THE UNIVERSE: SATELLITES, COMETS AND ASTEROIDS

Refresh Can you name the planets in our Solar System? What is the difference between a comet and an asteroid?

2 The Universe

The Universe is everything that exists in time and space. It contains energy and matter. It also includes the planets, stars and galaxies. The Universe is so large that scientists have not yet found where it ends. Scientists believe that the Universe appeared with an explosion and that it is still expanding. This is called the Big Bang Theory. It explains why celestial bodies are in constant movement.

Because the universe is so big, distances cannot be measured in kilometres. The unit we use to measure distances in space is the light year. This is the number of kilometres that light can travel in one year. Light travels at almost 300 000 kilometres per second. Did you know that some of the stars we can see do not exist anymore? They are just so far away that their light is still reaching us.

1 Are these sentences about the Universe true or false? Copy them in your notebook and correct the false sentences.

a) The Universe is very big.

b) The Universe started with an explosion.

c) The Universe continues forever.

d) We measure the distance between planets in kilometres.

e) Light travels at almost 300 000 kilometres per second.

3 Celestial bodies

Celestial bodies are the objects that exist in the Universe. Matter takes different forms, so there are many types of celestial bodies: nebulae, stars, galaxies, comets, asteroids, meteorites, satellites, planets and dwarf planets.

• A nebula is a giant cloud of gas and dust located between stars. Nebulae contain different types of gases that form new stars.

• Stars are spheres of gas (mostly hydrogen and helium) that emit light and energy. The Sun is a star.

• Groups of millions of stars and nebulae form a galaxy. Our Solar System, together with other stars and systems, is in the Milky Way.

1 Write the name of each type of celestial body in your notebook.

2 Work in groups. Do some research and agree on the definitions of the following words. Then, copy and complete the table in your notebook.

planets satellites comets asteroids meteorites dwarf planets

Term Definition

? Celestial bodies that orbit around bigger objects, like planets.

?

Large spherical bodies that can be made of rock or gas. They orbit around stars and form planetary systems such as the Solar System.

? Objects that leave their orbit and burn when they enter the atmosphere.

? Balls of ice and dust that orbit the Sun. Near the Sun they have a tail.

? They orbit stars. Pluto is a very small one.

? Small objects made of rock that orbit the Sun.