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Chapter 11:
Characterizing
Stars
The Inverse-Square Law
The same amount of radiation from a light source must illuminate an
ever-increasing area as the distance from the light source
increases. The decrease in brightness follows the inverse-square
law. This means, for example, that tripling the distance decreases
the brightness by a factor of 9.
Temperature and Color
The intensity of light emitted by three hypothetical stars is plotted against
wavelength. The range of visible wavelengths is indicated. Where the peak
of a star’s intensity curve lies relative to the visible light band determines
the apparent color of its visible light. The insets show stars of about these
surface temperatures.
Hertzsprung-Russell Diagram
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Luminosities of stars are
plotted against their spectral
types.
Luminosity and spectral
type are correlated.
Main-sequence stars fall
along the red curve.
Giants are to the right and
supergiants are on the top.
White dwarfs are below the
main sequence.
Spectral Classes
“Oh, Be A Fine Guy/Girl, Kiss Me!”
Types of Stars and Their Sizes
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Stellar luminosities are
graphed against the
surface temperatures.
Dashed diagonal lines
indicate stellar radii.
For stars of the same
radius, hotter stars are
more luminous than
cooler stars.
The Mass-Luminosity Relation
Luminosities and
masses are plotted
using logarithmic
scales.
 The more massive
a star, the more
luminous the star.
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The Mass-Luminosity Relation
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Each dot represents a
main-sequence star.
The number next to
each dot is the mass of
that star in solar
masses.
Mass, luminosity, and
surface temperature of
main-sequence stars
increase from lower
right to upper left
WHAT DO YOU THINK?
How near is the closest star other than the
Sun?
 Is the Sun brighter than other stars, or just
closer?
 What colors are stars?
 Are brighter stars hotter?
 What sizes are stars?
 Are most stars isolated from other stars,
as the Sun is?
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You will discover…
• that the distances to many nearby stars can be
measured directly, while the distances to farther ones are
determined indirectly
• the observed properties of stars on which
astronomers base their models of stellar evolution
• how astronomers analyze starlight to determine a
star’s temperature and chemical composition
• how the total energy emitted by stars and their
surface temperatures are related
• the different classes of stars
• the variety and importance of binary star systems
• how astronomers calculate stellar masses
Our eyes change angle as
we look at things that are
different distances away.
As the Earth orbits the
Sun, a nearby star appears to
shift its position against the
background of distant stars.
Our eyes are adjusting for
the parallax of the
things we see.
The closer the star is to us,
the greater the parallax angle p.
Apparent Magnitude Scale
Several stars in and around the
constellation Orion labeled with
their names and apparent
magnitudes
Astronomers denote the
brightness of objects in the sky by
apparent magnitudes. Stars visible
to the naked eye have magnitudes
between m = –1.44 and about
m = +6.
Principal Types of Stellar Spectra
Classifying the Spectra of Stars
Williamina Fleming (standing)
Annie Jump Cannon
Luminosity Classes
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Luminosity classes permit
finer distinctions between
giants and supergiants.
Ia and Ib encompass the
supergiants.
II, III, and IV indicate giants
of different brightness.
V is the main-sequence
stars.
White dwarfs do not have a
luminosity class.
Binary Star System
ζ Ursae Majoris is a binary system with stars
separated by only about 0.01 arcseconds.
Center of Mass of a Binary Star System
Spectral Line Motion in
Binary Star Systems
The radial-velocity curves of the binary HD 171978.
Double-Line Spectroscopic
Binary
Kappa Arietis has spectral lines that shift back and
forth as the two stars revolve about each other.
Light Curves of
Eclipsing Binaries
(a) a partial eclipse (b) a total eclipse (c) The binary star NN Serpens
WHAT DID YOU THINK?
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How near is the closest star other than the Sun?
Proxima Centauri is about 40 trillion kilometers (25 trillion
miles) away. It takes light about 4 years to reach the
Earth from there.
How luminous is the Sun compared with other stars?
The most luminous stars are about a million times
brighter and the least luminous stars are about a
hundred thousand times dimmer than the Sun.
What colors are stars?
Stars are found in a wide range of colors, from red
through violet, as well as white.
WHAT DID YOU THINK?
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Are brighter stars hotter than dimmer stars?
Not necessarily. Many brighter stars, such as red giants,
are cooler but larger than hotter, dimmer stars, such as
white dwarfs.
What sizes are stars?
Stars range from more than 1000 times the Sun’s
diameter to less than 1/100 the Sun’s diameter.
Are most stars isolated from other stars, as the Sun is?
No. In the vicinity of the Sun, two-thirds of the stars are
found in pairs or larger groups.
Key Terms
absolute magnitude
apparent magnitude
binary star
center of mass
close binary
dwarf star
eclipsing binary
giant star
Hertzsprung-Russell (H-R)
diagram
initial mass function
inverse-square law
light curve
luminosity
luminosity class
main sequence
mass-luminosity relation
OBAFGKM sequence
optical double
photometry
radial-velocity curve
red giant
spectral types
spectroscopic binary
spectroscopic parallax
stellar evolution
stellar parallax
stellar spectroscopy
supergiant
visual binary
white dwarf