Everything about H-r Diagram totally explained
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'.
Diagram
Forms of diagram
There are several forms of the Hertzsprung-Russell diagram, and the nomenclature isn't 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 (for example spectral type) is difficult to determine unambiguously and is therefore often replaced by the
B-V colour index of the stars. This type of diagram is called a Hertzsprung-Russell diagram, or colour-magnitude diagram, and it's 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 isn't 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 can be 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.
Interpretation
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 (
for example 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.
Cepheid variables reside in the upper section of the
instability strip.
The H-R diagram can also be 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.
The diagram's role in the development of stellar physics
Contemplation of the diagram led astronomers to speculate that it might demonstrate stellar evolution, a main suggestion being that stars collapsed from red giants to dwarf stars, then moving down along the line of the main sequence in the course of their lifetimes. However, following Russell's presentation of the diagram to a meeting of the
Royal Astronomical Society in 1912,
Arthur Eddington was inspired to use it as a basis for developing ideas on
stellar physics (Porter, 2003). In 1926, in his book
The Internal Constitution of the Stars he explained the physics of how stars fit on the diagram. This was a particularly remarkable development since at that time the major problem of stellar theory, the source of a star's energy, was still unsolved.
Thermonuclear energy, and even that stars are largely composed of
hydrogen, had yet to be discovered. Eddington managed to sidestep this problem by concentrating on the
thermodynamics of
radiative transport of energy in stellar interiors (Smith, 1995). So, Eddington predicted that dwarf stars remain in an essentially static position on the main sequence for most of their lives. In the 1930's and 1940's, with a understanding of hydrogen fusion, came a physically-based theory of evolution to red giants, and white dwarfs. By this time, study of the Hertzsprung-Russell diagram didn't drive such developments but merely allowed stellar evolution to be presented graphically.
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