130 PART 2^ |^ THE STARS
wavelengths as the photons absorbed by hydrogen atoms in
the gases of a star.
Most modern astronomy books display spectra as graphs of
intensity versus wavelength. Be sure you recognize the con-
nection between the dark absorption lines and the dips in
the graphed spectrum.Whatever kind of celestial object’s spectrum astronomers look at,
the most common spectral lines are the Balmer lines of hydro-
gen. In the next section, you will see how Balmer lines can tell
you the temperature of a star’s surface.The Balmer Thermometer
You can use the Balmer absorption lines as a thermometer to fi nd
the temperatures of stars. From the discussion of blackbody
radiation, you already know how to estimate temperature using
color, but the Balmer lines give you much greater accuracy.
Recall that astronomers use the Kelvin temperature scale
when referring to stellar temperatures. Th ese temperatures range
from about 40,000 K to about 2500 K and refer to the tempera-
ture of the star’s surface. Compare these extremes with the sur-
face temperature of the sun, about 5800 K. Th e centers of stars
are much hotter—millions of degrees—but the colors and spec-
tra of stars tell you only about the surface because that’s where
the light comes from.
Th e Balmer thermometer works because the strength of the
Balmer lines depends on the temperature of the star’s surface
layers. Both hot and cool stars have weak Balmer lines, but
medium-temperature stars have strong Balmer lines.
Th e Balmer absorption lines are produced only by atoms
with electrons in the second energy level. If a star is cool, there
are few violent collisions between atoms to excite the electrons,
so the electrons of most atoms are in the ground state, not the
second level. Electrons in the ground state can’t absorb photons
in the Balmer series. As a result, you should expect to fi nd weak
Balmer absorption lines in the spectra of cool stars.
In the surface layers of hot stars, on the other hand, there are
many violent collisions between atoms. Th ese collisions can
excite electrons to high energy levels or ionize some atoms by
knocking electrons out of the atoms. Consequently, there are few
hydrogen atoms with their electrons in the second orbit to form
Balmer absorption lines. Hot stars, like cool stars, have weak
Balmer absorption lines.
In stars of an intermediate temperature, roughly 10,000 K,
the collisions are just right to excite large numbers of electrons
into the second energy level. Hydrogen gas at that temperature
absorbs Balmer wavelength photons very well and produces
strong Balmer lines.
Th eoretical calculations can predict exactly how strong the
Balmer lines should be for stars of various temperatures. Such
calculations are the key to fi nding temperatures from stellar3
SCIENTIFIC ARGUMENT
The infrared radiation coming out of your ear can tell a doctor
your temperature. How does that work?
You know two radiation laws, so your argument must use the right
one. Doctors and nurses use a handheld device to measure body
temperature by observing the infrared radiation emerging from a
patient’s ear. You might suspect the device depends on the Stefan–
Boltzmann law and measures the intensity of the infrared radiation.
A person with a fever will emit more energy than a healthy person.
However, a healthy person with a large ear canal would emit more
than a person with a small ear canal, so measuring intensity would
not be helpful. The device actually depends on Wien’s law, fi nding
temperature by measuring the “color” of the infrared radiation. A
patient with a fever will emit at a slightly shorter wavelength of
maximum intensity, and the infrared radiation emerging from his or
her ear will be a tiny bit “bluer” than that emitted by a person with
normal temperature.
Astronomers can measure the temperatures of stars the same
way. Adapt your argument for stars. Use Figure 7-6 to explain
how the colors of stars reveal their temperatures.Stellar Spectra
Science is a way of understanding nature, and the spectrum
of a star can tell you a great deal about the star’s temperature,
motion, and composition. In later chapters, you will use spectra
to study many more astronomical objects such as galaxies and
planets, but you can begin by looking at the spectra of stars,
including that of the sun.
The Formation of a Spectrum
Th e spectrum of a star is formed as light passes outward through
the gases near its surface. Read Atomic Spectra on pages
132–133 and notice that it describes three important properties
of spectra and defi nes 12 new terms that will help you discuss
astronomical spectra:
Th ere are three kinds of spectra: (i) continuous spectra; (ii)
absorption or dark-line spectra, which contain absorption
lines; and (iii) emission or bright-line spectra, which contain
emission lines. Th ese spectra are described by Kirchhoff ’s laws.
When you see one of these types of spectra, you can recog-
nize the kind of matter that emitted the light.
Photons are emitted or absorbed when an electron in an
atom makes a transition from one energy level to another.
Th e wavelengths of the photons depend on the energy
diff erence between the two levels. Hydrogen atoms can
produce many spectral lines in series such as the Lyman,
Balmer, and Paschen series. Only three lines in the Balmer
series are visible to human eyes. Th e emitted photons
coming from a hot cloud of hydrogen gas have the same7-3