definition |
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.
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