Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).

1
Note that the following lectures include
animations and PowerPoint effects such as fly ins
and transitions that require you to be in
PowerPoint's Slide Show mode (presentation mode).
2
Starlight and Atoms

• Chapter 7

3
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
4
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.
5
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
6
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
7
The Amazing Power of Starlight
Just by analyzing the light received from a star,
astronomers can retrieve information about a
stars

1. Total energy output
2. Surface temperature
3. Radius
4. Chemical composition
5. Velocity relative to Earth
6. Rotation period

8
Color and Temperature
Stars appear in different colors, from blue
(like Rigel) via green / yellow (like our sun)
to red (like Betelgeuse). These colors tell us
about the stars temperature.
Orion
Betelgeuse
Rigel
9
Black Body Radiation (1)
The light from a star is usually concentrated in
a rather narrow range of wavelengths. The
spectrum of a stars light is approximately a
thermal spectrum called a black body spectrum. A
perfect black body emitter would not reflect any
radiation. Thus the name black body.
10
Two Laws of Black Body Radiation
1. The hotter an object is, the more luminous it
is
L AsT4
where
A surface area
s Stefan-Boltzmann constant
2. The peak of the black body spectrum shifts
towards shorter wavelengths when the temperature
increases. ? Wiens displacement law
lmax 3,000,000 nm / TK (where TK is the
temperature in Kelvin).
11
The Color Index (1)
The color of a star is measured by comparing its
brightness in two different wavelength bands The
blue (B) band and the visual (V) band. We define
B-band and V-band magnitudes just as we did
before for total magnitudes (remember a larger
number indicates a fainter star).
B band
V band
12
The Color Index (2)
We define the Color Index B V (i.e., B
magnitude V magnitude). The bluer a star
appears, the smaller the color index B V. The
hotter a star is, the smaller its color index B
V.
13
Light and Matter
Spectra of stars are more complicated than pure
blackbody spectra.
? characteristic lines, called absorption lines.
To understand those lines, we need to understand
atomic structure and the interactions between
light and atoms.
14
Atomic Structure

• An atom consists of an atomic nucleus (protons
  and 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.

15
Atomic Density
If you could fill a teaspoon just with material
as dense as the matter in an atomic nucleus, it
would weigh 2 billion tons!!
16
Different Kinds of Atoms

• The kind of atom depends on the number of protons
  in the nucleus.

Different numbers of neutrons ? different isotopes

• Most abundant Hydrogen (H), with one proton ( 1
  electron).

• Next Helium (He), with 2 protons (and 2 neutrons
  2 el.).

Helium 4
17
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.

18
Atomic Transitions

• An electron can be kicked into a higher orbit
  when it absorbs a photon with exactly the right
  energy.

Eph E3 E1
Eph E4 E1
Wrong energy

• The photon is absorbed, and
• the electron is in an excited state.

(Remember that Eph hf)

• All other photons pass by the atom unabsorbed.

19
Kirchhoffs 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.

20
Kirchhoffs 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 frequencies
21
Kirchhoffs 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
Frequencies corresponding to the transition
energies are absorbed from the continuous
spectrum.
22
The Spectra of Stars
Inner, dense layers of a star produce a
continuous (blackbody) spectrum.
Cooler surface layers absorb light at specific
frequencies.
gt Spectra of stars are absorption spectra.
23
Kirchhoffs Laws
(SLIDESHOW MODE ONLY)
24
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 composition of gases.

By far the most abundant elements in the Universe
25
Lines of Hydrogen
Most prominent lines in many astronomical
objects Balmer lines of hydrogen
26
The Balmer Lines
n 1
n 4
Transitions from 2nd to higher levels of hydrogen
n 5
n 3
n 2
Ha
Hb
Hg
The only hydrogen lines in the visible wavelength
range.
2nd to 3rd level Ha (Balmer alpha line)
2nd to 4th level Hb (Balmer beta line)

27
Observations of the H-Alpha Line
Emission nebula, dominated by the red Ha line.
28
Absorption Spectrum Dominated by Balmer Lines
Modern spectra are usually recorded digitally and
represented as plots of intensity vs. wavelength
29
The Balmer Thermometer
Balmer line strength is sensitive to temperature
Most hydrogen atoms are ionized gt weak Balmer
lines
Almost all hydrogen atoms in the ground state
(electrons in the n 1 orbit) gt few transitions
from n 2 gt weak Balmer lines
30
Measuring the Temperatures of Stars
Comparing line strengths, we can measure a stars
surface temperature!
31
Spectral Classification of Stars (1)
Different types of stars show different
characteristic sets of absorption lines.
Temperature
32
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
33
Stellar Spectra
O
B
A
F
Surface temperature
G
K
M
34
The Composition of Stars
From the relative strength of absorption lines
(carefully accounting for their temperature
dependence), one can infer the composition of
stars.
35
The Doppler Effect
The light of a moving source is blue/red shifted
by
Dl/l0 vr/c
l0 actual wavelength emitted by the source Dl
Wavelength change due to Doppler effect vr
radial velocity
Blue Shift (to higher frequencies)
Red Shift (to lower frequencies)
vr
36
The Doppler Effect (2)
The Doppler effect allows us to measure the
sources radial velocity.
vr
37
The Doppler Effect (3)

• Take l0 of the Ha (Balmer alpha) line
• l0 656 nm

Assume, we observe a stars spectrum with the Ha
line at l 658 nm. Then, Dl 2 nm.
We find Dl/l0 0.003 310-3
Thus, vr/c 0.003, or vr 0.003300,000 km/s
900 km/s.
The line is red shifted, so the star is receding
from us with a radial velocity of 900 km/s.
38
Doppler Broadening
In principle, line absorption should only affect
a very unique wavelength.
In reality, also slightly different wavelengths
are absorbed.
? Lines have a finite width we say they are
broadened.
Blue shifted abs.
Red shifted abs.
One reason for broadening The Doppler effect!
vr
Observer
vr
Atoms in random thermal motion
39
Line Broadening

• Higher Temperatures
• Higher thermal velocities
• ? broader lines

Doppler Broadening is usually the most important
broadening mechanism.
40
New Terms
temperature Kelvin temperature scale absolute
zero thermal energy electron black body
radiation wavelength of maximum intensity
(?max) 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
(bright-line spectrum) emission line Kirchhoffs
laws transition Lyman series Balmer
series Paschen series spectral class or type
41
New Terms (continued)
spectral sequence L dwarf T dwarf Doppler
effect blue shift red shift radial velocity
(Vr) transverse velocity line profile Doppler
broadening collisional broadening density
42
Discussion Questions
1. In what ways is our model of an atom a
scientific model? How can we use it when it is
not a completely correct description of an atom?
2. Can you think of classification systems we
commonly use to simplify what would otherwise be
very complex measurements? Consider foods,
movies, cars, grades, clothes, and so on.
43
Quiz Questions
1. Which of the following statements is true
about the Celsius and Kelvin (Absolute)
temperature scales? a. Zero is at the same
temperature on both scales. b. The size of one
degree is the same on both scales. c. Zero
degrees Celsius is the same temperature as -273
K. d. The size of one Celsius degree is 5/9 that
of a Kelvin. e. The size of one Kelvin is 5/9
that of a Celsius degree.
44
Quiz Questions
2. The temperature of a gas is a measure of
the a. total amount of internal energy in the
gas. b. amount of heat that flows out of the
gas. c. total number of atoms in the gas. d.
density of the gas. e. average motion of its
atoms.
45
Quiz Questions
3. Which subatomic particle has a negative
charge? a. The electron. b. The proton. c. The
neutron. d. Both a and b above. e. Both a and c
above.
46
Quiz Questions
4. The wavelength of maximum intensity that is
emitted by a black body is a. proportional to
temperature. b. inversely proportional to
temperature. c. proportional to temperature to
the fourth power. d. inversely proportional to
temperature to the fourth power. e. Both a and c
above.
47
Quiz Questions
5. Of the following, which color represents the
lowest surface temperature star? a. Yellow. b.
Blue. c. Orange. d. Red. e. White.
48
Quiz Questions
6. The amount of electromagnetic energy radiated
from every square meter of the surface of a
blackbody each second is a. proportional to
temperature. b. inversely proportional to
temperature. c. proportional to temperature to
the fourth power. d. inversely proportional to
temperature to the fourth power. e. Both a and c
above.
49
Quiz Questions
7. The B - V color index of a star indicates
its a. density. b. total mass. c. radius. d.
chemical composition. e. surface temperature.
50
Quiz Questions
8. If a star appears brighter through a B filter
than it does through a V filter, its B - V color
index is a. negative. b. zero. c. positive. d.
greater than or equal to zero. e. less than or
equal to zero.
51
Quiz Questions
9. An atom that is ionized must have a. more
neutrons than protons. b. more protons than
neutrons. c. more electrons than protons. d. more
protons than electrons. e. Either c or d above.
52
Quiz Questions
10. Which of the following is true of an atomic
nucleus? a. It contains more than 99.9 of an
atoms mass. b. It contains all of an atom's
positive charge. c. It contains no electrons. d.
Both a and b above. e. All of the above.
53
Quiz Questions
11. At what energy level are the electrons in
hydrogen gas at a temperature of 25,000 K? a.
Most are in energy level 1 (also known as the
ground state). b. Most are in energy level 2. c.
Most are in levels higher than energy level 2. d.
Half are in energy level 1, and half are in level
2. e. None of the above.
54
Quiz Questions
12. What conditions produce a dark (absorption
line) spectrum? a. A hot solid, liquid, or
high-density gas. b. A hot low-density gas. c.
Light from a continuous spectrum source passing
through a cooler low-density gas. d. Both a and b
above. e. All of the above.
55
Quiz Questions
13. Where is the location of the cooler
low-density gas that yields the dark (absorption)
line stellar spectra that were studied by Annie
Jump Cannon? a. In the interior of the star. b.
In the star's lower atmosphere. c. In Earth's
atmosphere. d. Both a and b above. e. Both b and
c above.
56
Quiz Questions
14. Which electron energy level transition
corresponds to a hydrogen atom absorbing a
visible-light photon that has a wavelength of 656
nanometers? a. The electron makes the
transition from energy level 1 to energy level
2. b. The electron makes the transition from
energy level 2 to energy level 1. c. The electron
makes the transition from energy level 2 to
energy level 3. d. The electron makes the
transition from energy level 3 to energy level
2. e. The electron makes the transition from
energy level 3 to energy level 4.
57
Quiz Questions
15. What does the presence of molecular bands in
the spectrum of a star indicate? a. The star has
a low surface temperature. b. The star has a high
surface temperature. c. The star is about to go
supernova. d. The star is spectral type G. e. The
star is spectral type TiO.
58
Quiz Questions
16. Of the following spectral types, which one
represents a star with the highest surface
temperature? a. A b. B c. F d. K e. G
59
Quiz Questions
17. All stars are composed of mostly hydrogen and
helium, yet many stars have no lines for hydrogen
or helium in their spectrum. What causes this
apparent contradiction? a. Spectral lines are
created in the lower atmospheres of stars, and
for many stars hydrogen and helium are hidden
below the atmosphere. b. The upper layers of a
star contain hot low-density gases that produce
bright lines at precisely the same wavelengths as
the dark lines, thus making them invisible. c.
Hot hydrogen and helium gas in the interstellar
medium produces bright lines to fill in the dark
lines. d. The resolution of many spectrographs is
too poor to show the extremely thin spectral
lines for hydrogen and helium. e. The surface
temperature is such that the electrons are not at
the proper energy levels to produce spectral
lines at visible wavelengths.
60
Quiz Questions
18. You research the star Sirius and find that
its spectral lines are blue shifted. What does
this tell you about Sirius? a. Its surface
temperature is higher than that of the Sun. b. It
has a transverse velocity that is away from
us. c. It has a transverse velocity that is
toward us. d. It has a radial velocity that is
away from us. e. It has a radial velocity that is
toward us.
61
Quiz Questions
19. Suppose that you take the spectrum of several
stars and identify the 656-nanometer line of
hydrogen. You then measure against the reference
spectrum on the same image and find that some of
the 656-nm lines are shifted due to the Doppler
Effect. Of the following shifted locations of
this line, which one signals a star that is
moving away from us at the highest speed? a.
Star A _at_ 655 nm. b. Star B _at_ 657 nm. c. Star C _at_
658 nm. d. Star E _at_ 659 nm. e. Star D _at_ 654 nm.
62
Quiz Questions
20. What property of a star can broaden the width
of its spectral lines? a. Rapid rotation of the
star. b. High-density atmosphere. c.
High-temperature atmosphere. d. Both b and c
above. e. All of the above.
63
Answers
1. b 2. e 3. a 4. b 5. d 6. c 7. e 8. a 9. e 10. e
11. c 12. c 13. e 14. c 15. a 16. b 17. e 18. e 19
. d 20. e