen, on October 8, 1873. He was trained as a chemical engineer at the Frederiksberg Polytechnic, and spent a few years working in Russia. He came to astronomy indirectly, through his interest in photography. In 1902 he began making photographic measurements of starlight at the Copenhagen Observatory. He published two papers, one in 1905 and the other two years later, in which he described the relationship between the color of stars and their absolute magnitude.
When we look up at the sky and notice how bright a star is, we are thinking of the apparent magnitude; that is, the brightness as seen from Earth. Naturally, this is dependent upon how far away it is. Hertzsprung defined absolute magnitude as the brightness of a star at the distance of 10 parsecs (32.6 light years), without regard to how it appears from our vantage point. His work provided a means to approximate the absolute magnitude of a star by virtue of its color. Comparing the absolute and apparent magnitudes enabled astronomers to estimate the star's distance.
This important accomplishment led to Hertzsprung's recognition as an astronomer. In 1909 Karl Schwarzschild (1873-1916), director of the Potsdam Observatory, offered him a position as senior astronomer there. While at Potsdam, Hertzsprung studied Cepheid variable stars. These stars are another important distance gauge, because their period is related to their absolute magnitude.
The chart now known as the Hertzsprung-Russell diagram was first published in 1913. It shows temperatures increasing from right to left, and brightness increasing from bottom to top. The main sequence of stars extends diagonally from the hottest, brightest, shortest-lived stars in the upper left to the dim, cool, long-lived stars in the lower right. Our Sun is an "average" star, around the middle of the main sequence.
Stars "fall off" the main sequence in the later stages of their life cycle. They might be found with the large, bright, cool red giants in the upper right, or the small, dim, hot white dwarfs in the lower left. Plotting a diagram for an entire population of stars, such as a globular cluster, gives an indication of the population's age. For example, if it is fairly old, the upper left-hand corner will be empty, because the hottest stars burn themselves out relatively quickly.
Hertzsprung became assistant director of the Leiden Observatory in the Netherlands in 1919, and published a catalog of color measurements for almost 750 stars in 1922. He was appointed director of the observatory in 1935. After he retired in 1945, he returned to Denmark. Free of administrative responsibilities, he continued his astronomical research until 1966, when he was 93 years old. He died on October 21, 1967.
SHERRI CHASIN CALVO