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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Apparent Magnitude Scale – brightness
of a star as seen from Earth
Several stars in and around the
constellation Orion labeled with
their names and apparent
magnitudes
Astronomers give 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.
The Inverse-Square Law
The farther a star is from Earth, the dimmer it looks
to us. Doubling the distance makes the star look
one-fourth as bright. Tripling the distance
decreases the star’s brightness by a factor of 9.
Absolute Magnitude – the actual
brightness of a star
Absolute magnitude tells how bright a star
really is, no matter how far from Earth it is.
 Are the car lights actually dimmer as the
car moves away?

No. Their actual brightness (absolute
magnitude) is the same no matter the
distance.
 But they look dimmer (apparent
magnitude) to us when the car is farther
away.
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Temperature and Color (review)
Hottest = blue color
Medium = orange/yellow color
Coolest = red color
Spectral Classes (Color and Temperature)
“Oh, Be A Fine Guy/Girl, Kiss Me!”
HertzsprungRussell (HR)
Diagram
(Textbook
page 305)
Hertzsprung-Russell (HR)
Diagram
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Star brightness is plotted against star spectral
types (color / temperature).
Brightness and spectral type are related.
Main-sequence stars (fusing hydrogen to
helium) fall along the red curve.
Giants are to the upper right and super-giants
are on the top.
White dwarfs are below the main sequence.
HR Diagram Basics
Thanks to Dan Enriquez
Star Size Is Also Important
Hotter stars are brighter than cooler stars
(of the same size).
 Bigger stars are brighter than smaller stars
(of the same temperature).
 So the brightest stars are the biggest,
hottest ones.
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L = R2T4 (L = brightness, R = radius,
T = temperature)
Mass-Temperature-Brightness
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Each dot = a mainsequence star.
The dot’s number is
the mass of that
star in solar masses
(Sun = 1).
Mass, brightness,
and 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.