A scatter graph of stars showing the relationship between each star’s absolute magnitude and its estimated surface temperature, or between optical and perceptual proxies for these quantities.
Absolute Magnitude and Temperature Scales
The absolute magnitude and temperature can be described as follows:
A star’s absolute magnitude is the attenuation (in factors 10−0.4) of the star Vega’s power (as received at 10 pc viewing distance [32.6 light-years]) to equal that of the star (also corrected to 10 pc). The convention that dimmer stars have higher magnitude is a historical precedent that dates from Hipparchus (c. 190 BCE–c. 120 BCE), whose system of stellar magnitudes was based on visual assessment.
A star’s surface temperature is estimated in one of three ways: by the observed color (an old way), by a comparison of two sensor outputs such as blue and violet (a newer way), or by a model prediction of the temperature of a black-body radiator with the same radiation power per unit star-surface area (the most modern way). The third way requires independent inference of the star’s radius but assumes the star has an emissivity of 1. To acknowledge the lack of compensation for the true emissivity, the temperature on an H-R diagram is called “effective temperature.”
Here are the coordinate conventions of the H-R diagram: Temperature increases from right to left, and magnitude (i.e., dimness) increases from bottom to top. Hence dim red stars (red giants) appear near the upper right of the diagram, and bluish bright stars (such as white dwarfs) appear in the lower left. A long cluster of stars called the main sequence extends from the upper left to the lower right. Higher mass stars occur at the upper left of this sequence, and the Sun appears approximately in the middle. The main sequence is composed of stars that are currently dominated by hydrogen that is fusing into helium. According to currently accepted theory of stellar evolution, such stars will eventually migrate either to the red giants or white dwarf domain of the H-R diagram.
H-R diagrams are often depicted in color, either pseudocolor with a thermal code to show the temperature or coded according to star categories such as cluster membership.
The H-R diagram was originated by Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell (https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram). Originally the diagram was based on visual estimation of magnitude and color, and it was a research tool to help characterize stellar evolution before the mechanism of nuclear fusion was understood. After about the 1930s, the H-R diagram became based on objective measurements but was used less as a research tool and more as a way to illustrate the theoretically predicted evolution of stars through trajectories in the diagram.
- 1.Gribbin, J.: Companion to the Cosmos. Little, Brown, NY (1996)Google Scholar
- 2.Friedman H.: The Astronomer’s Universe: Stars, Galaxies, and Cosmos. W. W. Norton, NY (1990)Google Scholar
- 3.Moore, P.: Atlas of the Universe. Cambridge U. Press, NY (1998)Google Scholar
- 4.Pasachoff, J. M.: A Brief View of Astronomy. Saunders College Publ., NY (1986)Google Scholar
- 5.Ronin C. A.: The Natural History of the Universe: From the Big Bang to the End of Time. MacMillan, NY (1991)Google Scholar
- 6.Shapley, A. E.: Physical Properties of Galaxies from z = 2 – 4. http://arxiv.org/abs/1107.5060v2, accessed 26 Feb. 2013Google Scholar
- 7.Spence, P. (ed.): The Universe Revealed. Cambridge U. Press, NY (1998)Google Scholar