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Astronomical
Instruments
Astronomy: The Solar System and Beyond
5th edition
Michael Seeds
Chapter 6
Starlight and Atoms
Awake! for Morning in the Bowl
of Night
Has flung the stone that put the
Stars to Flight!
And lo! the Hunter of the East
has caught
The Sultan’s Turret in a Noose
of Light.
- EDWARD FITZGERALD
Translator
The Rubáiyát of Omar Khayyám
Starlight and Atoms
• No laboratory jar on Earth holds a
sample labeled ‘star stuff,’ and no
instrument has ever probed inside a star.
– The stars are far beyond human reach.
Starlight and Atoms
• The only information you can obtain
about stars comes hidden in light.
– Whatever you want to know about the stars you
must catch in a noose of light—a spectrum.
• Astronomers can study such varied
phenomena as the
atmosphere of Pluto,
distant stars, and
clouds of gas in deep
space by examining
their spectra.
Starlight and Atoms
• In this chapter, you will consider how
atoms interact with light to produce
spectral lines—dark or bright lines that
cross spectra at specific positions.
– The chapter begins with the hydrogen atom
because it is the most common atom in the universe
as well as the simplest.
– Other atoms are larger and more complicated but, in
many ways, their properties resemble those of
hydrogen.
– Properly analyzed, a spectrum can tell you a great
deal about an astronomical object and the object’s
motion relative to Earth.
Atoms
Starlight and Atoms
• To think about atoms and how they can
interact with light, you need a working
model of an atom.
– In Chapter 2, you used a working
model of the sky, the
celestial sphere.
– You identified and named the
important parts, and described
how they were located and
how they interacted.
– Now, it is time to build a
model of an atom.
A Model Atom
Starlight and Atoms
• The model atom has a positively charged
nucleus at the center.
• This nucleus consists of two kinds of
particles.
– Protons carry a positive electrical charge and
neutrons have no charge.
– Thus, the nucleus has a net positive charge.
A Model Atom
Starlight and Atoms
• The nucleus in this model atom is
surrounded by a whirling cloud of orbiting
electrons, low-mass particles with a
negative charge.
A Model Atom
Starlight and Atoms
• In a normal atom, the number of
electrons equals the number of protons,
and the positive and negative charges
balance to produce a neutral atom.
– Because each proton or neutron has a mass 1,836
times greater than that of an electron, most of the
mass of the atom lies in the nucleus.
A Model Atom
Starlight and Atoms
• The hydrogen atom is the simplest of all
atoms.
– The nucleus is a single proton orbited by a single
electron, with a total mass of only 1.67 x 10-27 kg,
about a trillionth of a trillionth of a gram.
A Model Atom
Starlight and Atoms
• An atom is mostly empty space.
– To see this, imagine constructing a simple scale
model.
– The nucleus of a hydrogen atom is a proton with a
diameter of about 0.0000016 nm, or 1.6 x 10-15 m.
– If you multiply this by 1 trillion
(1012), you can represent the
nucleus of your model atom
with a grape seed, which is
about 0.16 cm in diameter.
A Model Atom
Starlight and Atoms
• The region of a hydrogen atom
containing the whirling electron has a
diameter of about 0.4 nm, or 4 x 10-10 m.
– Multiplying by a trillion magnifies the diameter to
about 400 m, or about 4.5 football fields
laid end to end.
– When you imagine a grape
seed in the midst of a sphere
4.5 football fields in diameter,
you can see that an atom
is mostly empty space.
Different Kinds of Atoms
Starlight and Atoms
• There are over 100 chemical elements.
• Which element an atom is depends only
on the number of protons in the nucleus.
– A carbon atom has six protons in its nucleus.
– The element with one more proton is nitrogen.
– The element with one fewer proton is boron.
Different Kinds of Atoms
Starlight and Atoms
• Although the number of protons in an
atom is fixed, you can change the
number of neutrons in an atom’s nucleus
without changing the atom significantly.
– For instance, if you add a neutron to the carbon
nucleus, you still have carbon, but it is slightly
heavier than normal carbon.
Different Kinds of Atoms
Starlight and Atoms
• Atoms that have the same number of
protons but a different number of
neutrons are isotopes.
– Carbon has two stable isotopes.
– One form contains six protons and six neutrons for a
total of 12 particles and is thus called carbon-12.
– Carbon-13 has six protons and seven neutrons in its
nucleus.
Different Kinds of Atoms
Starlight and Atoms
• Protons and neutrons are bound tightly
into the nucleus.
• However, electrons are held loosely in the
electron cloud.
Different Kinds of Atoms
Starlight and Atoms
• Running a comb through your hair
creates a static charge by removing a
few electrons from their atoms.
• This process is called ionization.
– An atom that has lost one or more electrons is
an ion.
Different Kinds of Atoms
Starlight and Atoms
• A carbon atom is neutral if it has six
electrons to balance the positive
charge of the six protons in its nucleus.
– If you ionize the atom by removing one or more
electrons, the atom is left with a net positive
charge.
Different Kinds of Atoms
Starlight and Atoms
• Under some circumstances, an atom
may capture one or more extra electrons,
giving it a net negative charge.
– Such a negatively charged atom is also considered
an ion.
Different Kinds of Atoms
Starlight and Atoms
• Atoms that collide may form bonds
with each other by exchanging or
sharing electrons.
– Two or more atoms bonded together form a
molecule.
Different Kinds of Atoms
Starlight and Atoms
• Atoms do collide in stars, but the high
temperatures cause violent collisions that
are unfavorable for chemical bonding.
– Only in the coolest stars are the collisions gentle
enough to permit the formation of chemical bonds.
– Later, you will see that molecules can form in cool
gas clouds in space and in the atmospheres of
planets.
Electron Shells
Starlight and Atoms
• So far, you have been thinking of the
whirling cloud of orbiting electrons in a
general way.
• However, the specific way electrons
behave within the cloud is very important
in astronomy.
Electron Shells
Starlight and Atoms
• The electrons are bound to the atom by
the attraction between their negative
charge and the positive charge on the
nucleus.
– This attraction is known as the Coulomb force, after
the French physicist Charles-Augustin de Coulomb
(1736–1806).
Electron Shells
Starlight and Atoms
• To ionize an atom, you need a certain
amount of energy to pull an electron
away from its nucleus.
– This energy is the electron’s binding energy, the
energy that holds it to the atom.
Electron Shells
Starlight and Atoms
• Electrons may orbit the nucleus at
various distances.
• The size of an electron’s orbit is related
to the energy that binds it to the atom.
– If the orbit is small, the electron is close to the
nucleus, and a large amount of energy is needed
to pull it away. Therefore, its binding energy is
large.
– An electron orbiting farther from the nucleus is held
more loosely, and it takes less energy to pull it
away. It therefore has less binding energy.
Electron Shells
Starlight and Atoms
• Nature permits atoms only certain
amounts (quanta) of binding energy.
• The laws that describe how atoms
behave are called the laws of quantum
mechanics.
– Much of this discussion of atoms is based on the
laws of quantum mechanics.
Electron Shells
Starlight and Atoms
• As atoms can have only certain amounts
of binding energy, your model atoms can
have orbits of only certain sizes, called
permitted orbits.
– These are like steps in a staircase: you can stand on
the number-one step or the number-two step, but not
on the number-one-and-one-quarter step.
– The electron can occupy any permitted orbit, but not
orbits in between.
Electron Shells
Starlight and Atoms
• The arrangement of permitted orbits
depends primarily on the charge of the
nucleus, which in turn depends on the
number of protons.
– Thus, each element
has its own pattern
of permitted orbits.
Electron Shells
Starlight and Atoms
• Isotopes of the same elements have
nearly the same pattern because they
have the same number of protons.
• However, ionized atoms have orbital
patterns that differ from their non-ionized
forms.
– Thus, the arrangement of permitted orbits differs for
every kind of atom and ion.
Building Scientific Arguments
Starlight and Atoms
• How many hydrogen atoms would it take
to cross the head of a pin?
– This is not a frivolous question.
– In answering it, you will discover how small atoms
really are and you will see how powerful physics
and mathematics can be as a way to understand
nature.
– Indeed, many scientific arguments are convincing
because they have the precision of mathematics.
Building Scientific Arguments
Starlight and Atoms
• To begin, assume that the head of a pin is
about 1 mm in diameter, that is, it is 0.001
m.
– The size of a hydrogen atom is represented by the
diameter of the electron cloud, roughly 0.4 nm.
– As 1 nm equals 10-9 m, you can multiply and
discover that 0.4 nm equals 4 x 10-10 m.
Building Scientific Arguments
Starlight and Atoms
• To find out how many atoms would
stretch 0.001 m, you can divide the
diameter of the pinhead by the diameter
of an atom.
– That is, you divide 0.001 m by 4 x 10-10 m, and you
get 2.5x106.
– It would take 2.5 million hydrogen atoms lined up
side by side to cross the head of a pin.
Building Scientific Arguments
Starlight and Atoms
• Now, you can see how tiny an atom is
and also how powerful a bit of physics
and mathematics can be.
– It reveals a view of nature beyond the capability of
your eyes.
• Now, build an argument using another bit
of arithmetic.
– How many hydrogen atoms would you need to add
up to the mass of a paper clip (1 g)?
The Interaction of Light and Matter
Starlight and Atoms
• You should begin your study of light
and matter by considering the
hydrogen atom.
– As you read earlier, hydrogen is both simple and
common.
– Roughly 90 percent of all atoms in the universe
are hydrogen.
The Excitation of Atoms
Starlight and Atoms
• Each orbit in an atom represents a
specific amount of binding energy.
• So, physicists commonly refer to the
orbits as energy levels.
– Using this terminology, you can say that an electron
in its smallest and most tightly bound orbit is in its
lowest permitted energy level.
The Excitation of Atoms
Starlight and Atoms
• You can move the electron from one
energy level to another, by supplying
enough energy to make up the difference
between the two energy levels.
– It is like moving a flowerpot from a low shelf
to a high shelf: the greater the distance
between the shelves, the more energy you
need to raise the pot.
The Excitation of Atoms
Starlight and Atoms
• The amount of energy needed to move
the electron is the energy difference
between the two energy levels.
– If you move the electron from a low energy level
to a higher energy level, the atom is said to be an
excited atom.
– That is, you have added energy to the atom by
moving its electron.
– If the electron falls back to the lower energy level,
that energy is released.
The Excitation of Atoms
Starlight and Atoms
• An atom can become excited by collision.
– If two atoms collide, one or both may have electrons
knocked into a higher energy level.
– This happens very commonly in hot gas, where the
atoms move rapidly and collide often.
The Excitation of Atoms
Starlight and Atoms
• Another way an atom can get the energy
that moves an electron to a higher energy
level is to absorb a photon.
• However, only a photon with exactly the
right amount of energy can move the
electron from one level to another.
– If the photon has too much or too little energy, the
atom cannot absorb it.
The Excitation of Atoms
Starlight and Atoms
• As the energy of a photon depends on its
wavelength, only photons of certain
wavelengths can be absorbed by a given
kind of atom.
• The figure displays the lowest four energy
levels of the hydrogen
atom along with three
photons that the atom
could absorb.
The Excitation of Atoms
Starlight and Atoms
• The longest-wavelength photon only
has enough energy to excite the
electron to the second energy level.
• However, the shorter-wavelength
photons can excite the electron to
higher levels.
The Excitation of Atoms
Starlight and Atoms
• A photon with too much or too little
energy cannot be absorbed.
– As the hydrogen atom has many more energy
levels than those displayed, it can absorb photons
of many different wavelengths.
The Excitation of Atoms
Starlight and Atoms
• Atoms, like humans, cannot exist in an
excited state forever.
– The excited atom is unstable and must eventually
(usually within 10-6 to 10-9 s) give up the energy it
has absorbed and return its electron to the lowest
energy level.
– As the electrons eventually tumble down to this
bottom level, physicists call it the ground state.
The Excitation of Atoms
Starlight and Atoms
• When the electron drops from a higher
to a lower energy level, it moves from a
loosely bound level to one more tightly
bound.
• Then, the atom has a surplus of
energy—the energy difference between
the levels—that it can emit as a photon.
The Excitation of Atoms
Starlight and Atoms
• The sequence of events depicted in
the figure illustrates how an atom can
absorb and emit photons.
The Excitation of Atoms
Starlight and Atoms
• As each type of atom or ion has its
unique set of energy levels, each type
absorbs and emits photons with a unique
set of wavelengths.
– Thus, you can identify the elements in a gas by
studying the characteristic wavelengths of light
absorbed or emitted.
The Excitation of Atoms
Starlight and Atoms
• The process of excitation and emission
is a common sight in urban areas at
night.
• A neon sign glows when atoms of the
neon gas in the tube are excited by
electricity flowing through the tube.
– As the electrons in the electric current flow
through the gas, they collide with the neon atoms
and excite them.
The Excitation of Atoms
Starlight and Atoms
• Immediately after an atom is excited, its
electron drops back to a lower energy
level, emitting the surplus energy as a
photon of a certain wavelength.
– The photons emitted by excited neon produce a
reddish-orange glow.
– Signs of other colors, erroneously called ‘neon,’
contain other gases or mixtures of gases instead of
pure neon.
Radiation from a Heated Object
Starlight and Atoms
• To begin thinking about the interaction of
light and matter, consider a simple but
very important phenomenon.
– When you heat up an object, it begins to glow.
• Think of a blacksmith heating a
horseshoe in a forge.
– The hot iron glows bright yellow-orange.
– What produces this light?
Radiation from a Heated Object
Starlight and Atoms
• Light is a changing electric and
magnetic field.
– Whenever you change the motion of an electron,
you generate electromagnetic waves.
Radiation from a Heated Object
Starlight and Atoms
• If you run a comb through your hair, you
disturb electrons in both hair and comb,
producing static electricity.
– As each electron is surrounded by an electric field,
any sudden change in the electron’s motion gives rise
to electromagnetic radiation.
• Running a comb through your hair while
standing near an AM radio produces radio
static.
– To see what this has to do with a heated object, think
of what you mean when you say an object is hot.
Radiation from a Heated Object
Starlight and Atoms
• The molecules and atoms in an object
are in constant motion.
• In a hot object, they are more agitated
than in a cool object.
• You can refer to this agitation as thermal
energy.
– When you touch a hot object, you feel heat as the
thermal energy flows into your fingers.
Radiation from a Heated Object
Starlight and Atoms
• 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 contain
more thermal energy and can burn your tongue.
• Thus, heat refers to the flow of thermal
energy, and temperature refers to the
intensity of the atomic motion.
Radiation from a Heated Object
Starlight and Atoms
• When astronomers refer to the
temperature of astronomical objects such
as stars, they use the Kelvin temperature
scale.
Radiation from a Heated Object
Starlight and Atoms
• On the Kelvin scale, zero degrees Kelvin
(written 0 K) is absolute zero (-459.7°F),
the temperature at which an object
contains no heat energy that can be
extracted.
– Water freezes at 273 K and boils at 373 K.
• The Kelvin temperature scale is useful in
astronomy because it is based on absolute
zero and thus is related to the motion of
the particles in an object.
Radiation from a Heated Object
Starlight and Atoms
• Now, you can understand why a hot
object glows.
– The hotter an object is, the more motion there is
among its particles.
– The agitated particles collide with electrons and,
when electrons are accelerated, part of the energy
is carried away as a photon.
Radiation from a Heated Object
Starlight and Atoms
• The radiation emitted by a heated object is
called black body radiation, which refers to
the way a perfect emitter of radiation
would behave.
• A perfect emitter would also be a perfect
absorber.
– So, at room temperature, it would look black.
– Thus, the term black body is often used to refer to
objects that glow brightly.
Radiation from a Heated Object
Starlight and Atoms
• Black body radiation is quite common.
– In fact, it is the way in which light is emitted by an
incandescent light bulb.
– Electricity flowing through the filament of the light
bulb heats it to high temperature, and it glows.
– You can also recognize the light emitted by a heated
horseshoe in a blacksmith’s forge as black body
radiation.
Radiation from a Heated Object
Starlight and Atoms
• Many objects in astronomy, including
stars, emit radiation almost as if they
were black bodies.
• Hot objects emit black body radiation, but
so do objects that seem cold.
– Ice cubes are cold, but their temperature is higher
than absolute zero. So, they contain some thermal
energy and must emit some black body radiation.
– The coldest gas drifting in space has a temperature
only a few degrees above absolute zero, but it too
emits black body radiation.
Radiation from a Heated Object
Starlight and Atoms
• There are two important features of
black body radiation.
• First, the hotter an object is, the more
black body radiation it emits.
– Hot objects emit more radiation because
their agitated particles collide more often
and more violently with electrons.
– Thus, a glowing coal from a fire emits more
total energy than an ice cube of the same
size.
Radiation from a Heated Object
Starlight and Atoms
• The second feature is the relationship
between the temperature of the object
and the wavelengths of the photons it
emits.
– The wavelength of a photon emitted when a particle
collides with an electron depends on the violence of
the collision.
– Only a violent collision can produce a shortwavelength (high-energy) photon.
Radiation from a Heated Object
Starlight and Atoms
• As extremely violent collisions don’t occur
very often, short-wavelength photons are
rare.
• Similarly, most collisions are not
extremely gentle. So, long-wavelength
(low-energy) photons are also rare.
– Consequently, black body radiation is made up of
photons with a distribution of wavelengths, and very
short and very long wavelengths are rare.
Radiation from a Heated Object
Starlight and Atoms
• The wavelength of maximum intensity
(λmax), the wavelength at which the object
emits the most radiation, occurs at some
intermediate wavelength.
• Remember, λmax does not refer to the
maximum wavelength but to the
wavelength of maximum intensity.
Radiation from a Heated Object
• The figure depicts the
intensity of radiation
versus wavelength for
three objects of
different
temperatures.
Starlight and Atoms
Radiation from a Heated Object
• The curves are high in
the middle and low at
either end, confirming
that these objects
emit most intensely at
intermediate
wavelengths.
Starlight and Atoms
Radiation from a Heated Object
• The total area under
each curve is
proportional to the
total energy emitted.
– Notice that the hotter
object emits more total
energy than the cooler
objects.
Starlight and Atoms
Radiation from a Heated Object
Starlight and Atoms
• Closer examination reveals that the
wavelength of maximum intensity depends
on temperature.
– Even though the most violent collisions remain rare, the
most common collision in a hotter body is more violent
than the most common collision in a cooler body.
– The hotter the object, the shorter the wavelength of
maximum intensity.
Radiation from a Heated Object
Starlight and Atoms
• Notice how temperature determines the
color of a glowing black body.
– The hotter object emits more blue light than red, and
thus looks blue.
– The cooler object emits more red than blue, and
consequently looks red.
Radiation from a Heated Object
Starlight and Atoms
• You can see that phenomenon
among stars.
– Hot stars look blue and cool stars red.
Radiation from a Heated Object
Starlight and Atoms
• Notice that cool objects may emit little
visible radiation but are still producing
black body radiation.
– For example, the human body has a temperature of
310 K and emits black body radiation mostly in the
infrared part of the spectrum.
– Thus, infrared security cameras can detect burglars
by the radiation they emit.
Radiation from a Heated Object
Starlight and Atoms
• Humans emit very few visible-wavelength
photons and almost never emit X-ray or
gamma-ray photons.
• Humans’ wavelength of maximum
intensity lies in the infrared part of the
spectrum.
Building Scientific Arguments
Starlight and Atoms
• How could a doctor measure
someone’s temperature without
touching him or her?
– You just learned that red-hot horseshoes and
cold clouds of gas in space emit black body
radiation.
– Use what you know about black body radiation
to construct an argument specific to human
beings.
Building Scientific Arguments
Starlight and Atoms
• Doctors and nurses use a handheld
device to measure body temperature,
by observing the infrared radiation
emerging from a patient’s ear.
Building Scientific Arguments
Starlight and Atoms
• 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 energy than a person with a small
ear canal.
– Thus, measuring intensity would not be accurate.
Building Scientific Arguments
Starlight and Atoms
• The device actually depends on Wien’s
law in that it measures the ‘color’ of the
infrared radiation.
– A patient with a fever will emit energy at a shorter
wavelength of maximum intensity.
– The infrared radiation emerging from his or her ear
will be a tiny bit ‘bluer’ than normal.
– Astronomers can measure the temperature of stars
in much the same way.
Building Scientific Arguments
Starlight and Atoms
• Now, modify your argument to refer to
stars instead of humans.
– Look at the figure and explain how astronomers
could measure the temperature of stars.
Information from Spectra
Starlight and Atoms
• A spectrum tells you a great deal about
such phenomena as temperature,
motion, and composition.
• The spectrum of a star is formed as
light passes outward through the gases
near its surface.
The Formation of a Spectrum
Starlight and Atoms
• Notice three important properties of
spectra.
• One, there are three kinds of spectra
described by three simple rules.
– When you see one of these types of spectra,
you can recognize the kind of matter that
emitted the light.
The Formation of a Spectrum
Starlight and Atoms
• Two, the wavelengths of the photons that
are absorbed or emitted are determined
by the atomic energy levels in the atoms.
– The emitted photons from a hot cloud of hydrogen
gas have the same wavelengths as the photons
absorbed by hydrogen atoms in the gases of a star.
– Although the hydrogen atom produces many
spectral lines from the ultraviolet to the infrared, only
three are visible to human eyes.
The Formation of a Spectrum
Starlight and Atoms
• Three, most modern astronomy books
display spectra as graphs of intensity
versus wavelength.
– You need to see the connection between dark
absorption lines and dips in the graphed spectrum.
The Formation of a Spectrum
Starlight and Atoms
• Spectra are filled with information about
the sources of the light.
– To extract that information, astronomers must be
experts on the interaction of light and matter.
– Electrons moving among their orbits within atoms
can reveal the secrets of the stars.
The Analysis of a Spectrum
Starlight and Atoms
• Kirchhoff ’s laws give astronomers
a powerful tool in the analysis of a
spectrum.
The Analysis of a Spectrum
Starlight and Atoms
• If a spectrum is an emission spectrum,
astronomers know immediately that
they are observing an excited, lowdensity gas.
The Analysis of a Spectrum
Starlight and Atoms
• Nebulae in space such as that displayed
produce emission spectra and so do
some tails of comets.
– This tells astronomers that some comet tails are
made of ionized gas.
The Analysis of a Spectrum
Starlight and Atoms
• If astronomers see an absorption
spectrum, they know that they are
seeing light that has traveled through a
cloud of gas.
– The spectra of stars and so, of course, the
spectrum of the sun are absorption spectra.
The Analysis of a Spectrum
Starlight and Atoms
• At visual wavelengths, astronomers
almost never see a continuous spectrum.
– However, all planetary objects emit black body
radiation.
The Analysis of a Spectrum
Starlight and Atoms
• Observing in the infrared, astronomers
can measure the wavelength of
maximum intensity in the radiation
emitted by an object, and deduce the
temperature of the object.
– You will see how this can be applied to phenomena
such as dust in space and the moons of planets.
The Analysis of a Spectrum
Starlight and Atoms
• The chemical composition of a star can
be determined by a spectrum.
– If a star’s spectrum contains lines specific to a
certain atom, the star must contain that atom.
– For example, the sun’s spectrum contains the
spectral lines of sodium. Thus, the sun must
contain sodium.
– Later, you will learn that stars are mostly hydrogen
and helium with only traces of heavier elements.
The Analysis of a Spectrum
Starlight and Atoms
• Deducing the relative amounts of the
different atoms revealed by a spectrum is
quite difficult.
• However, the mere presence of spectral
lines means that certain atoms must be
present.
– For example, the spectrum of Mars contains
absorption lines produced by carbon dioxide gas.
– Thus, astronomers know its atmosphere must
contain that gas.
The Doppler Effect
Starlight and Atoms
• Not only can a spectrum tell you about the
object that emitted the light, but it can also
tell you about motion.
• The Doppler effect is an apparent change
in the wavelength of radiation caused by
the motion of the source.
– Astronomers use it to measure the velocity of stars
relative to Earth.
The Doppler Effect
Starlight and Atoms
• When astronomers talk about the Doppler
effect, they are talking about a shift in the
wavelength of electromagnetic radiation.
• However, the Doppler shift can occur in all
forms of radiation, including sound waves.
– So, you probably hear the Doppler effect everyday
without really noticing.
The Doppler Effect
Starlight and Atoms
• The pitch of a sound is determined
by its wavelength.
– Sounds with long wavelengths have low
pitches.
– Sounds with short wavelengths have higher
pitches.
The Doppler Effect
Starlight and Atoms
• You hear a Doppler shift every time a car
or truck passes you and the pitch of its
engine noise drops.
– Its sound is shifted to shorter wavelengths and higher
pitches while it is approaching.
– It is shifted to longer wavelengths and lower pitches
after it passes.
The Doppler Effect
Starlight and Atoms
• To see why the sound waves are shifted
in wavelength, consider a fire truck
approaching you with a bell clanging
once a second.
– When the bell clangs, the sound travels ahead of
the truck to reach your ears.
The Doppler Effect
Starlight and Atoms
• One second later, the bell clangs again.
– During that one second, the fire truck has moved
closer to you.
– So, the bell is closer at its second clang.
– Now, the sound has a shorter distance to travel, and
reaches your ears a little sooner than it would have
if the fire truck were not approaching.
The Doppler Effect
Starlight and Atoms
• If you timed the clangs, you would find that
the clangs are slightly less than one
second apart.
– When the fire truck passes you and moves away, you
hear the clangs sounding slightly more than one
second apart.
– That is because, now, each successive clang of the
bell occurs farther from you.
The Doppler Effect
Starlight and Atoms
• The figure depicts a fire truck moving
toward one observer and away from
another observer.
– The position of the bell at each clang is represented
by a small black bell.
– The sound spreading outward is represented by
black circles.
The Doppler Effect
Starlight and Atoms
• You can see how the clangs are
squeezed together ahead of the fire
truck and stretched apart behind.
The Doppler Effect
Starlight and Atoms
• Now, you can substitute a source of light
for the clanging bell.
– Imagine the light source emitting waves
continuously as it approaches and passes you.
The Doppler Effect
Starlight and Atoms
• Each time the source emits the peak of a
wave, it will be slightly closer to you than
when it emitted the peak of the previous
wave.
– From your vantage point, the successive peaks of
the wave will seem closer together—in the same
way that the clangs of the bell seemed closer
together.
– The light will appear to
have a shorter
wavelength,
making it slightly bluer.
The Doppler Effect
Starlight and Atoms
• As the light is shifted slightly toward
the blue end of the spectrum, this is
called a blueshift.
The Doppler Effect
Starlight and Atoms
• As the light source moves away from
you, the peaks of successive waves
seem farther apart.
– So, the light has a longer wavelength and is redder.
– This is a redshift.
The Doppler Effect
Starlight and Atoms
• The terms blueshift and redshift are used
to refer to any range of wavelengths.
– The light does not actually have to be red or blue.
– Also, the terms apply equally to wavelengths in the
radio, X-ray, or gamma-ray parts of the spectrum.
– ‘Red’ and ‘blue’ refer to the direction of the shift, not
to actual color.
The Doppler Effect
Starlight and Atoms
• The amount of change in wavelength
and, thus, the magnitude of the Doppler
shift depend on the velocity of the source.
– A moving car has a smaller Doppler shift than a jet
plane.
– A slow moving star has a smaller Doppler shift than
one that is moving more quickly.
The Doppler Effect
Starlight and Atoms
• You can measure the velocity of an
object by measuring the size of the
Doppler shift.
– The police measures Doppler shifts of passing cars
by using radar guns.
– Astronomers measure the shift of dark lines on a
star’s spectrum.
The Doppler Effect
Starlight and Atoms
• If a star is moving toward Earth, it is
blueshifted, and each of its spectral lines
is shifted very slightly closer to the blue
end of the spectrum.
• If a star is moving away from Earth, it is
redshifted, and each of its spectral lines
is shifted very slightly toward the red end
of the spectrum.
– The shifts are much too small to change the color of
a star, but they are easily detected in spectra.
The Doppler Effect
Starlight and Atoms
• When you think about the Doppler
effect, it is important to remember two
points.
• One, Earth itself moves. So, a
measurement of Doppler shift really
measures the relative motion between
Earth and the star.
The Doppler Effect
Starlight and Atoms
• The figure depicts the Doppler effect in
two spectra of the star Arcturus.
– Lines in the top spectrum are slightly blueshifted
because the spectrum was recorded when Earth,
in the course of its
orbit, was moving
toward Arcturus.
The Doppler Effect
Starlight and Atoms
– Lines in the bottom spectrum are redshifted
because it was recorded six months later,
when Earth was moving away from Arcturus.
The Doppler Effect
Starlight and Atoms
• The second point to remember is that the
Doppler shift is sensitive only to the part
of the velocity directed away from you or
toward you.
• This is the radial velocity (Vr).
The Doppler Effect
Starlight and Atoms
• You cannot use the Doppler effect to
detect any part of the velocity that is
perpendicular to your line of sight.
– For example, a star moving to the left would have no
blueshift or redshift because its distance from Earth
would not be decreasing or increasing.
– That is why police using radar guns park right next to
the highway.
– They want to measure your full velocity as you come
toward them down the highway, not just part of your
velocity.
Building Scientific Arguments
Starlight and Atoms
• Why would you be unable to use the
Doppler effect to detect the motion of a
star that is moving in a direction
perpendicular to an imaginary line
connecting Earth to that star?
• Imagine again a light source that
approaches and then passes you.
Building Scientific Arguments
Starlight and Atoms
• As the light source moves toward you,
each time it emits the peak of a wave, it is
slightly closer to you than when it emitted
the peak of the previous wave.
• As it moves away from you, it is slightly
farther away from you for each
successive wave peak it emits.
– This is what causes the
Doppler shift in the
light you observe.
Building Scientific Arguments
Starlight and Atoms
• Now, imagine that the light source is a
distant star moving in a direction
perpendicular to the imaginary line
connecting Earth to that star.
– Now, the light source is neither approaching nor
retreating.
– Each time the star emits the peak of a wave, it is
the same distance from Earth.
– Thus, there is no Doppler shift in its spectrum.
Building Scientific Arguments
Starlight and Atoms
• The Doppler effect only captures the radial
velocity of an object.
• However, it has another important
limitation.
– As measured from
Earth, the Doppler
shift of Arcturus is
different at different
times of year.
– Why is this?