s8pe-41402-ca
10/31/05
7:11 PM
MAZER
Page 466
Star Systems
Unlike our Sun, most stars do not exist alone. Instead, they are
grouped with one or more companion stars. The stars are held together
by the force of gravity between them. A binary star system consists of
two stars that orbit each other. A multiple star system consists of more
than two stars.
In many star systems, the stars are
too close together to be seen individually. However, astronomers have
developed ways of detecting such systems. For example, in a binary star
system, one of the stars may orbit in
front of the other when viewed from
Earth. The star that orbits in front
will briefly block some of the other
star’s light, providing a clue that
more than one star is present. The
illustration at right shows a binary
star system that can be detected this
way. Sometimes astronomers can also
figure out whether a star is really a
star system by studying its spectrum.
Binary Star System
Some binary star systems appear
to dim briefly when one star
orbits in front of the other and
blocks some of its light.
When neither star is in front
of the other, the star system
appears to give off more light.
Star systems are an important
source of information about star
masses. Astronomers cannot measure the mass of a star directly.
However, they can figure out a star’s mass by observing the effect
of the star’s gravity on a companion star.
Check Your Reading
Why are star systems important to astronomers?
KEY CONCEPTS
CRITICAL THINKING
1. Why must astronomers figure
out a star’s distance to calculate
its actual brightness? (8.4.c)
4. Analyze Some of the brightest stars are red supergiants.
How can stars with cooler red
surfaces be so bright?
2. How are color and temperature related in stars? (8.4.b)
3. How does a star’s mass affect
its life cycle? (8.4.b)
466 Unit 4: Space Science
5. Infer Will the Sun eventually
become a black hole? Why or
why not?
CHALLENGE
6. Infer At what stage in the life
cycle of the Sun will it be
impossible for life to exist on
Earth? Explain.
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