Text 1 

The Universe contains everything that exists – not only the Earth and everything on it, but also all the planets, stars, and galaxies, and the space in between them. The sun, at the center of the solar system, is just one of about 100 billion stars in our galaxy, or collection of stars, called the Milky Way. Astronomers have estimated that there are about 100 billion other galaxies in the universe. Most scientists think that the universe formed about 15 billion years ago in an enormous explosion called the big bang. They also think that the universe is expanding.

Text 2 

The sun and the nine planets in orbit around it make up the solar system. The nine planets, which are drawn to the sun by its gravity, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The sun is the heaviest of these bodies.

Text 3 

There are billions of stars in the universe. Stars are massive, energy-filled globes of fiery gases. The force of gravity holds these gases together. At a star's core, atoms of hydrogen join together to form helium in a process called nuclear fusion. The energy generated by this process produces a star's heat and light. Collections of stars are called galaxies, and each galaxy contains many different types of stars.

Text 4 

The Earth is one of nine planets that travel around the sun in the solar system. Of these planets, the Earth is the third nearest the sun. It is the only planet known to support life. About 5 billion years ago, a cloud of gas and dust began to condense into a solid mass – the young Earth. At first, this mass was very cold, but later it was melted by radioactivity. Heavy metals collected at the center and rocks floated near the surface. After millions of years, the rocks formed a hard crust and the oceans and the atmosphere appeared.

Text 5 

Air is a mixture of gases that surrounds the Earth and supports all life on it. We breathe in air to stay alive, and rely on it for heat, as fire cannot burn without it. The layer of air that we breathe stretches for only 7 miles above the Earth's surface. Oxygen and nitrogen are the key elements, with water vapour, carbon dioxide, and other gases making up the total. The amount of water vapour in the air varies and is referred to as humidity.

Text 6 

Of all species, humans have the greatest effect on the environment. Some of our activities threaten the world's habitats and therefore the Earth's biodiversity (its range of plants and animals). The balance of nature is upset by activities such as overfishing, overhunting, and cutting down too many trees. We also damage the environment by polluting the land, the air, and oceans, rivers, and lakes. One of the causes of over-activity is the sheer number of people in the world. This number is rising.

Text 7 

The part of the Earth and its atmosphere that can support living organisms is called the biosphere. This consists of the air and land, as well as lakes, rivers, and oceans. Organisms usually live alongside other members of the same species to form a population. Lots of different populations share the same habitat. Together, they all form a community. The members of a community constantly react with one another and their environment, creating a balanced, living system called an ecosystem, such as an ocean or a rain forest. The study of all these interactions is called ecology.