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Phys1403 – Introductory Astronomy
Instructor: Dr. Goderya
Topics for Today’s Class
Foundations of Astronomy | 13e
Seeds
• H-R Diagram
• Luminosity
Classes
Chapter 9
• Worksheet
Family of the Stars
© Cengage Learning 2016
Luminosity Equation
The Hertzsprung-Russell Diagram Part 1
• We know that stars have different:
Quantitatively:
L = 4  R2  T4
Surface area of the star
Surface flux due to a
blackbody spectrum
The Heart of Astrophysics
– Temperatures
– Luminosities
– Sizes
• To order them, we use the HertzsprungRussell (H-R) diagram
– Luminosity (or absolute magnitude) versus
temperature (or spectral type)
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The Hertzsprung-Russell Diagram
• In an H–R diagram, a star is
represented by a dot at a
position that shows the star’s
luminosity and temperature.
• The background color in this
diagram indicates the
temperature of the stars.
• The Sun is a yellow-white G2
star.
• Most stars including the Sun
have properties along the mainsequence strip running from hot
high-luminosity stars at upper
left to cool low-luminosity stars
at lower right.
Fig. 9-8, p. 179
The Hertzsprung-Russell Diagram
The Hertzsprung-Russell Diagram
Same
temperature,
but much
brighter than
MS stars
 Must be
much larger
 Giant
Stars
Same
Luminosity
but hotter
than Sun
The Radii of Stars in the H-R
Diagram
• Observations with interferometers
can resolve the size and shape of
some nearby stars.
• The stars of the upper main
sequence are indeed larger than
the Sun, as predicted by the H–R
diagram.
• The examples shown here are
flattened by rapid rotation, but most
stars rotate slower and are more
nearly spherical.
• On the scale of this diagram, the
supergiant Betelgeuse would have
a diameter of about 7 meters ( 23
feet ) .
ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu)
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The Relative Sizes of Stars in
the HR Diagram
• An H–R diagram showing the
luminosity and temperature of
many well-known stars.
• Individual stars that orbit each
other are designated A and B,
for example Spica A and Spica
B.
• The dashed lines are lines of
constant radius; star sizes on
this diagram are not to scale.
• To visualize the size of the
largest stars, imagine that the
Sun is the size of a tennis ball.
• Then, the largest supergiants
would be the size of a sports
stadium and white dwarfs the
size of grains of sand.
ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu)
Masses of Stars in the H-R
Diagram
• The more massive a star is, the brighter it is
– High-mass stars have much shorter lives than
low-mass stars
L = M3.5
ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu)
Comparison of Spectral Lines
• These model spectra show how the widths of spectral lines reveal a
star’s luminosity class.
• Supergiants have very narrow spectral lines, and main-sequence
stars have broad lines. Certain spectral lines are more sensitive to
this effect than others;
• Careful inspection of a star’s spectrum can determine its luminosity
classification.
Luminosity Class
• See Figure 9-12
– Ia Bright Supergiants
– Ib Supergiants
– II Bright Giants
– III Giants
– IV Subgiants
– V Main-Sequence Stars
• Our Sun: G2 star on the Main Sequence:
G2V
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Example Luminosity Classes
Luminosity Classes and H-R Diagram
Ia Bright Supergiants
• Our Sun: G2 star on the Main Sequence:
Ia
Ib
G2V
Ib Supergiants
II
III
IV
• Polaris: G2 star with Supergiant luminosity:
II Bright Giants
III Giants
IV Subgiants
V
G2Ib
V Main-Sequence
Stars
Attendance
ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu)
Campus.kellerisd.net
ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu)
4
HR-Diagram Worksheet
• Please work on the worksheet provided to
you for the next 15-20 minutes.
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