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VOCABULARY 1

1 Listen and read. TR: 4.1

A comet is a cloud of rock, ice, and gas that
2 Listen and repeat. TR: 4.2 orbits the sun. Many earth years pass in its

journey around the sun. Scientists keep data
Earth is a planet that moves around the sun. Other planets also orbit on comets to know when they will appear.
the sun. The sun and planets make up our solar system. The sun is
a star like the stars you see in the sky at night. Some stars have solar
systems with planets, too. There may be another planet out there that space
has an atmosphere with oxygen to breathe.
a comet

a galaxy

A star and the planets that orbit around it make up
a solar system. Stars and solar systems make up a
galaxy. Our galaxy is the Milky Way. It has about
100 billion stars. The Milky Way isn’t the only galaxy.
There are more galaxies in the universe than there are
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stars in a galaxy! How many? We don’t know. There are

too many to count, and many are too far away to see.
Think of the many galaxies in the universe. Think
of the many stars in each galaxy. Think of the
many planets that orbit the stars. Do you think
that extraterrestrials may live on one of the
planets? Many people debate this question.

an orbit a planet

3 Discuss. Work with a partner.

What did you learn?

The sun is one of the
100 billion stars in Our galaxy is called the
our galaxy. Milky Way.

62 Unit 4 6363