Hertzsprung-Russell diagram (HR diagram)


Hertzsprung-Russell diagram. A schematic diagram showing the main sequence and areas occupied by stars at different stages of evolution. Absolute magnitude is plotted against colour index in this example. The dashed lines link stars of different luminosities with the same spectral types but different colour indices.
A graph displaying, for any sample of stars, the relationship between their spectral type and luminosity (see illustration).
Colour, temperature or some other comparable quantity may be substituted for spectral type as the quantity plotted along the horizontal axis. Temperature conventionally decreases towards the right. Either magnitude or luminosity relative to the Sun are frequently used for the vertical scale. The resulting graph may also be called a colour-magnitude diagram or colour-luminosity diagram according to the actual quantities used.
What is now known as a Hertzsprung-Russell diagram was first plotted by Henry Norris Russell in 1913. It was later recognized that Ejnar Hertzsprung had independently put forward similar ideas at around the same time.
Any star whose spectral type and luminosity are known may be plotted as a single point on the HR diagram, but the diagram acquires particular significance when plotted for a related group of stars, such as a star cluster. For any sample of stars, the points are not distributed randomly: most lie on a band running diagonally from the upper left to the lower right, the so-called main sequence. It arises because the most significant factor determining a star's spectral type and luminosity is its mass - the main sequence is in effect a mass sequence. The idea once held that it is an evolutionary sequence is known to be wrong. Nevertheless, hot stars are still often called "early-type stars" and cooler ones "late-type stars". These misnomers are a legacy from the misinterpretation of the main sequence.
The effects of evolution in fact move stars away from the main sequence, which represents stars burning hydrogen in thermonuclear reactions in their cores. When the central hydrogen is exhausted, a sequence of internal changes leads to a great expansion of the star, coupled with a decrease in surface temperature. Such evolved stars are found in the giant and supergiant branches lying above the main sequence. The highly evolved white dwarfs form a group well below the main sequence.
The HR diagram for a star cluster immediately makes clear how many stars there are at each stage of evolution. This, coupled with the theoretical knowledge of how evolution rate increases with stellar mass, gives an important key to the ages of clusters. Plotting apparent magnitude on the vertical axis rather than absolute magnitude for a cluster provides a method of measuring a cluster's distance.
HR diagrams are also useful for displaying the sequence of changes in colour and luminosity that take place in an individual star in the course of its evolution - before, on and after the main sequence. The result is an evolutionary track.

Herzsprung-Russel diagrams produced by the Hipparcos and Tycho data reproduced by kind permission of ESA.
(MV, B-V) diagram for the 41 704 single stars from the Hipparcos Catalogue with relative distance precision sp /p< 0.2 and sB-V 0.05 mag. Colours indicate the number of stars in a cell of 0.01 mag in B-V and 0.05 mag in MV . (MHp , V-I) diagram for the 41 453 single stars from the Hipparcos Catalogue with relative distance precision sp /p< 0.2 and sV-I 0.05 mag. Colours indicate the number of stars in a cell of 0.01 mag in V-I and 0.05 mag in MHp.

See also: stellar evolution.