Chapter 4
Radiation and Spectra
The Sun
in ultraviolet
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Radiation from Space 
Information from the Stars
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The Nature of Light
At least 95% of the celestial information we
receive is in the form of light.
Astronomers have devised many techniques to
decode as much of the encoded information as
possible from the small amount of light that
reaches Earth.
This includes information about the object's
temperature, motion, chemical composition, gas
density, surface gravity, shape, structure, and
more!
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The Nature of Light (cont’d)
Most of the information in light is revealed by
using spectroscopy:
Spectroscopy is the separation of light into its
different constituent colors (or wavelengths) for
analysis.
The resulting components are called the spectrum
of the light.
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Electric and Magnetic Fields
Light is composed of electric
fields and magnetic fields.
Electric charges and magnets
alter the region of space around
them so that they can exert
forces on distant objects.
This altered space is called a
force field (or just a field).
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Electromagnetism
Connection between electric and magnetic
fields was discovered in the 19th century.
A moving electric charge or an electric current
creates a magnetic field.
Coils of wire are used to make the large
electromagnets used in car junk yards or the
tiny electromagnets in your telephone receiver.
Electric motors used to start your car or spin a
computer's hard disk around are other
applications of this phenomenon.
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How it works…
A changing magnetic field creates electrical current--an electric field.
Concept used in power generators---large coils of wire
are made to turn in a magnetic field
The coils of wire experience a changing magnetic field
and electricity is produced.
Computer disks and audio/video tapes encode
information in magnetic patterns...
When the magnetic disk or tape material passes by
small coils of wire, electrical currents/fields are
produced.
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James Clerk Maxwell
Born/Educated in Scotland
Lived 1831—1879
Achieved a synthesis of knowledge of
electricity and magnetism of his time.
Hypothesis:
If a changing magnetic field can make an electric
field, then a changing electric field should make a
magnetic field.
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Electric and Magnetic Fields
Consequence:
Changing electric and magnetic fields should trigger
each other.
The changing fields move at a speed equal to the
speed of light.
Maxwell’s conclusion.
Light is an electromagnetic wave.
Later experiments confirmed Maxwell's theory.
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The electric and magnetic fields oscillate at right angles
to each other and the combined wave moves in a
direction perpendicular to both of the electric and
magnetic field oscillations.
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Electromagnetic Radiation
Light, electricity, and magnetism are
manifestations of the same thing called
electromagnetic radiation.
Electromagnetic radiation is a form of
energy.
This energy exists in many forms not
detectable by our eyes such as infrared (IR),
radio, X-rays, ultraviolet (UV), and gamma rays.
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EM Waves General Properties
Travels through empty space.
Most other types of waves don’t
The speed of light (EM radiation) is constant in
space.
All forms of light have the same speed of 299,800
kilometers/second in space
This number is abbreviated as c.
C = f, f = c/,  = c/f
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Wave Characteristics (1)
Amplitude (A)
8
A measure of the
strength/size of the wave.
Period (P)
Rate of repetition of a
periodic phenomenon.
f=1/P
Units: Hertz (Hz), or
cycle/s.
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Wave Amplitude
4
Duration of a cycle
Units: year, day, hours,
seconds, …
Frequency (f)
Period
6
Amplitude
2
0
0
5
10
15
20
-2
-4
-6
-8
Time
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Period/Frequency Examples
Phenomenon
Period
Frequency
Earth’s orbit around
the Sun
365 days
0.00274 days-1
Earth’s rotation
1 day or
86400 sec
1 day-1 or
1.16x10-5 Hz
Electrical Power (US)
0.0167 sec
60 Hz
Blue light
1.67x10-15 sec 6.0x1014 Hz
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Wave Characteristics (2)
Wavelength ()
Velocity (v)
Speed at which the wave
propagate through space.
v=fx
Units: m/s, miles/hour,
km/hour, etc.
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Wavelength
6
4
Wave Amplitude
Size of one cycle of the wave
in space.
Units: meter (m), centimeter
(cm), micrometer (mm),
nanometer (nm), angstrom
(A).
2
0
0
5
10
15
20
-2
-4
-6
-8
Distance (m)
15
Distance Units
1 meter = 1 m
= 100 cm (centimeter)
= 1000 mm (millimeter)
= 1000000 mm (micrometer)
= 1000000000 nm (nanometer)
= 10000000000 Å (Angstrom)
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Visible Light
color
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f (*1014 Hz)
(Å)
Energy (*10-19 J)
violet
4000
4600
7.5
6.5
5.0
4.3
indigo
4600
4750
6.5
6.3
4.3
4.2
blue
4750
4900
6.3
6.1
4.2
4.1
green
4900
5650
6.1
5.3
4.1
3.5
yellow
5650
5750
5.3
5.2
3.5
3.45
orange
5750
6000
5.2
5.0
3.45
3.3
red
6000
8000
5.0
3.7
3.3
2.5
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Color Composition
White light is made of different colors
(wavelengths).
White light passing through a prism or
diffraction grating, is spread out into its different
colors.

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First discovered by Newton
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Light Dispersion by refraction
Refraction Angle or dispersion function of the
wavelength (color)
Incidence angle
Refraction angle
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Max Planck’s Photon
Max Planck (lived 1858--1947)
Discovered that if one considers light
as packets of energy called photons,
one can accurately explain the shape
of continuous spectra.
A photon is a particle of electromagnetic
radiation.
Bizarre though it may be, light is both a
particle and a wave.
Whether light behaves like a wave or like a
particle depends on how the light is observed
it depends on the experimental setup!
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Albert Einstein’s Photon
Energy Interpretation.
Albert Einstein (lived 1879--1955)
A few years after Planck's discovery
Albert Einstein found a very simple
relationship between the energy of a light
wave (photon) and its frequency:
Energy of light = h  f = (h  c)/ 
where h is a universal constant of nature
called ``Planck's constant'' = 6.63  10-34
J·sec.
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Characterizing Light
We now have three ways to characterize
electromagnetic radiation:
wavelength
frequency
energy
Astronomers use these interchangeably.
We also divide the spectrum of all possible
wavelengths/frequencies/energies into bands
that have similar properties. Light is the most
familiar of these.
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Visible Spectrum
Small wavelength
High frequency
High energy
large wavelength
low frequency
low energy
Remember the Spectrum: ROY G BIV
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The Full Spectrum
From the highest to lowest energy
Gamma rays
X-rays
Ultraviolet
Visible
Infrared
Microwave
Radio
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EM Waves General Properties (cont’d)
A wavelength of light is defined similarly to that
of water waves
distance between crests or between troughs.
Visible light (what your eye detects)
has wavelengths 400-800 nanometers. 1 nm = 10-9
m.
Radio wavelengths are often measured in
centimeters: 1 centimeter = 10-2 meter = 0.01
meter.
The abbreviation used for wavelength is the
Greek letter lambda: 
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The Full E-M Spectrum
EM Radiation Reaching Earth
Not all wavelengths of light from space make it
to the surface.
Only long-wave UV, visible, parts of the IR and
radio bands make it to surface.
More IR reaches elevations above 9,000 feet
(2765 meters) elevation.
That is one reason why modern observatories are
built on top of very high mountains.
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Earth’s atmosphere is a shield
Blocks gamma rays, X-rays, and most UV.
Good for the preservation of life on the planet…
An obstacle for astronomers who study the sky in
these bands.
Blocks most of the IR and parts of the radio.
Astronomers unable to detect these forms of energy
from celestial objects from the ground
Must resort to very expensive satellite observatories
in orbit.
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Types of Spectra
Continuous spectra consist of all frequencies
(colors) - the thermal or blackbody spectrum is
the most common example we will see.
Absorption line spectra are continuous spectra
with certain missing certain frequencies.
Emission line spectra are a series of discrete
frequencies (with or without a continuous
spectra).
Often astronomers deal with combinations of
the above.
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Black Body Spectrum
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Star Color vs.
Temperature
Discrete Spectra
Close examination of the spectra from the Sun and
other stars reveals that the rainbow of colors has
many dark lines in it, called absorption lines.
They are produced by the cooler thin gas in the upper
layers of the stars absorbing certain colors of light
produced by the hotter dense lower layers.
The spectra of hot, thin (low density) gas clouds are a
series of bright lines called emission lines.
In both of these types of spectra you see spectral
features at certain, discrete wavelengths (or colors)
and no where else.
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Absorption Line Spectrum
Spectra
The type of spectrum you see depends on the
temperature of the thin gas.
If the thin gas is cooler than the thermal source in
the background, you see absorption lines.
Since the spectra of stars show absorption lines, it
tells you that the density and temperature of the
upper layers of a star is lower than the deeper
layers.
In a few cases you can see emission lines on top of
the thermal spectrum. This is produced by thin gas
that is hotter than the thermal source in the
background.
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Spectra (cont’d)
The spectrum of a hydrogen-emission nebula
(``nebula'' = gas or dust cloud) is just a series of
emission lines without any thermal spectrum
because there are no stars visible behind the
hot nebula.
Some objects produce spectra that are a
combination of a thermal spectrum, emission
lines, and absorption lines simultaneously!
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Spectra (cont’d)
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The Structure of the Atom
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Bohr atom
Explanation for the discrete line spectra …
Niels Bohr (lived 1885--1962) provided the
explanation in the early 20th century.
Electrons only exist in certain energy levels and as
long as an electron stays in a particular energy
level, it doesn’t emit any energy (photons).
If an electron changes energy levels, it emits or
absorbs energy in the form of a photon.
Set of energies (light frequencies) uniquely identify
the type of atom!
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Bohr’s Model
In Bohr's model of the atom, the massive but small positivelycharged protons and massive but small neutral neutrons are
found in the tiny nucleus.
The small, light negatively-charged electrons move around the
nucleus in certain specific orbits (energy levels).
In a neutral atom the number of electrons = the number of
protons.
The arrangement of an atom's energy levels (orbits) depends
on the number of protons (and neutrons) in the nucleus and the
number of electrons orbiting the nucleus.
Because every type of atom has a unique arrangement of
energy levels, they produce a unique pattern of absorption or
emission lines.
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Isotopes
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How is light produced?
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Absorption Line Spectra
Doppler Effect
The wave nature of light means there will be a shift in the
spectral lines of an object if it is moving.
Sound Waves: pitch  frequency of wave
Changes the pitch of the sound coming from something moving
toward you or away from you
train whistle, police siren
Sounds from objects moving toward you are at a higher pitch
because the sound waves are compressed together, shortening
the wavelength of the sound waves.
Sounds from objects moving away from you are at a lower pitch
because the sound waves are stretched apart, lengthening the
wavelength.
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Sound Waves
• Spread uniformly from a sound source
• Circles -- crests of the sound waves
• Think of waves spreading from a pebble
dropped into a pool
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Doppler Shift
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Moving towards
Moving away
Short wavelength
longer wavelength
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Doppler Shift  Speed of Source
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Red shift, Blue shift
Motion of the light source causes the spectral lines to
shift positions.
Which way the spectral lines shift tells you if the object
is moving toward or away from you.
Blue shift: If the object is moving toward you, the waves are
compressed, so their wavelength is shorter. The lines are
shifted to shorter (bluer) wavelengths.
Red shift: If the object is moving away from you, the waves
are stretched out, so their wavelength is longer. The lines
are shifted to longer (redder) wavelengths.
The doppler effect will not affect the overall color of an
object unless it is moving at a significant fraction of the
speed of light (VERY fast!)
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Doppler Shifted Spectra
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Doppler Shift (5)
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Doppler Shift (6)
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Expanding Universe
The doppler effect tells us about the relative
motion of stars with respect to us.
The spectral lines of nearly all of the galaxies in
the universe are shifted to the red end of the
spectrum.
These galaxies are moving away from our
Milky Way galaxy.
This is evidence for the expansion of the
universe.
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