Spectral Classes

When you compare the spectra of stars like Polaris or Vega with the Sun's spectrum (Figure 3.8), you see that some look the same while others look quite different. Absorption spectra are used to classify stars into nine principal types, called spectral classes.

Hydrogen lines are much stronger in the spectra of some stars than in the Sun's spectrum. Astronomers once mistakenly thought that these stars had more hydrogen than other stars. They classified stars by the strength of the hydrogen lines in their spectra, in alphabetical order, from the strongest (called Class A) to the weakest (called Class Q).

U.S. astronomer Annie J. Cannon (1863-1941), who examined and classified the spectra of 225,300 stars, modified this classification system to its still-

used form: O B A F G K M L T. (Astronomy students remember this order by saying: "Oh Be A Fine Girl/Guy Kiss Me Love To")

All visible stars are roughly uniform in composition, made mostly of hydrogen and helium. U.S. astronomer Cecilia Payne-Gaposhkin (1900-1979) showed that the differences in the dark line patterns of stars are due primarily to their enormously different surface temperatures.

The sequence of spectral classes is a temperature sequence. The O stars are hottest, with the temperature continuously decreasing down to the coolest T stars. Each spectral class is arranged in 10 subclasses numbered 0 to 9, also in order of decreasing temperature.

Today discoveries necessitate extra classes. L and T were added recently for dwarfs cooler than class M stars.

What property determines the spectral class of a star?_

Answer: Surface temperature.

Figure 3.8. Seven main classes of spectra of visible stars, arranged in order of decreasing temperature. Intensity versus wavelengths of spectral lines shown in nanometers (nm). (He: neutral helium; H: hydrogen; Ca: calcium; Fe: iron; TiO: titanium oxide; Na: sodium; He+: helium ion.)

3.7 TEMPERATURE

The spectrum of a hot star and that of a cool star look very different. Examine Figure 3.8, which displays the spectral classes of visible stars. Each spectral class has key characteristics that serve, like numbers on a thermometer, to indicate a star's temperature.

Today's spectral classes of stars in order from highest to lowest temperatures, the approximate surface temperatures of these classes, and the main class characteristics are summarized in Table 3.1.

You can identify a new star's spectral class and probable temperature by comparing its spectrum to the images in Figure 3.8 and the class characteristics in Table 3.1.

TABLE 3.1 Spectral Class Characteristics

Approximate Spectral Temperature

Class (K) Main Class Characteristics

Approximate Spectral Temperature

Class (K) Main Class Characteristics

O

>30,000

Relatively few lines; lines of ionized helium

B

10,000-30,000

Lines of neutral helium

A

7,500-10,000

Very strong hydrogen lines

F

6,000-7,500

Strong hydrogen lines; ionized calcium lines; many metal lines

G

5,000-6,000

Strong ionized calcium lines; many strong lines of ionized and neutral iron and other metals

K

3,500-5,000

Strong lines of neutral metals

M

2,000-3,500

Bands of titanium oxide molecules

L

1,300-2,000

Bands of iron hydride molecules

T

~700-1,300

Methane and water vapor

Figure 3.9. Star spectrums may be displayed as a graph of intensity versus wavelength or as a picture that looks like an old-fashioned photographic spectrum.

List the spectral class and probable temperature of each of the stars whose spectrum is shown in Figure 3.9. (a)_; (b)_

Answer: (a) A type (7500 K -10,000 K); (b) M type (<3500 K).

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