Kaler, James B. Department of Astronomy, College of Liberal Arts and Sciences, University of Illinois, Urbana-Champaign, Illinois.
- Classic HR diagram
- Color-magnitude diagrams
- Calibration and spectroscopic distances
- Third dimension
- Variation on the HR diagram
- Links to Primary Literature
- Additional Readings
In its original and classic form, a plot of stellar luminosity, expressed through absolute visual magnitude, against temperature, as given by the Harvard spectral class (OBAFGKM). The Hertzsprung-Russell (HR) diagram, developed separately by the Danish astronomer Ejnar Hertzsprung (1873–1967) and Princeton University's Henry Norris Russell (1877–1957) between 1911 and 1913, is the fundamental tool of stellar astronomy through which it is possible to understand and follow the evolution of stars from birth to death. There are many modern variants that can be divided into two broad forms, the observational and the theoretical. Observational Hertzsprung-Russell diagrams commonly use color as a quantitative substitute for spectral class, while the theoretical version plots the logarithm of total luminosity (L) against the logarithm of temperature (T), and hence is known as the log L–log T plane. Transformation between the two forms requires the difficult calibration of spectral class or color against stellar temperature as well as the relation between whatever magnitude system is being used and the total luminosity, which also involves temperature. The observational forms clearly require stellar distance. See also: Magnitude (astronomy); Spectral type
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