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classboundaries have been noted along the abscissa in
Figure 13-2. We see that the U-B value separates the
supergiants from the main sequence stars rather well in
the spectral range F0 through G5, but that confusion
exists for the K and M stars. Giant stars lie on a curve
intermediate to the two curves shown, and the confusion
of these with main sequence stars is correspondingly
worse.
U-B
1.2 t I T T
•8 REDDENING LINE
.8
i
1.7'#"
SUPERGIANTS
,\
\\O
AXIS
is simply its distance above or below the Q-axis and is
independent of the amount of reddening. On a plot of Q
versus B-V, reddening would produce a horizontal shift
in the star's position, but the confusion of spectral and
luminosity classes produced by reddening would be just
as great as on the standard two-color diagram.
We see that reddening will shift F0 and later type
supergiants along the existing locus and will have little
effect on the total confusion in the K region. Type A
supergiants will be shifted into the F and G part of the
main sequence curve, but these stars are very rare. More
serious is the reddening of B3 through B8 main sequence
stars into the F0 through K0 part of the main sequence
curve.
It is of interest to see if adding a fourth wavelength in
the red (R) or infrared (/) would enable the amount of
reddening to be determined and thus eliminate reddened
stars masquerading as F through K main sequence stars.
Interstellar absorption is usually assumed to be proportional
to l/h. Figure 13-3 compares the curves of
black-body radiation from a 6000 ° K (GO) star with the
black-body radiation from a 10,000 ° K source that has
had enough 1]_ absorption to give it the same B-V value.
We see that the difference between the curves is slight,
amounting to less than 0.1 magnitude in the lp region.
On this basis, the addition of a fourth wavelength would
hardly seem worthwhile. On the other hand, work by
1.6
2.0
-.4
BO B3 BB AO FO GO KD K5 WO
i[ AIL L J 1 , I
0 .4 .8 1.2 1.6
B-V
Figure 13-2. Two-color relation for main sequence
stars and supergiants.
A further source of confusion arises from interstellar
IZ
°i
Q:
! \\ _ ,o,ooo° K
# '_", fix REDDENED
1 4
absorption, which increases with decreasing wavelength
and thus reddens the light of distant stars seen in the
galactic plane. The reddening decreases both U-B and
B-V, the former by about 0.72 times as much as the
latter, and thus shifts the positions of stars on the
two-color diagram along a line having a slope of-0.72 as
shown by the "reddening line" in Figure 13-2. The
"Q-axis," also shown in the figure, is a line of slope
-0.72 drawn through the point U - B = 0, B - V = 0.
The Q-value of a star, defined as
Q = (U- B)- 0.72(B- V) (4)
I I I0
WAVELENGTH,
microns
Figurel3-3. Effect of reddening on black-body
radiation.
Johnson (ref. 2) seems to indicate that the absorption
falls more rapidly than 1/_. beyond 0.6p. According to
his published curves, the reddened 10,000 ° K radiation
158