CHAPTER 7 | ATOMS AND STARLIGHT 127
of an electron or other charged particle, you generate electromag-
netic waves. If you run a comb through your hair, you disturb
electrons in both hair and comb, producing static electricity.
Th at produces electromagnetic radiation, which you can hear as
snaps and crackles if you are standing near an AM radio. Stars
don’t comb their hair, of course, but they are hot, and they are
made up of ionized gases, so there are plenty of electrons zipping
around.
Th e molecules and atoms in any object are in constant
motion, and in a hot object they are more agitated than in a cool
object. You can refer to this agitation as thermal energy. If you
touch an object that contains lots of thermal energy, it will feel
hot as the thermal energy fl ows into your fi ngers. Th e fl ow of
thermal energy is called heat. In contrast, temperature refers to
the average speed of the particles. Hot cheese and hot green
beans can have the same temperature, but the cheese can con-
tain more thermal energy and can burn your tongue. Th us, heat
refers to the fl ow of thermal energy, and temperature refers to the
intensity of the agitation among the particles (Focus on
Fundamentals 3).
When astronomers refer to the temperature of a star, they
are talking about the temperature of the gases in the photo-
sphere, and they express those temperatures on the Kelvin
temperature scale. On this scale, zero degrees Kelvin (written
0 K) is absolute zero (−459.7°F), the temperature at which an
object contains no thermal energy that can be extracted. Water
freezes at 273 K and boils at 373 K. Th e Kelvin temperature
scale is useful in astronomy because it is based on absolute zero
and consequently is related directly to the motion of the parti-
cles in an object.
Now you can understand why a hot object glows, or to put
it another way, why a hot object emits photon bundles of elec-
tromagnetic energy. Th e hotter an object is, the more motion
there is among its particles. Th e agitated particles, including
electrons, collide with each other, and when electrons acceler-
ate—change their motion–part of the energy is carried away as
electromagnetic radiation. Th e radiation emitted by a heated
object is called blackbody radiation, a name translated from a
German term that refers to the way a perfectly opaque object
would behave. A perfectly opaque object would be both a perfect
emitter and a perfect absorber of radiation. At room tempera-
ture, such a perfect absorber and emitter would look black, but
at higher temperatures it would glow at wavelengths visible to a
human eye. In astronomy and physics you will see the term
blackbody referring to objects that glow brightly.
Blackbody radiation is quite common. In fact, it is respon-
sible for the light emitted by an incandescent light bulb.
Electricity fl owing through the fi lament of the bulb heats it to
high temperature, and it glows. You can also recognize the light
emitted by a heated horseshoe as blackbody radiation. Many
objects in the sky, including the sun and stars, primarily emit
blackbody radiation because they are mostly opaque.emits photons with a unique set of wavelengths. As a result, you
can identify the elements in a gas by studying the characteristic
wavelengths of light that are absorbed or emitted.
Th e process of excitation and emission is a common sight in
urban areas at night. A neon sign glows when atoms of neon gas
in a glass tube are excited by electricity fl owing through the tube.
As the electrons in the electric current fl ow through the gas, they
collide with the neon atoms and excite them. Almost immedi-
ately after a neon atom is excited, its electron drops back to a
lower energy level, emitting the surplus energy as a photon of a
certain wavelength. Th e photons emitted by excited neon blend
to produce a reddish-orange glow. Signs of other colors, generi-
cally called “neon signs,” contain other gases or mixtures of gases
instead of pure neon. Whenever you look at a neon sign, you are
seeing atoms absorbing and emitting energy in the form of pho-
tons with specifi c colors determined by the structure of electron
orbits in those atoms.
Neon signs are simple, but stars are complex. Th e colors of
stars are not determined by the gases they contain. In the next
section, you will discover why some stars are red and some are
blue, and that will give you a new insight into how light interacts
with matter.
Radiation from a Heated Object
If you look closely at the stars in the constellation Orion, you will
notice that they are not all the same color (see Figure 2-4). One
of your Favorite Stars, Betelgeuse, in the upper left corner of
Orion, is quite red; another Favorite Star, Rigel, in the lower
right corner, is blue. Th ese diff erences in color arise from diff er-
ences in temperature.
Th e starlight that you see comes from gases that make up the
visible surface of the star, its photosphere. (Recall that you met
the photosphere of the sun in Chapter 3.) Layers of gas deeper
inside the star also emit light, but that light is reabsorbed before
it can reach the surface. Th e gas above the photosphere is too
thin to emit much light. Th e photosphere is the visible surface of
a star because it is dense enough to emit lots of light but transpar-
ent enough to allow that light to escape.
Stars produce their light for the same reason heated horse-
shoes glow in a blacksmith’s forge—because they are hot. If a
horseshoe is not too hot, it glows ruddy red, but as it heats up it
grows brighter and yellower. Yellow-hot is hotter than red-hot
but not as hot as white-hot.
Th e light from stars and from glowing horseshoes is pro-
duced by the acceleration of charged particles. Usually the accel-
erated particles are electrons because they are the least massive
charged particles, and they are on the outsides of atoms, so they
are the easiest to get moving. An electron is surrounded by an
electric fi eld; and, if you disturb an electron, the change in its
electric fi eld spreads outward at the speed of light as electromag-
netic radiation. You learned in Chapter 5 that “acceleration”
means any change in motion. Whenever you change the motion