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Spectroscopic Atlas for Amateur Astronomers 41
10 Spectral Class B
10.1 Overview
Below the extreme O-category seamlessly follows the B-class. Several of the bright Orion
stars are early B0-types and differ therefore from the above presented late O9.5 stars, only
by small nuances within the spectral profiles. These blue-white luminous B-stars are less
massive and hot and therefore living much longer. Further they are much more numerous,
dominating a considerable part of the brighter constellations. Here some examples: all
bright members of the Pleiades (see sect. 27.6), all the bright Orion stars except Alnitak,
Mintaka and Betelgeuse, all bright stars in the head of the Scorpion, except the reddish
Antares. Further mentionable are Regulus, Spica, Alpheratz, and the weaker blue compo-
nent of the well known double star Albireo B (B8V). Even the famous, unstable giant P
Cygni, as well as most of the Be- and Be-Shell stars (Tables 14 – 15) are members of the B-
class.
10.2 Parameters of the Early to Late B-Class Stars
The following table shows the data, exclusively valid for the Main Sequence Stars of the
B-class, compared to the Sun ( ) and according to [701] and other sources (*). Here, the
enormous spread becomes evident, covered by this class from the B0–B9 types. Particu-
larly impressive is the difference in the luminosity – that is about a factor of 550! The huge
luminosity is one of the reasons, why the some 1000 ly distant, mostly early B-class stars in
the Orion constellation, are able to show such a great brightness. Within this class, in the
range of about 8–10 solar masses, it’s decided whether single stars explode as a SN or end
up as White Dwarfs.
Mass Stay on Main Temperature Radius Luminosity L/L
M/M Sequence [y] photosphere [K] R/R
18 – 3 10M – 400M 25,000 (*) – 10,500 8.4 – 3.0 52,000 – 95
10.3 Spectral Characteristics of the B-Class
This class is characterised by the absorption lines of neutral helium, He I, reaching their
maximum intensity at about class B2 and weakening downwards the subclasses to B9. Fur-
ther dominating are spectral lines of singly ionised metals O II, Si II, Mg II. Towards later
subclasses, the Fraunhofer K line of Ca II becomes faintly visible and the H-Balmer series
gets significantly stronger. Due to lower temperatures and thereby decreasing degree of
ionisation the simply ionised He II is only visible in the top B0 subclass, but limited here to
the Main Sequence Stars of the luminosity class V. Absorption lines of higher ionised silicon
Si III and Si IV appear until down to type B2.
Higly abundant in the B–class are so called "Fast Rotators". Such stars with high rotation
speed (about 150–400 km/s) influence the appearance of the spectrum significantly. For
details, see the comments to the tables below.
The maximum intensity of the real continuum is still in the UV range, but with a significantly
higher share in the visible spectrum. The following graph shows the theoretical continuum
for a B6 IV standard star (Vspec Tools/Library). Marked with a red arrow is here the small
kink of the emerging but still very weak Fraunhofer K-line of singly ionised calcium Ca II.
