A display of stellar properties using a plot of
→ effective temperature (or instead
→ color or → spectral type)
along the abscissa versus
→ luminosity
(or → absolute magnitude). The temperature is plotted
in the inverse direction, with high temperatures on the left and low temperatures on
the right. On the diagram the majority of stars are concentrated in a diagonal strip running
from upper left to lower right, i.e. from high temperature-high luminosity
→ massive stars to low
temperature-low luminosity → low-mass stars.
This feature is known as the
→ main sequence. This is the locus of stars burning hydrogen in
their cores (→ proton-proton chain).
The lower edge of this strip, known as the
→ zero age main sequence (ZAMS), designates the positions
where stars of different mass first begin to burn hydrogen in their cores. Well below
the main sequence there is a group of stars that, despite
being very hot, are so small that their luminosity is very small as a
consequence. These are the class of → white dwarfs.
These objects represent old and very evolved
stars that have shed their outer layers to reveal a very small but
extremely hot inner core. They are no longer generating energy
but are merely emitting light as they cool
(→ white dwarf cooling track).
Stars with high luminosities but relatively low temperatures occupy a wide region
above the main sequence. The majority of them have used up all
the hydrogen in their cores and have expanded and cooled as a result of internal
readjustment. Called → red giants, they are still
burning helium in their cores (→ helium burning,
→ carbon burning).
There are also stars with very high luminosities, resulting from their
enormous outputs of energy, because they are burning their fuel at a prodigious rate.
These are the → supergiants. They can be hot or cool,
hence blue or red in color. Same as → H-R diagram.
See also:
→ asymptotic giant branch,
→ blue horizontal branch star,
→ extreme horizontal branch star,
→ field horizontal branch star,
→ Hayashi track,
→ horizontal branch,
→ post-asymptotic giant branch star,
→ red giant branch,
→ supra-horizontal branch star,
→ zero age horizontal branch star,
→ Humphreys-Davidson limit.
See also: Named after the Danish Ejnar Hertzsprung (1873-1967) and the American
Henry Norris Russell (1877-1957). However,
the first H-R diagram was published not by Hertzpurung neither Russell,
but by a PhD student of Karl Schwarzschild at Göttingen. The student was
Hans Rosenberg (1879-1940), who in 1910 published the diagram for stars in the
→ Pleiades
(Astronomische Nachrichten, Vol. 186 (4445), p. 71, 1910).
Although Hertzpurung had a very preliminary diagram in
1908, his first proper diagram was published in 1911. Likewise, Russell published his
version only in 1915 with the better and more numerous data then available
(Nielsen, A.V., 1969, Centaurus 9, 219;
Valls-Gabaud, D., 2002, Observed HR diagrams and stellar evolution,
ASP Conf. Proceedings, Vol. 274. Edited by Thibault Lejeune and João Fernandes);
→ diagram.
