Chapter 7
Starlight and Atoms
Guidepost
Some chapters in textbooks do little more than present
facts. The chapters in this book attempt to present
astronomy as organized understanding. But this chapter
is special. It presents us with a tool. The interaction of
light with matter gives astronomers clues about the
nature of the heavens, but the clues are meaningless
unless astronomers understand how atoms leave their
traces on starlight. Thus, we dedicate an entire chapter
to understanding how atoms interact with light.
This chapter marks a transition in the way we look at
nature. Earlier chapters described what we see with our
eyes and explained those observations using models
and theories. With this chapter, we turn to modern
astrophysics, the application of physics to the study of
Guidepost (continued)
the sky. Now we can search out secrets of the stars that
lie beyond the grasp of our eyes.
If this chapter presents us with a tool, then we should
use it immediately. The next chapter will apply our new
tool to understanding the sun.
Outline
I. Starlight
A. Temperature and Heat
B. The Origin of Starlight
C. Two Radiation Laws
D. The Color Index
II. Atoms
A. A Model Atom
B. Different Kinds of Atoms
C. Electron Shells
III. The Interaction of Light and Matter
A. The Excitation of Atoms
B. The Formation of a Spectrum
Outline (continued)
IV. Stellar Spectra
A. The Balmer Thermometer
B. Spectral Classification
C. The Composition of the Stars
D. The Doppler Effect
E. Calculating the Doppler Velocity
F. The Shapes of Spectral Lines
The Amazing Power of Starlight
Just by analyzing the light received from a
star, astronomers can retrieve information
about a star’s
1. Total energy output
2. Surface temperature
3. Radius
4. Chemical composition
5. Velocity relative to Earth
6. Rotation period
Color and Temperature
Stars appear in
different colors:
• blue (like Rigel)
Orion
Betelgeuse
• green / yellow
(like our sun)
• red (like
Betelgeuse).
These colors
tell us about
the star’s
temperature.
Rigel
Black Body Radiation
The light from a star is mostly
ultraviolent (sunburn!), visual
(roygbiv!), and infrared (heat).
The star’s light is nearly a
black body spectrum.
Two laws of blackbodies:
1. The hotter an object is,
the more luminous it is
2. The hotter the object is the
more the black body spectrum
shifts towards shorter
wavelengths.
The Color Index
The color of a star is
measured by comparing its
brightness in the blue (B)
band and the visual (V) band.
A color index measures both
B-band & V-band magnitude,
and takes the difference
(B minus V, or simply B – V).
The hotter (more blue) a star,
the smaller the color index:
Blue stars: B – V = -0.4
50,000 degrees Kelvin!
Red stars: B – V = +2.0
2,000 degrees Kelvin
B band
V band
Light and Matter
Spectra of stars are
more complicated
than pure blackbody
spectra because of
characteristic
absorption lines.
To understand
those lines, we
need to
understand atomic
structure and the
interactions
between light and
atoms.
Atomic Structure
video clip
video clip
• An atom consists of
an atomic nucleus
The nucleus has
positive protons and
neutral neutrons, and
a cloud of electrons
surrounding it.
• Almost all of the
mass is contained
in the nucleus,
while almost all of
the space is
occupied by the
electron cloud.
• The nucleus is so dense
that a teaspoon of it would
weigh about 2 billion tons!!
Different Kinds of Atoms
The kind of atom
depends on the number
of protons in the nucleus:
• Hydrogen (H), has one
proton (and 1 electron).
• Helium (He), has 2
protons (and 2 neutrons
& 2 electrons).
Atoms can collide and
bond into molecules,
but only in “cool” stars.
If an atom loses or
gains electrons, it is
called an ion.
Different
numbers of
neutrons =
different
isotopes
Electron Orbits
• Electron orbits in the electron cloud are
restricted to very specific radii and energies.
r3, E3
r2, E2
r1, E1
• These characteristic electron energies are
different for each individual element.
Atomic Transitions
• An electron can
be kicked into a
higher orbit in a
collision or when
it absorbs a
photon with the
right
energy/wavelen
• gth.
The photon is
absorbed, and
the electron is in
an excited state
• When the electron
returns to the
ground state it will
emit a photon.
video clip
Ephoton = E3 – E1
Ephoton = E4 – E1
• The spectrum of a star forms
as light passes outward
through gases near its surface
Kirchhoff’s Laws of Radiation (1)
1. A solid, liquid, or dense gas excited to emit
light will radiate at all wavelengths and thus
produce a continuous spectrum.
Kirchhoff’s Laws of Radiation (2)
2. A low-density gas excited to emit light will
do so at specific wavelengths and thus
produce an emission spectrum.
Light excites electrons in
atoms to higher energy states
Transition back to lower states emits light at specific wavelengths
Kirchhoff’s Laws of Radiation (3)
3. If light comprising a continuous spectrum
passes through a cool, low-density gas,
the result will be an absorption spectrum.
Light excites electrons in
atoms to higher energy states
Wavelengths of light corresponding to the transition
energies are absorbed from the continuous spectrum.
The Spectra of Stars
video clip
Inner, dense layers of a
star produce a continuous
(blackbody) spectrum.
Cooler surface layers absorb light at specific frequencies.
Therefore, spectra of stars are absorption spectra.
Kirchhoff’s Laws
(SLIDESHOW MODE ONLY)
Analyzing Absorption Spectra
• Each element produces a specific set of
absorption and emission lines.
• Comparing the relative strengths of these sets
of lines, we can study the gases from stars.
Where to
start?
With the
most
abundant
elements
in the
universe!
Lines of Hydrogen
Most prominent lines
in many astronomical
objects are Balmer
lines of hydrogen
Observations of the H-Alpha Line
Emission nebula, dominated by
the red hydrogen alpha (H ) line.
Absorption Spectrum Dominated
by Balmer Lines
Modern spectra are usually
recorded digitally and
represented as plots of intensity
vs. wavelength
The Balmer Thermometer
Balmer line strength tells us star temperature
In medium temp. stars (10,000K),
Balmer lines are strong
In cooler stars
(<<10,000 K)
almost all
hydrogen atoms
are in the ground
state, so Balmer
lines are weak.
In hotter stars (>> 10,000K) most hydrogen
atoms are ionized, so Balmer lines are weak
Measuring the Temperatures of Stars
A star’s surface temperature is
measured by comparing many lines
A very hot star (40,000K)
has weak Balmer lines and
strong ionized helium lines.
A very cool star (3,000K)
has weak Balmer lines and
strong titanium oxide lines.
Spectral Classification of Stars (1)
Temperature
Each spectral class divides Different types of stars show different
into 10 subclasses (0 to 9) characteristic sets of absorption lines.
Spectral Classification of Stars (2)
Mnemonics to
remember the
spectral
sequence:
Oh
Oh
Only
Be
Boy,
Bad
A
An
Astronomers
Fine
F
Forget
Girl/Guy
Grade
Generally
Kiss
Kills
Known
Me
Me
Mnemonics
Stellar Spectra
F
G
K
M
Surface temperature
O
B
A
The Composition of Stars
From the relative strength of absorption lines (carefully
accounting for their temperature dependence), one can
infer the composition of stars.
The Doppler Effect
higher
pitch
lower
pitch
Blue Shift (to higher
frequencies)
Red Shift (to lower
frequencies)
Waves from a
source are shifted
in observed
frequency when
the
source/observer
move toward each
other.
Light of different
frequency is seen
as a different color.
Increase in
observed
frequency is called
a blue shift.
Decrease in
observed
frequency is called
a red shift.
Doppler Shift
If a star is moving toward Earth, the lines in its spectrum are
shifted slightly toward shorter wavelength (higher frequency).
This shifts the absorption lines toward the blue end of the
spectrum, so it’s called a blue shift.
If a star is moving away from Earth, the lines in its spectrum are
shifted slightly toward the longer wavelength (lower frequency).
This creates a red shift in the absorption spectrum.
New Terms
temperature
Kelvin temperature scale
absolute zero
thermal energy
electron
black body radiation
wavelength of maximum
intensity (λmax)
color index
nucleus
proton
neutron
isotope
ionization
ion
molecule
Coulomb force
binding energy
quantum mechanics
permitted orbit
energy level
excited atom
ground state
continuous spectrum
absorption spectrum
(dark-line spectrum)
absorption line
emission spectrum (brightline spectrum)
emission line
Kirchhoff’s laws
transition
Lyman series
Balmer series
Paschen series
spectral class or type