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Ultraviolet Visible Infrared TABLE 14.2

Major stellar spectral types and temperatures
Type Color Temperature (K) Comment
Intensity of radiation 6,000 K helium and hydrogen but
10,000 K
O
Bluish
30,000–80,000 Spectrum with ionized
little else; short-lived and
rare stars

helium, none ionized

3,000 K B Bluish 10,000–30,000 Spectrum with neutral
A Bluish 7,500–10,000 Spectrum with no helium,
strongest hydrogen, some
magnesium and calcium
F White 6,000–7,500 Spectrum with ionized
2,000 4,000 7,000 10,000 12,000 14,000 calcium, magnesium,
neutral atoms of iron
Wavelength (angstroms)
G Yellow 5,000–6,000 The spectral type of the
sun. Spectrum shows
FIGURE 14.7 The distribution of radiant energy emitted is
67 elements in the sun
different for stars with different surface temperatures. Note that the
peak radiation of a cooler star is more toward the red part of the K Orange-red 3,500–5,000 Spectrum packed with
spectrum, and the peak radiation of a hotter star is more toward the lines from neutral metals
blue part of the spectrum. M Reddish 2,000–3,500 Band spectrum of molecules,
e.g., titanium oxide; other
related spectral types
(R, N, and S) are based on
color, and still others have a yellowish color. This color difference other molecules present in
is understood to be a result of the relationship that exists between each spectral type
the color and the temperature of an incandescent object. The col-
ors of the various stars are a result of the temperatures of the stars.
You see a cooler star as reddish and comparatively hotter stars SOLUTION 7
___
as bluish white. Stars with in-between temperatures, such as the λ Peak = 3,500 T = 2.897 × 1 0 K∙angstroms

Sun, appear to have a yellowish color (Figure 14.7). λ Peak
7
Astronomers analyze starlight to measure the tempera- angstroms T = ? ___
2.897 × 1 0 K∙angstroms

ture and luminosity as well as the chemical composition of a = 3500 angstroms
star. When the starlight is analyzed in a spectroscope, specific _ 7 K∙angstroms
2.897 × 1 0 __

elements can be identified from the unique set of spectral lines = 3500
angstroms
that each element emits. Temperature and spectra are used as
3
the basis for a star classification scheme. Originally, the clas- = 8.3 × 1 0 K
sification scheme was based on 16 categories according to the
strength of the hydrogen line spectra. The groups were identi-
fied alphabetically with A for the group with the strongest hy- EXAMPLE 14.5
drogen line spectrum, B for slightly weaker lines, and on to the A star emits light with a peak wavelength of 5050 angstroms. What
last group with the faintest lines. Later, astronomers realized type of star is this and what is its color?
that the star temperature was the important variable, so they (Answer: Type G, yellow)
rearranged the categories according to decreasing temperatures.
The original letter categories were retained, however, resulting
in classes of stars with the hottest temperature first and the cool- CONCEPTS Applied
est last with the sequence O B A F G K M. Table 14.2 compares
the color, temperature ranges, and other features of the stellar Seeing Spectra
spectra classification scheme.
Inexpensive diffraction grating on plastic film is available
from many scientific materials supply houses. If available,
EXAMPLE 14.4 view the light from gas discharge tubes to see bright line
spectra. Use the grating to examine light from different
The temperature of a star in Kelvins can be determined from
wattage incandescent lightbulbs, fluorescent lights, lighted
the peak wavelength of the light emitted by a relationship called
“neon” signs of different colors, and streetlights. Describe
Wien’s displacement law. The temperature is equal to a constant of
7
2.897 × 10 K∙angstroms divided by the peak wavelength in ang- the type of spectrum each produces. If it has lines, see if
you can identify the elements present.
stroms. What is the temperature of a star emitting light with a peak
wavelength of 3,500 angstroms?
14-7 CHAPTER 14 The Universe 357