THE STARS
STELLAR SPECTRA
Type Spectrum Surface temperature, °C Example
W Many bright lines. Divided into WN up to 80,000 ÁVelorum (WC7)
(nitrogen sequence) and WC (carbon
sequence). Rare.
O Both bright and dark lines. Rare. 40,000–35,000 ̇Orionis (09.5)
B Bluish-white. Prominent lines due 25,000–12,000 Spica, ‚Crucis
to helium.
AWhite. Prominent hydrogen lines. 10,000–8000 Sirius, Vega
F White or very slightly yellowish. 7500–6000 Canopus, Polaris
Calcium lines very prominent.
G Yellowish; weaker hydrogen lines, Giants 5500–4200
many metallic lines. Dwarfs 6000–5000 Capella, Sun
K Orange. Strong metallic lines. Giants 4000–3000 Arcturus, Aldebaran
Dwarfs 5000–4000 ÂEridani, ÙCeti
M Orange-red. Complicated spectra, Giants 3400 Betelgeux, Antares
with many bands due to molecules. Dwarfs 3000 Proxima Centauri
R Reddish. 2600 T Lyrae
N Reddish; strong carbon lines. 2500 R Leporis
S Red; prominent bands of titanium oxide 2600 ̄Cygni, R Cygni
and zirconium oxide.
L and T Very cool red dwarfs (‘brown dwarfs’) 2000▲ HR Diagram. In this
version of a typical HR
diagram, the stars are
plotted according to their
spectral types and surface
temperatures (horizontal
axis, x) and their luminosities
in terms of the Sun (vertical
axis, y). The Main Sequence
is obvious at first glance,
from the hot and powerful
W and O stars (1), through
to the dim red dwarfs oftype M (8). Also shown are
the supergiants and giants
(2, 3); Cepheid variables (4);
RR Lyrae variables (5);
subgiants (6); subdwarfs
(7); and white dwarfs (9).
Originally it was believed
that a star began its career
as a large, cool red giant,
and then heated up to join
the Main Sequence; it then
cooled and shrank as it
passed down the MainSequence from top left to
bottom right. This theory
has been found to be
completely wrong. The red
giants are at an advanced
stage in their evolution.
1 Rigel: type B8. A very
massive and luminous star,
at the upper end of the Main
Sequence. It is 60,000 times
as luminous as the Sun, and
is white, with a temperature
of more than 12,000°C.2 Betelgeux: type M. A red
supergiant, 15,000 times
as luminous as the Sun,
with a greater diameter
than that of the Earth’s
orbit. It is surrounded
by a very tenuous ‘shell’
of potassium.
3 Aldebaran:type K. A giant
star, orange in colour, not
as large as Betelgeux,
although it is 100 times as
powerful as the Sun andhas a diameter believed to
be at least 50 million km
(30 million miles).
4 The Sun:type G2. A typical
Main Sequence star. It is
officially ranked as a dwarf,
while Capella, also of type G
(G8), is a giant. (Capella
is not a single star, but a
close binary.)
5 Sirius B: a white dwarf
which has used up all
its nuclear ‘fuel’, and has
no reserves left. It is of
planetary size, but is
amazingly dense, and is as
massive as the Sun.
6 Wolf 339: type M. A dim
red dwarf, with a surface
temperature of 3000°C but
a luminosity only 0.00002
that of the Sun. Yet its
spectral type is the same
as that of Betelgeux.6
5
4
3
2
1
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