Hertzsprung–Russell diagram

The Hertzsprung -Russell diagram, short HRD, was developed in 1913 by Henry Norris Russell and builds on work by Ejnar Hertzsprung. It roughly shows the development of the distribution of the stars. If to the spectral type versus absolute magnitude, resulting in a sufficient number of registrations characteristic line-like clusters.

Characteristic areas

Main sequence

The diagram shows most of the stars in the area of the so-called main sequence (main sequence or Zwergenast ), which differ from the O- stars with an absolute magnitude of about magnitude -6 to the M- stars with an absolute magnitude of Magnitude 9 - 16 drags. The stars of the main sequence are the luminosity class V.

The Sun is a main sequence star of spectral type G2. Other examples of main-sequence stars are Vega (A0) and Sirius (A1).

Giant branch

The stars of a second distinct branch fall into the spectral classes G0 to M and have an absolute brightness of Magnitude 0 In comparison to stars of the main sequence with the same spectral type (and thus the same temperature and surface brightness ) they possess a greater absolute magnitude and thus a greater luminous surface. So you have a larger diameter than main sequence stars, are therefore classified as (normal ) Giant Stars ( Giants ) and form the so-called giant branch and the luminosity class III.

Other areas

Between the main sequence and the giant branch, the rarer subgiant find the luminosity class IV Their diameter is between that of the stars of the main sequence and the giant stars.

In the field of spectral classes A5 to G0, left, above the main sequence, the so-called Hertzsprung gap is ( on an illustration not shown ), an area with few conspicuous stars. It can be explained by the fact that massive stars only need a very short time to develop into giants that rise relatively quickly in the giant branch. Therefore, the range of the Hertzsprung- gap appears to be relatively empty.

In addition to the densely populated main sequence and the giant branch, there are still areas of bright giant (bright giants ) with the luminosity class II and the supergiants ( supergiants ) with luminosity class I. These areas are occupied relatively thin but uniform.

Below the main group contains the divisions of the dwarfs with lower by about 1-3 magnitude, as well as the isolated lying in the region of spectral classes B to G group of white dwarfs with lower by about 8-12 Magnitude than the stars of the main group and a very small diameter.

Interpretation

The concentration of stars on the different groups can be explained from the theory of stellar evolution. The development status of the stars are separated from each other more or less clear and can be found at specific points of the HRD again. Over time changing the two state variables of the temperature and of the effective luminance of a star as a function of the nuclear processes in its interior, so that each star passes through a certain path of development through the HRD. This occurs at a different rate. Development states that last a long time are, accordingly observed more frequently (eg in the main sequence ) as a rapid, short -lasting developmental stages ( eg in the area of the Hertzsprung gap). Beyond effective temperatures of about 3000-5000 Kelvin can be found in the HRD no more stars, because this is the area of ​​the protostars, which have a very high rate of development. This almost perpendicular "line" is called Hayashi - line.

Since the spectral type roughly related to the temperature of the star, the HRD can be viewed as a temperature -luminosity diagram

Instead of the spectral type can also be the color index of the stars to apply, which is also a measure of its temperature. Instead of HRD as the color-magnitude diagram is obtained.