PART 2 | THE STARS
Summary
▶ An atom consists of a nucleus (p. 123) surrounded by a cloud of
electrons (p. 123). The nucleus is made up of positively charged
protons (p. 123) and uncharged neutrons (p. 123).
▶ The number of protons in an atom determines which element it is.
Atoms of the same element (that is, having the same number of pro-
tons) with different numbers of neutrons are called isotopes (p. 124).
▶ A neutral atom is surrounded by a number of negatively charged elec-
trons equal to the number of protons in the nucleus. An atom that has
lost or gained an electron is said to be ionized (p. 124) and is called
an ion (p. 124).
▶ Two or more atoms joined together form a molecule (p. 124).
▶ (^) The electrons in an atom are attracted to the nucleus by the Coulomb
force (p. 124). As described by quantum mechanics (p. 124), the
binding energy (p. 124) that holds electrons in an atom is limited to
certain energies, and that means the electrons may occupy only certain
permitted orbits (p. 124).
▶ (^) The size of an electron’s orbit depends on its energy, so the orbits
can be thought of as energy levels (p. 126) with the lowest possible
energy level known as the ground state (p. 126).
▶ (^) An excited atom (p. 126) is one in which an electron is raised to
a higher orbit by a collision between atoms or the absorption of a
photon of the proper energy.
▶ (^) The agitation among the atoms and molecules of an object is called
thermal energy (p. 127), and the fl ow of thermal energy is heat
(p. 127). In contrast, temperature (p. 127) refers to the intensity of
the agitation and can be expressed on the Kelvin temperature scale
(p. 127), which gives temperature above absolute zero (p. 127).
▶ (^) Collisions among the particles in a body accelerate electrons and cause
the emission of blackbody radiation (p. 127). The hotter an object
is, the more it radiates and the shorter is its wavelength of maximum
intensity, max (p. 128). This allows astronomers to estimate the
temperatures of stars from their colors.
▶ (^) Density (p. 137) is a measure of the amount of matter in a given volume.
▶ (^) Kirchhoff’s laws (p. 132) explain that a hot solid, liquid, or dense
gas emits electromagnetic radiation at all wavelengths and produces a
continuous spectrum (p. 132). An excited low-density gas produces
an emission (bright-line) spectrum (p. 132) containing emission
lines (p. 132). A light source viewed through a low-density gas
produces an absorption (dark-line) spectrum (p. 132) containing
absorption lines (p. 132).
▶ (^) An atom can emit or absorb a photon when an electron makes a
transition (p. 133) between orbits.
▶ (^) Because orbits of only certain energy differences are permitted in
an atom, photons of only certain wavelengths can be absorbed or emit-
ted. Each kind of atom has its own characteristic set of spectral lines.
The hydrogen atom has the Lyman (p. 133) series of lines in the
ultraviolet, the Balmer series (p. 133) partially in the visible, and the
Paschen series (p. 133) (plus others) in the infrared.
▶ (^) The strength of spectral lines depends on the temperature of the star.
For example, in cool stars, the Balmer lines are weak because atoms are
not excited out of the ground state. In hot stars, the Balmer lines are
weak because atoms are excited to higher orbits or are ionized. Only at
medium temperatures are the Balmer lines strong.
▶ (^) A star’s spectral class (or type) (p. 131) is determined by the
absorption lines in its spectrum. The resulting spectral sequence
(p. 131), OBAFGKM, is important because it is a temperature
What Are We? Stargazers
Do you suppose chickens ever look at the
sky and wonder what the stars are?
Probably not. Chickens are very good at the
chicken business, but they are not known
for big brains and deep thought. Humans,
in contrast, have highly evolved, sophisti-
cated brains and are extremely curious. In
fact, curiosity may be the most reliable
characteristic of intelligence, and curiosity
about the stars is a natural extension of
our continual attempts to understand the
world around us.
For early astronomers like Copernicus and
Kepler, the stars were just points of light.
There seemed to be no way to learn anything
about them. Galileo’s telescope revealed
surprising details about the planets; but,
even viewed through a large telescope, the
stars are just points of light. Even when
later astronomers began to assume that the
stars were other suns, the stars seemed
forever beyond human knowledge.
As you have seen, the key is understand-
ing how light interacts with matter. In the
last 150 years or so, scientists have
discovered how atoms and light interact to
form spectra, and astronomers have applied
those discoveries to the ultimate object of
human curiosity—the stars.
Chickens may never wonder what the
stars are, or even wonder what chickens
are, but humans are curious animals, and
we do wonder about the stars and about
ourselves. Our yearning to understand the
stars is just part of our quest to under-
stand what we are.