CHAPTER 7 | ATOMS AND STARLIGHT 123
sun’s atmosphere, and found that the spectra of other stars have
similar patterns of lines, a window fi nally opened to a scientifi c
understanding of the sun and stars. In this chapter you will look
through that window, seeing how the sun and other stars pro-
duce light, and how atoms interact with light to cause spectral
lines. Once you understand that, you will know how astrono-
mers determine the chemical composition of stars, as well as
measure motions of gas at their surfaces and in their
atmospheres.Atoms
Stars are great balls of hot gas, and the atoms in that gas
leave their marks on the light the stars emit. By understanding
what atoms are and how they interact with light, you can decode
the spectra of the stars and learn their secrets.A Model Atom
To think about atoms and how they interact with light, you need
a working model of an atom. In Chapter 2, you used a model of
the sky, the celestial sphere. In this chapter, you will begin your
study of atoms by creating a model of an atom. Remember that
a model can have practical value without being true. Th e stars are
not actually attached to a sphere surrounding Earth, but to navi-
gate a ship or point a telescope, it is useful to pretend they are.
Th e electrons in an atom are not actually little beads orbiting the
nucleus the way planets orbit the sun, but for some purposes it
is useful to picture them as such.
Your model atom contains a positively charged nucleus at
the center, which consists of two kinds of particles: Protons carry
a positive electrical charge, and neutrons have no charge. Th is
means the nucleus has a net positive charge.
Th e nucleus of this model atom is surrounded by a cloud of
orbiting electrons, low-mass particles with negative charges.
Normally the number of electrons equals the number of protons,
and the positive and negative charges balance to produce a neu-
tral atom. Protons and neutrons have masses about 1840 times
greater than that of an electron, so most of the mass of an atom
lies in the nucleus. Even so, a single atom is not a massive object.
A hydrogen atom, for example, has a mass of only 1.67 10 ^27
kg, about a trillionth of a trillionth of a gram.
An atom is mostly empty space. To see this, imagine con-
structing a simple scale model of a hydrogen atom. Its nucleus
is a single proton with a diameter of about 0.0000016 nm, or
1.6 10 −15 m. If you multiply this by one trillion (10^12 ), you can
represent the nucleus of your model atom with something about
0.16 cm in diameter—a grape seed would do. Th e region of a
hydrogen atom that contains the electron has a diameter of about
0.4 nm, or 4 10 −10 m. Multiplying by a trillion increases the
diameter to about 400 m, or about 4.5 football fi elds laid end to
end (■ Figure 7-2). When you imagine a grape seed in the middle7-1
Awake! for Morning in the Bowl of Night
Has fl ung the Stone that puts the Stars to
Flight:And Lo! the Hunter of the East has caught
Th e Sultan’s Turret in a Noose of Light.
— THE RUBÁIYÁT OF OMAR KHAYYÁM, TRANS. EDWARD FITZGERALDT
he universe is filled with fabulously beautiful clouds
of glowing gas illuminated by brilliant stars, but they are
all hopelessly beyond reach. No laboratory jar on Earth
holds a sample labeled “star stuff ,” and no space probe has ever
visited the inside of a star. Th e stars are far away, and the only
information you can obtain about them comes hidden in star-
light (■ Figure 7-1).
Earthbound humans knew almost nothing about stars until
the early 19th century, when the German optician Joseph von
Fraunhofer studied the solar spectrum and found it interrupted
by some 600 dark lines representing colors that are missing from
the sunlight Earth receives. When scientists realized that those
spectral lines are related to the presence of various atoms in the
■ Figure 7-1
What’s going on here? The sky is fi lled with beautiful and mysterious objects
that lie far beyond your reach—in the case of the nebula NGC 6751, about
6500 ly beyond your reach. The only way to understand such objects is by
analyzing their light. Such an analysis reveals that this object is a dying star
surrounded by the expanding shell of gas it ejected a few thousand years
ago. You will learn more about this phenomenon in a later chapter. (NASA
Hubble Heritage Team/STScI/AURA)
Visual-wavelength image