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Chapter 11:
Characterizing
Stars
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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Distance
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Parallax
As the Earth orbits the
Sun, a nearby star appears to
shift its position against the
background of distant stars.
The closer the star is to us,
the greater the parallax angle p.
Telescopes on earth can
measure stars up to 100pc and
space based telescopes can
measure stars up to 150pc
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Distance (in parsecs) = 1 / parallax angle in arcseconds
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The nearest star, Proxima Centauri, has a parallax angle
of .77 arcsec. What is its distance?
Apparent Magnitude
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Brightness of a star
without regard to
distance
The more negative
the brighter it is
Stars visible to the
naked eye have
magnitudes between
m = –1.44 and about
m = +6.
Absolute Magnitude
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Brightness of a star at
a distance of 10pc
Can be calculated
using the star’s
distance and
apparent magnitude
using the inverse
square law
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
tripling the distance
decreases the brightness by a factor of 9.
law. This means, for example, that
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.
Principal Types of Stellar Spectra
Classifying the Spectra of Stars
Williamina Fleming (standing)
Annie Jump Cannon
Spectral Classes
“Oh, Be A Fine Guy/Girl, Kiss Me!”
Hertzsprung-Russell Diagram
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Luminosities (absolute
magnitude) of stars are
plotted against their spectral
types.
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.
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.
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.
Stellar Mass
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Found by its
gravitational effects
on other objects
Keplers 3’rd Law
(a3/p2)
Most stars are binary
stars
Ursae Majoris
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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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 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.