CHAPTER 7 | ATOMS AND STARLIGHT 135
lines superimposed. Th e wavelength of maximum intensity is in
the infrared for the coolest stars and in the ultraviolet for the hot-
test stars. Look carefully at these graphs, and you can see that
helium is visible only in the spectra of the hottest classes and tita-
nium oxide bands only in the coolest. Two lines of ionized cal-
cium increase in strength from A to K and then decrease from K
through M. Because the strengths of these spectral lines depend
on temperature, it requires only a few moments to study a star’s
spectrum and determine its temperature.
Now you can learn something new
about your Favorite Stars. Sirius, brilliant in
the winter sky, is an A1 star; and Vega, bright
overhead in the summer sky, is an A0 star.
Th ey have nearly the same temperature and
color, and both have strong Balmer lines in
their spectra. Th e bright red star in Orion is
Betelgeuse, a cool M2 star, but blue-white
Rigel is a hot B8 star. Polaris, the North Star,
is an F8 star a bit hotter than our sun, and
Alpha Centauri, the closest star to the sun, is
a G2 star just like the sun.
Th e study of spectral types is a century
old, but astronomers continue to discover
new types. Th e L dwarfs, found in 1998, are
cooler and fainter than M stars, and are
understood to be objects smaller than stars
but larger than planets called “brown dwarfs”
that you will learn about in a later chapter.
Th e spectra of M stars contain bands pro-
duced by metal oxides such as titanium
oxide (TiO), but L dwarf spectra contain
bands produced by molecules such as iron
hydride (FeH). Th e T dwarfs, discovered in
2000, are an even cooler and fainter type of
brown dwarf than L dwarfs. Th eir spectra
show absorption by methane (CH 4 ) and
water vapor (■ Figure 7-10). Th e develop-
ment of giant telescopes and highly sensitive
infrared cameras and spectrographs is allow-
ing astronomers to fi nd and study these cool
objects.weaker but for a diff erent reason. Th e lower temperature cannot
excite many electrons to the second energy level, so few hydrogen
atoms are capable of absorbing Balmer wavelength photons.
Although these spectra are attractive, astronomers rarely
work with spectra as color images. Rather, they display spectra as
graphs of intensity versus wavelength that show dark absorption
lines as dips in the graph (■ Figure 7-9). Such graphs allow more
detailed analysis than photographs. Notice, for example, that the
overall curves are segments of blackbody curves with spectral
■ Figure 7-9
Modern digital spectra are often represented as
graphs of intensity versus wavelength with dark abs-
orption lines appearing as sharp dips in the curves.
The hottest stars are at the top and the coolest at
the bottom. Hydrogen Balmer lines are strongest at
about A0, while lines of ionized calcium (Ca II) are
strong in K stars. Titanium oxide (TiO) bands are
strongest in the coolest stars. Compare these spectra
with Figures 7-7c and 7-8. (Courtesy NOAO, G. Jacoby,
D. Hunter, and C. Christian)CaΙΙTiO TiO TiO TiOM0
K0
G1
A1
O5
F0
B0
M5
UV Yellow RedHδ
HγHβHeSodiumHα400 500
Wavelength (nm)600 700IntensityBlue