Project Cyclops, A Design... - Department of Earth and Planetary ...

MAIN SEQUENCE SPECTRAL CLASS
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MASS, SOIOrmosses
Figure 2-4. The mass-luminosity relation.
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SURFACE TEMPERATURE, °KxIO 3
Luminosity Function. This is the generic term applied to
the distribution of number of stars per unit volume of
space versus their luminosity or absolute magnitude. The
observed distribution is shown in Figure 2-5. At the high
Figure 2-3. The Hertzsprung-Russell (HR ) diagram.
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The significant feature of the HR diagram is that
almost all stars fall into the four shaded areas shown.
This grouping indicates a restricted set of relationships
between luminosity, temperature, and radiusrelationships
that are now understood to represent
different epochs in the life of a star. The main sequence
accounts for _bout 90 percent of all stars, giants and
supergiants [or about 1 percent, and white dwarfs for
about 9 percent. (see Appendix A).
Mass-Luminosity Relation. The masses of binary stars
can be determined by applying Newton's derivation of
Kepler's Third Law relating mass, mean separation, and
orbital period. If the luminosities of the stars are plotted
against their masses, determined in this way, the
distribution shown in Figure 2-4 is found for main
sequence stars. The stars shown in this plot have the
usual spread of chemical composition associated with
different ages and populations. Nevertheless, the points
do not depart very far from the dashed line. This
indicates that the luminosity of a main sequence star is
determined primarily by its mass. Although the true
relation is more complicated, we can approximate it
quite well over most of the range by the simple
expression:
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ABSOLUTE MAGNITUDE
Figure 2-5. The luminosity ]im('ti(m