ICSE Class 8 Physics Chapter 01 Universe

Universe

The earth, the sun, the moon and all the stars in the night sky belong to the universe. In fact, they form only a small portion of the universe. Most of the universe is so far away from us that we do not see it. Even the star closest to the solar system, called Proxima Centauri, is 4.2 light years away. The light year is a unit of distance used by astronomers, or scientists who study the heavenly bodies. One light year is the distance travelled by light in a year. This works out to 9.46 trillion km (9.46 x 10^12 km), so you can try to imagine how far Proxima Centauri is. Then, knowing that it is the star closest to us, you can try to grasp how vast the universe is.

Cosmologists, or scientists who study the universe, believe that about 15 billion years ago, everything contained in the universe was concentrated at one point. A gigantic explosion, called the Big Bang, which occurred at that time, caused the concentrated matter to move outwards, or expand. This is what gave birth to the universe. Slowly, the clouds of gases that expanded outwards, clumped together here and there due to gravitational force, and stars were formed. In fact, the universe is still expanding and stars are still being born in clouds of gases called nebulas.

Just as stars are born, they also die when they run out of fuel. The fuel of a star is usually hydrogen. The heat and light of a star is generated by nuclear reactions in which hydrogen changes into helium (a gas). Stars are of different sizes and brightnesses. They live for different spans of time and end their lives in different ways. The sun is a medium-sized star in its middle age. It has been shining for about 4.6 billion years and will probably shine for another 5 billion years.

Fig. 1.1 Stars are still being born in clouds of gases called nebulas.

Galaxies

Stars are not evenly distributed in the universe. They occur in clusters called galaxies. There are countless galaxies in the universe. Each galaxy consists of billions of stars and other heavenly bodies held together by gravitational force. They also contain nebulas in which new stars are forming.

Galaxies are of different shapes and sizes. Many of them are elliptical. Some look like rings, while others look like spirals, with arms coming out of a central band. There are yet others that are irregular, or do not have a clearly defined shape.

Milky Way Galaxy

We are in the central disc of a spiral galaxy called the Milky Way Galaxy. What we see of our galaxy when we look up at the sky is a faint whitish band stretching across the sky. Ancient Romans called this band of stars Via Galactica, or the 'path of milk'. That is how our galaxy got its name.

There are about 100 billion stars in our galaxy, most of which are concentrated in the central disc. All the stars move slowly around the centre of the galaxy.

Constellations

Some stars in the sky seem to form patterns that never change. The patterns move across the sky as the earth moves around the sun. But the stars that form a pattern do not seem to shift with respect to each other. These patterns formed by stars are called constellations.

Down the ages, people from different civilisations gave different names to the constellations and made up stories about them. Some constellations that are visible in the Northern Hemisphere are Orion, the hunter (Kalpurush), Scorpius, the scorpion (Vrishchik), Cancer, the crab (Karkat) and Taurus, the bull (Vrishabh). These constellations are not visible in the Southern Hemisphere, and the constellations that are visible there are not visible to us.

A very prominent constellation in the northern sky is the Great Bear (Ursa Major). As the name suggests, its stars roughly form the shape of a bear. Seven of the brightest stars of this constellation form the Big Dipper (Saptarshi), which looks like a long-handled spoon. In ancient times, travellers would use the Big Dipper to find the direction. This is because an imaginary line joining the two brightest stars of the Big Dipper points towards the Pole Star (Dhruv tara). The Pole Star, as you have already learnt, seems fixed, while all the other stars move across the sky. The reason is that it is situated almost directly above the North Pole, the northern tip of the earth's axis of rotation.

One thing that you must remember is that the stars of a constellation are not necessarily close together. They just seem to form a group though they might be many light years away from each other. The following activity will show you how this is possible.

Teacher's Note

When we look at the night sky, constellations help us navigate and understand our place in the cosmos, much like how we use landmarks to navigate our local towns and cities.

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