The Hertzsprung-Russell diagram is a chart that shows the relationship between a star's luminosity or absolute magnitude versus its spectral type, stellar classification, and effective temperature. It plots each star on a graph of magnitude against temperature and color. Most stars occupy the main sequence area of the diagram where they are fusing hydrogen. The diagram can also be used to measure the approximate distance between a star cluster and Earth by comparing apparent and absolute magnitudes.

1. In astronomy the Hertzprung-Russell diagram is a
distribution chart showing the relationship
between the absolute magnitude or luminosity
versus spectral type or stellar classification and
effective temperature.
The Hertzsprung-Russell diagrams are not
pictures or maps showing the location of stars.
Instead, they put each star a graph showing the
magnitude (or brightness) against its temperature
and color.
Hertzsprung-Russell Diagram

2. Hertzsprung-Russell Diagram
HR Diagram
adapted from
Powell. Shown
are 22,000 stars
of the catalog of
Hipparchus and
1,000 of the
Gliese Catalogue
of nearby stars.

3. Most stars occupies the area of the diagram
along line called the main sequence. During
this stage stars are fusing hydrogen nuclei.
The concentration of stars is in the next
horizontal branch (helium fusion and burning
hydrogen in the layer covering the core).
Hertzsprung-Russell Diagram

4. Hertzsprung-Russell Diagram
HR Diagram of
two open
clusters M6
and NGC188,
showing off
the main
sequence at
different ages.

5. Another important area is the failure of
Hertzsprung located in the area between A5 and
G0 spectral types and between +1 and -3
absolute magnitudes (i.e., between the top of the
main sequence and giant the horizontal branch),
where the density of stars is smaller.
The RR Lyrae variable stars are found to the left of
this failure. The variables of Cepheids located in
the upper section of the instability strip (one
almost vertical region of the diagram occupied by
pulsating variable stars).
Hertzsprung-Russell Diagram

6. Hertzsprung-Russell Diagram

7. The sun is on the main sequence, the magnitude 1.
The HR diagram can also be used by scientists to
measure approximately the distance between a star
cluster and earth. This can be done by comparing the
apparent magnitudes of the stars of the cluster with
the absolute magnitudes of stars with known distances
(stars or model).
The group observed is then moved in vertical direction
until the two major sequences coincide. The difference
in magnitude that was covered to match the two
groups is called the distance modulus and is a direct
measure for the distance. This technique is known as
sequence matching primary or spectroscopic parallax.

8. Hertzsprung-Russell Diagram