The Solar System

134 PART 2^ |^ THE STARS

Th e Balmer lines are strongest in A stars, where the temperature

is moderate but still high enough to excite the electrons in hydro-

gen atoms to the second energy level, where they can absorb

Balmer wavelength photons. In the hotter stars (O and B), the

Balmer lines are weaker because the higher temperature excites

the electrons to energy levels above the second or ionizes the

atoms. Th e Balmer lines in cooler stars (F through M) are also

G star, but a G2 star. ■ Table 7-1 breaks down some of the infor-
mation contained in Figure 7-7c and presents it in tabular form
according to spectral class. For example, if a star has weak Balmer
lines and lines of ionized helium, it must be an O star.
Th irteen stellar spectra are arranged in ■ Figure 7-8 from the
hottest at the top to the coolest at the bottom. You can easily
see how the strength of spectral lines depends on temperature.

G star, but a G2 star.■ Table 7-1 breaks down some of the infor- Th e Balmer lines are strongest in A stars, where the temperature

■ Table 7-1 ❙ Spectral Classes

Spectral Approximate Hydrogen Other Spectral Naked-Eye

Class Temperature (K) Balmer Lines Features Example

O 40,000 Weak Ionized helium Meissa (O8)

B 20,000 Medium Neutral helium Achernar (B3)

A 10,000 Strong Ionized calcium weak Sirius (A1)

F 7500 Medium Ionized calcium weak Canopus (F0)

G 5500 Weak Ionized calcium medium Sun (G2)

K 4500 Very weak Ionized calcium strong Arcturus (K2)

M 3000 Very weak TiO strong Betelgeuse (M2)

■ Figure 7-8

These spectra show stars ranging from hot O stars at the top to cool M stars at the bottom. The Balmer lines of hydrogen are strongest at spectral type A0,
but the two closely spaced lines of sodium in the yellow are strongest for very cool stars. Helium lines appear only in the spectra of the hottest stars. Notice
that the helium line visible in the top spectrum has nearly but not exactly the same wavelength as the sodium lines visible in cooler stars. Bands produced
by the molecule titanium oxide are strong in the spectra of the coolest stars. (AURA/NOAO/NSF)

06.5

B0

B6

A1

A5

F0

F5

G0

G5

K0

K5

M0

M5

39,000 K

3200 K

400 nm 500 nm

Wavelength (nm)

Temperature

600 nm 700 nm

Hδ Hγ He Hβ He Hα

TiO TiO TiO Sodium TiO TiO TiO