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Announcements:
-Skipping section 12.4
Extra Credit opportunity!!
Friday, April 17, 2007
Telescope Viewing at Evergreen Valley College
for more information and directions:
http://www.evc.edu
You will need to get handout from class (or on
class website) and complete it at the event in
order to get credit!!
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Chapter 12
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Measuring the Properties of Stars (Chp. 12)
• Distance from the Sun
• Temperature
• Luminosity (magnitudes)
• Mass
• Composition
• Rotation
• How we classify and categorize them
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Astronomers can determine the composition of a
star many light-years away by:
a) sending probes to collect atmospheric
material from the star and analyzing it with
a mass spectrometer.
b) Observing the spectrum of the star and
applying Wien’s law.
c) Observing the absorption line spectrum and
matching up lines with the laboratory
spectra of various elements.
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How would you design a mission that could
exceed the performance of the Hipparcos
satellite?
a) Use a larger baseline: the orbital diameter
of Mars, for example.
b) Use a larger baseline: select more distant
stars, for example.
c) Use a smaller baseline for higher accuracy.
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Which unit of distance is larger, the light-year or the
parsec?
a) light-year
b) parsec
c) The parsec is a measure of angle, not distance
d) They are the same.
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READING QUIZ
Stars are classified according to the types of
absorption lines they have. The classification
nomenclature is
a) Based upon a numeric scheme running
backwards from 25 to 1 (and missing some
numbers).
b) Based upon roman numerals one through ten
(I, II, III, IV, V, VI, VII, VIII, IX, and X)
c) Based upon an alphabetic scheme that does
not include all of the letters and is not
necessarily in alphabetic order.
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Inverse-Square Law of Brightness
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The Luminosity of a Star
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Stefan-Boltzmann law
Figure 12.5
L  Energy emitted by 1 sq meter  Number of sq meters on its surface
=
10
 T4

4 R 2
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Brightness: energy per unit area
-intrinsic
-apparent
Luminosity: Total energy emitted
-intrinsic
-apparent
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Star A has a radius R and a temperature T. Star
B has the same temperature and is at the same
distance from the Sun as star A. However, it is 4
times larger than star A. How many times more
luminous is star B?
a) 16
b) 4
c) both have the same luminosity
d) not enough information to tell
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Star A has a radius R and a temperature T. Star
B is at the same distance from the Sun as star A.
However, it is 4 times larger than star A and two
times cooler. How many times more luminous is
star B?
a) 16
b) 4
c) both have the same luminosity
d) not enough information to tell
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Classification of Stellar Spectra
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Stellar spectra are not all the same!
A
B
C
What causes these differences?
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Plotting a Spectrum
Absorption lines
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Annie Jump Cannon:
• Hired as an assistant in 1896
by Edward Pickering of
Harvard College Obervatory.
• Classified stars by their
spectra.
• Looked at hundreds of
thousands of spectra.
• Published the Draper
Catalogue.
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Letters ordered by decreasing temperature:
hot
cold
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What causes such large differences in spectra?
• 1920’s: Cecilia Payne
• PhD thesis, Harvard
College (1925)
• Differences due to
temperature, not
composition!
• All stars made mostly of
hydrogen (very
controversial!)
•
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O:
B:
A:
F:
G:
K:
M:
Ionized helium
Neutral helium, stronger hydrogen
Very strong hydrogen lines
Absorption lines from other elements (metals) begin
to appear (like ionized Calcium)
Weaker hydrogen, neutral metals
Some lines from molecules begin to appear
Lots of lines from molecules like TiO, water
Sun is a G2!
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The HR Diagram
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The Hertzsprung-Russell Diagram
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Constructing an H-R diagram requires that you
know the absolute (intrinsic) luminosity.
You can compute this quantity by measuring
the apparent luminosity and determining the
distance to the star.
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The H-R diagram
Figure 12.16
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Stellar luminosity classes
I
II
III
IV
V
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Supergiants
Bright Giants (not widely used)
Giants
Subgiants
Dwarfs (Main Sequence Stars)
White Dwarfs
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