WHAT DO YOU THINK?
1.
2.
3.
Which is hotter, a “red-hot” or a “blue-hot”
object?
What color does the Sun emit most brightly?
How can we determine the age of space debris
found on Earth?
In this chapter you will discover…

properties of electromagnetic radiation related to
TEMPERATURE, COMPOSITION, & MOTION

the structure of atoms
stars with different surface temperatures emit different
intensities of electromagnetic radiation


astronomers determine chemical compositions of stars
and interstellar clouds by studying the wavelengths of
electromagnetic radiation that they absorb or emit

The importance of the DOPPLER shift to indicate motion
Different COLOR indicates different temperature
Properties of Thermal Radiation
1. Radiation is emitted in ALL wavelengths without
breaks.
“Continuous Spectrum”
Properties of Thermal Radiation
2. Hotter objects emit much, much, much, MUCH more
light at all frequencies per unit area.
3. Hotter objects emit bluer photons with a higher
average energy.
What are these?
Absorption Spectrum of Sun
Emission Line spectrum of Iron in laboratory on Earth
Absorption Spectrum of Hydrogen Gas
Emission Spectrum of Hydrogen Gas
Spectral “Fingerprints”
Absorption
Emission
Three TYPES of Spectra!
Continuous Spectrum

The spectrum of a common (incandescent)
light bulb spans all visible wavelengths,
without interruption.
Emission Line Spectrum

A thin or low-density cloud of gas emits light only at
specific wavelengths that depend on its composition
and temperature, producing a spectrum with bright
emission lines.
Absorption Line Spectrum

A cloud of gas between us and a light bulb can
absorb light of specific wavelengths, leaving dark
absorption lines in the spectrum.
What are the three basic types
of spectra?
Continuous Spectrum
Emission Line Spectrum
Absorption Line Spectrum
Spectra of astrophysical objects are usually combinations of
these three basic types.
Chemical Fingerprints
Energy levels of hydrogen

Each type of
atom has a
unique set of
energy levels.

Each transition
corresponds to a
unique photon
energy,
frequency, and
wavelength.
Why Spectral Lines???
Atoms have nuclei (protons & neutrons) &
electrons in specific energy level patterns
 Each element has different NUMBERS of
electrons, in different energy level patterns
 Transitions of electrons absorbing or
emitting energy produce different
wavelengths of light we see.

Simple “Model” of Atom
Electron Energy
Levels
Nucleus
Chemical Fingerprints

Electron
Transitions to
LOWER energy
states produce a
unique pattern of
emission lines.
Chemical Fingerprints

Atoms can also
absorb photons
with same
energies.

Transitions to
higher energy
states produce a
pattern of
absorption lines
at the same
wavelengths.
Spectroscopy works at ALL wavelengths!
Examples of Spectra!
Emission Spectra from Hydrogen & Oxygen
Thought Question
Which letter(s) labels absorption lines?
A
B
C
D E
Thought Question
Which letter(s) labels absorption lines?
A
B
C
D
E
Thought Question
Which letter(s) labels the peak (greatest
intensity) of infrared light?
A
B
C
D E
Thought Question
Which letter(s) labels the peak (greatest
intensity) of infrared light?
A
B
C
D E
Thought Question
Which letter(s) labels emission lines?
A
B
C
D E
Thought Question
Which letter(s) labels emission lines?
A
B
C
D E
Interpreting an Actual
Spectrum

By carefully studying the features in a
spectrum, we can learn a great deal
about the object that created it.
What is this object?
Reflected Sunlight: Continuous spectrum of visible light is
like the Sun’s except that some of the blue light has been
absorbed—object must look red
What is this object?
Thermal Radiation: Infrared spectrum peaks at a
wavelength corresponding to a temperature of 225 K
What is this object?
Carbon Dioxide: Absorption lines are the
fingerprint of CO2 in the atmosphere
What is this object?
Ultraviolet Emission Lines:
Indicate a hot upper
atmosphere
What is this object?
Mars!
How does light tell us the speed
of a distant object?
The Doppler Effect
Doppler shift tells us ONLY about the part of an
object’s motion toward or away from us.
Measuring the Shift
Stationary
Moving Away
Away Faster
Moving Toward
Toward Faster

We generally measure the Doppler effect from
shifts in the wavelengths of spectral lines.
Thought Question
I measure a line in the lab at 500.7 nm. The
same line in a star has wavelength 502.8 nm.
What can I say about this star?
•
•
•
It is moving away from me.
It is moving toward me.
It has unusually long spectral lines.
Thought Question
I measure a line in the lab at 500.7 nm. The
same line in a star has wavelength 502.8 nm.
What can I say about this star?
•
•
•
It is moving away from me.
It is moving toward me.
It has unusually long spectral lines.
Summary of Key Ideas
By studying the wavelengths of electromagnetic
radiation emitted and absorbed by an astronomical
object, astronomers can learn about its
temperature, chemical composition, rotation rate,
companion objects, and movement through space.
Blackbody Radiation


A blackbody is a hypothetical object that perfectly
absorbs electromagnetic radiation at all wavelengths.
The relative intensities of radiation that it emits at
different wavelengths depend only on its temperature.
Stars closely approximate blackbodies.
Wien’s law states that the peak wavelength of radiation
emitted by a blackbody is inversely proportional to its
temperature—the higher its temperature, the shorter the
peak wavelength. The intensities of radiation emitted at
various wavelengths by a blackbody at a given
temperature are shown as a blackbody curve.
Blackbody Radiation


The Stefan-Boltzmann law shows that a hotter blackbody
emits more radiation at every wavelength than does a
cooler blackbody.
The motion of an object toward or away from an
observer causes the observer to see all of the colors
from the object to blueshift or redshift, respectively. This
is generically called a Doppler shift.
Discovering Spectra



Spectroscopy—the study of electromagnetic spectra—
provides important information about the chemical
composition of remote astronomical objects.
Kirchhoff’s three laws of spectral analysis describe the
conditions under which absorption lines, emission lines,
and a continuous spectrum can be observed.
Spectral lines serve as distinctive “fingerprints” that
identify the chemical elements and compounds
comprising a light source.
Atoms and Spectra




An atom consists of a small, dense nucleus (composed
of protons and neutrons) surrounded by electrons. Atoms
of different elements have different numbers of protons,
while different isotopes have different numbers of
neutrons.
Quantum mechanics describes the behavior of particles
and shows that electrons can only be in certain allowed
orbits around the nucleus.
The nuclei of some atoms are stable, while others
(radioactive ones) spontaneously split into pieces.
The spectral lines of a particular element correspond to
the various electron transitions between allowed orbits of
that element with different energy levels. When an
electron shifts from one energy level to another, a photon
of the appropriate energy (and hence a specific
wavelength) is absorbed or emitted by the atom.
Atoms and Spectra




The spectrum of hydrogen at visible wavelengths
consists of part of the Balmer series, which arises from
electron transitions between the second energy level of
the hydrogen atom and higher levels.
Every different element, isotope, and molecule has a
different set of spectral lines.
When an atom loses or gains one or more electrons it is
said to be charged. The atom loses an electron when the
electron absorbs a sufficiently energetic photon, which
rips it away from the nucleus.
The equation that describes the Doppler effect states
that the size of a wavelength shift is proportional to the
radial velocity between the light source and the observer.
Key Terms
absorption line
absorption line spectrum
atom
atomic number
blackbody
blackbody curve
blueshift
continuous spectrum
diffraction grating
Doppler shift
electromagnetic force
electron
element
emission line
emission line spectrum
energy flux
excited state
ground state
ion
ionization
isotope
Kirchhoff’s laws
luminosity
molecule
neutron
nucleus (of an atom)
periodic table
Planck’s law
proper motion
proton
quantum mechanics
radial velocity
radioactive
redshift
spectral analysis
spectrograph
spectroscope
Stefan-Boltzmann law
strong nuclear force
transition (of an
electron)
transverse velocity
weak nuclear force
Wien’s law
WHAT DID YOU THINK?


Which is hotter, a “red-hot” or a “blue-hot” object?
Of all objects that glow visibly from heat generated inside
them, those that glow red are the coolest.
WHAT DID YOU THINK?


What color does the Sun emit most brightly?
The Sun emits all wavelengths of electromagnetic
radiation. The colors it emits most intensely are in the
blue-green part of the spectrum. Because the human
eye is less sensitive to blue-green than to yellow, and
Earth’s atmosphere scatters blue-green wavelengths
more readily than longer wavelengths, we normally see
the Sun as yellow.
WHAT DID YOU THINK?


How can we determine the age of space debris found on
Earth?
We measure how much the long-lived radioactive
elements, such as 238U, have decayed in the object.
Carbon dating is only reliable for things that formed
within the past 100,000 years. It cannot be used for
determining the age of debris found on Earth, which was
all formed more than 4.5 billion years ago.