Download Main Sequence Stars

Main Sequence Stars
The rst classication of stars was suggested by Einar Hertzsprung in
Denmark, and Henry Norris Russell at Princeton University, around 1913.
They plotted the locations of stars on a graph with the horizontal coordinate
being spectral type (equivalent to temperature) and the vertical coordinate
being absolute magnitude (equivalent to luminosity). The result, called the
Hertzsprung-Russell diagram, or H-R diagram, is widely used in modern astrophysics.
The location of most (about 90%) of the stars are clustered in a relatively
thin curved band that stretches from the upper left to the lower right. The
band is called the main sequence.
The luminosity-mass and radius-mass relations of the main sequence stars
(obtained for binary systems) can be approximated as simple power laws
L
L
R
R
where L,
spectively.
M
=
M
M
=
M
M
!3 5
:
;
(1)
;
(2)
!0 8
:
and R are the luminosity, mass and radius of the Sun re-
The central temperature of a main sequence star is approximately
Tc
1
0
M=M
A = 1:6 107K
= 1:6 107K @
R=R
M
M
!0 2
:
(3)
:
that implies that the central temperature of a main sequence star is extremely
insensitive to the star's mass. For example, a blue giant star with mass
7
M = 100M will have T = 4 10 K, while a red dwarf with mass M = 0:1M
will have T c = 1 107 K. While M ranges over a factor of 1000, T ranges
over a factor less than 4!
c
c
August 24, 1999
1