Download Lesson 16.3

Name
Lesson 3 Summary
Use with pp. 524–529
Lesson 3: How are stars grouped together?
Vocabulary
galaxy a huge system of stars, dust, and gas held together by gravity
constellation an area of the sky and all the stars seen in that area
A galaxy is a huge group of stars, dust,
and gases. Gravity holds a galaxy together.
There are billions of galaxies. Earth is part
of the Milky Way galaxy.
About three-fourths of the galaxies are
spiral galaxies with wide centers. They also
have thin arms that stretch out from the
center like a pinwheel. Stars are in these
arms. The stars move around the center
of the galaxy. The Milky Way is a spiral
galaxy.
Elliptical galaxies are round or oval. They
can look like footballs. The largest galaxies
are elliptical. There are also elliptical galaxies
that are smaller than the Milky Way.
Some galaxies are called irregular
because they are not spiral or elliptical.
They do not have a definite shape.
Irregular galaxies are probably young
galaxies with their stars are still forming.
Constellations
Ursa Major is part of the Milky Way
galaxy. Ursa Major is a constellation. A
constellation is an area of the sky and all
the stars seen in that area. A constellation
is like a star’s address. Scientists use
constellations to help them locate stars.
For example, the Big Dipper is part of
the constellation Ursa Major. There are
88 constellations.
128
Chapter 16, Lesson 3 Summary
From Earth, the stars in a constellation
may look close together. They may look
this way because they are in the same
direction from Earth. But the stars might
actually be very far apart.
People in different parts of the world
see different constellations. Earth can be
divided into two parts along the equator.
The half to the north of the equator is called
the Northern Hemisphere. The half to the
south of the equator is called the Southern
Hemisphere. The United States is in the
Northern Hemisphere. Ursa Major can
be seen in the Northern Hemisphere. But
people in the Southern Hemisphere cannot
see it. The constellation Centaurus can be
seen only in the Southern Hemisphere.
Stars on the Move
You can see Ursa Major all year. But
other constellations can only be seen at
certain times of the year. Canis Major is a
constellation we see only in the winter.
The constellations change with the
seasons. This is because Earth rotates, or
spins. It takes Earth one year to move
around the Sun. We see different parts of the
sky as Earth moves.
Nothing in the universe stands still. Stars
hurtle though space at different speeds and
in different directions. We cannot see this
movement because the stars are too far
away. Over time, the patterns of the stars
will change. Some stars will move closer
together or farther apart.
Quick Study
© Pearson Education, Inc. 5
Galaxies
Name
Lesson 3 Checkpoint
Use with pp. 524–529
Lesson 3 Checkpoint
1. What type of galaxy is the Milky Way?
2. Which constellation is the Big Dipper in?
3. Which type of galaxy is almost round or football shaped?
4. How many constellations are there?
© Pearson Education, Inc. 5
5. Compare and Contrast Ursa Major and the Milky Way.
Quick Study
Chapter 16, Lesson 3 Checkpoint
129