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The 'Hertzsprung-Russell diagram' (usually referred to by the abbreviation 'H-R diagram' or 'HRD', also known as a 'Colour-Magnitude diagram', or 'CMD') shows the relationship between absolute magnitude, luminosity, classification, and effective temperature of stars. The diagram was created circa 1910 by Ejnar Hertzsprung and Henry Norris Russell, and represented a huge leap forward in understanding stellar evolution, or the 'lives of stars'.

Contents

Diagram

Interpretation

See also

External links

Diagram

Interpretation

There are several forms of the Hertzsprung-Russell diagram, and the nomenclature is not very well defined.
The original diagram displayed the spectral type of stars on the horizontal axis and the absolute magnitude on the vertical axis. The first quantity (i.e. spectral type) is difficult to determine unambiguously and is therefore often replaced by the colour index of the stars. This type of diagram is called a Hertzsprung-Russell diagram, or colour-magnitude diagram, and it is often used by observers. However, colour-magnitude diagram is also used in some cases to describe a plot with the vertical axis depicting the apparent, rather than the absolute, magnitude.
Another form of the diagram plots the effective temperature of the star on one axis and the luminosity of the star on the other. This is what theoreticians calculate using computer models that describe the evolution of stars. This type of diagram should probably be called ''temperature-luminosity diagram'', but this term is hardly ever used, the term ''Hertzsprung-Russell diagram'' being preferred instead. Despite some confusion regarding the nomenclature, astrophysicists make a strict distinction between these types of diagrams.
The reason for this distinction is that the exact transformation from one to the other is not trivial, and depends on the stellar-atmosphere model being used and its parameters (like composition and pressure, apart from temperature and luminosity). Also, one needs to know the distance to the observed objects and the interstellar reddening. Empirical transformation between various colour indices and effective temperature are available in literature. (Sekiguchi 2000, Casagrande 2006)
The H-R diagram is used to define different types of stars and to match theoretical predictions of stellar evolution using computer models with observations of actual stars. It is then necessary to convert either the calculated quantities to observables, or the other way around, thus introducing an extra uncertainty.
Most of the stars occupy the region in the diagram along the line called main sequence. During that stage stars are fusing hydrogen in their cores. The next concentration of stars is on the horizontal branch (helium fusion in the core and hydrogen burning in a shell surrounding the core). Another prominent feature is the Hertzsprung gap located in the region between A5 and G0 spectral type and between +1 and −3 absolute magnitudes (''i.e.'' between the top of the main sequence and the giants in the horizontal branch). RR Lyrae stars can be found in the left of this gap. In the upper section of the instability strip Cepheid variables are residing.
The H-R diagram is also used by scientists to roughly measure how far away a star cluster is from Earth. This can be done by comparing the apparent magnitudes of the stars in the cluster to the absolute magnitudes of stars with known distances (or of model stars). The observed group is then shifted in the vertical direction, until the two main sequences overlap. The difference in magnitude that was bridged in order to match the two groups is called the distance modulus and is a direct measure for the distance. This technique is known as ''main-sequence fitting'', or, confusingly, as the ''spectroscopic parallax''.

See also

★ Asymptotic Giant Branch

★ Hayashi track

★ Henyey track

★ Hess diagram

★ Red clump

★ Stellar birthline

★ Stellar classification

★ Tip of the Red Giant Branch

External links

★ A Study of the B-V Color-Temperature Relation, , Maki, Sekiguchi, The Astronomical Journal, 2000

★ Accurate fundamental parameters for lower main-sequence stars, , L., Casagrande, MNRAS, 2006

★ JavaHRD an interactive Hertzsprung-Russell diagram as a Java applet