The Solar System

CHAPTER 7 | ATOMS AND STARLIGHT 131

Temperature has a similar eff ect on the spectral lines of other

elements, but the temperature at which the lines reach their

maximum strength diff ers for each element (Figure 7-7b). If you

add a number of chemical elements to your graph, you will have

a powerful aid for fi nding the stars’ temperatures (Figure 7-7c).

Now you can determine a star’s temperature by comparing

the strengths of its spectral lines with your graph. For instance, if

you found medium-strength Balmer lines and strong helium

lines in a star’s spectrum, you could conclude that it had a tem-

perature of about 20,000 K. But if the star had weak hydrogen

lines and strong lines of ionized iron, you would assign it a tem-

perature of about 5800 K, similar to that of the sun.

Th e spectra of stars cooler than about 3500 K contain dark

bands produced by molecules such as titanium oxide (TiO).

Because of their structure, molecules can absorb photons at

many wavelengths, producing numerous, closely spaced spec-

tral lines that blend together to form bands. Th ese molecular

bands appear in the spectra of only the coolest stars because,

as mentioned before, molecules in cool stars are not subject to

the violent collisions that would break them apart in hotter

stars.

Spectral Classifi cation

You have seen that the strengths of a star’s various spectral lines

depend on its surface temperature. From this you can conclude

that all stars of a given temperature should have similar spectra.

If you learn to recognize the pattern of spectral lines produced by

a 6000 K star, for instance, you need not use Figure 7-7c every

time you see that kind of spectrum. You can save time by classify-

ing stellar spectra rather than analyzing each one individually.

Th e fi rst widely used classifi cation system was devised by

astronomers at Harvard during the 1890s and 1900s. One of the

astronomers, Annie J. Cannon, personally inspected and classi-

fi ed the spectra of over 250,000 stars. Th e spectra were fi rst clas-

sifi ed into groups labeled A through Q, but some groups were

later dropped, merged with others, or reordered. Th e fi nal clas-

sifi cation includes the seven major spectral classes, or types, still

used today: O, B, A, F, G, K, M.*

Th is sequence of spectral types, called the spectral sequence, is

important because it is a temperature sequence. Th e O stars are the

hottest, and the temperature decreases along the sequence to the

M stars, the coolest. For increased precision, astronomers divide

each spectral class into ten subclasses. For example, spectral class A

consists of the subclasses A0, A1, A2,... A8, A9. Next come F0,

F1, F2, and so on. Th is fi ner division gives a star’s temperature to

an accuracy of about 5 percent. Th e sun, for example, is not just a

■ Figure 7-7

The strength of spectral lines can tell you the temperature of a star. (a) Balmer
hydrogen lines alone are not enough because they give two answers. Balmer
lines of a certain strength could be produced by a hotter star or a cooler star.
(b) Adding another atom to the diagram helps, and (c) adding many atoms
and molecules to the diagram creates a precise aid to fi nd the temperatures
of stars.

10,000 6000 4000

Temperature (K)

Line strength

c

10,000 6000 4000

Temperature (K)

Hydrogen

Ionized

helium

Helium

Ionized

calcium

Hydrogen

Hydrogen

High

Low

High

Low

High

Low

Ionized

calcium

Titanium

oxide

Ionized

iron

Line strength

b

10,000 6000 4000

Temperature (K)

Line strength

a

The lines of each atom or

molecule are strongest at

a particular temperature.

Lines of ionized calcium are

strongest at lower temperatures

than the hydrogen Balmer lines.

Hydrogen Balmer lines

are strongest for medium-

temperature stars.

spectra. Th e curve in ■ Figure 7-7a shows the strength of the
Balmer lines for various stellar temperatures. But you can see
from the graph that a star with Balmer lines of a certain strength
might have either of two temperatures, one high and one low.
How do you know which is right? You must examine other spec-
tral lines to choose the correct temperature.

*Generations of astronomy students have remembered the spectral sequence

using the mnemonic “Oh, Be A Fine Girl (Guy), Kiss Me.” More recent

suggestions from students include “Oh Boy, An F Grade Kills Me” and “Only

Bad Astronomers Forget Generally Known Mnemonics.”