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UNIT NINE: Matter and Motion
in the Universe
 Chapter 26 The Solar System
 Chapter 27 Stars
 Chapter 28 Exploring the Universe
Chapter Twenty-Seven: Stars
 27.1 The Sun
 27.2 Stars
 27.3 The Life Cycle of Stars
Chapter 27.2 Learning Goals
 Compare the Sun to other stars.
 Interpret an H-R diagram.
 Explain how stars are classified.
Investigation 27A
Stars and Spectroscopy
 Key Question:
What are stars
made of?
27.2 Classifying Stars
 Astronomers classify
stars according to:
color, and
27.2 The size of stars
Stars come in a range
of sizes and masses.
Our Sun is a mediumsized star.
The largest stars,
giant stars have a
mass of about 60
times the mass of the
27.2 The size of stars
There are two types of
giant stars.
Red giants are cooler
than white stars.
Blue giant stars are hot
and much more
massive than our sun.
27.2 The size of stars
 Stars that are smaller
than the sun come in
two main categories,
dwarfs and neutron
 Sirius, the Dog Star, is
the largest known
white dwarf.
27.2 Temperature and color
 If you look closely at the stars on a clear night,
you might see a slight reddish or bluish tint to
some stars.
 This is because their surface temperatures are
27.2 Temperature and color
 The color of light is related to its
 White light is a mixture of all colors at
equal brightness.
27.2 Brightness and luminosity
 Brightness, also
called intensity,
describes the
amount of light
energy per second
falling on a surface.
27.2 Brightness and luminosity
 For a distant source of light like a star,
the brightness decreases as the
inverse square of the distance.
27.2 Brightness and luminosity
 Luminosity is the total
amount of light given off
by a star in all directions.
 Luminosity is a
fundamental property of a
star whereas brightness
depends on both
luminosity and distance.
27.2 Temperature and luminosity
 In the early 1900s, Danish astronomer
Ejnar Hertzsprung and American
astronomer Henry Russell developed
an important tool for studying stars.
 Their graph showed luminosity on the
y axis…
 …and surface temperature on the
x axis
27.2 Temperature and luminosity
 H-R diagrams are useful because they help
astronomers categorize stars into groups:
 Main sequence stars, like the Sun, are in a very
stable part of their life cycle.
 White dwarfs are hot and dim and cannot be seen
without a telescope.
 Red giants are cool and bright and some can be
seen without a telescope.
Can you locate blue giants on the H-R diagram?