When we look at the sky, we see different kinds of natural bodies like the sun, the stars, the moon, and so on. The natural bodies in the sky are called celestial bodies or heavenly bodies. They are part of our universe. The universe is a huge space which contains everything that exists. The celestial bodies that we see are just a small fraction of the bodies that exist in the universe. One of the reasons why we do not see more of them is that they are very, very far away.
To measure the large distances in the universe, scientists use a unit of length called the light year. A light year is the distance travelled by light in one year. Light travels 9.46 trillion km in a year (one trillion is 1 followed by 12 zeroes).
One light year represents this huge distance. Proxima Centauri, the star closest to our solar system, is 4.2 light years from us. This means that light from this star takes 4.2 years to reach us. In this article, we shall learn a bit about stars and our solar system. But before that, let us see how the universe was formed.
Scientists believe that the universe was born after a massive explosion called the 'big bang'. A long time after the big bang, stars like our sun were formed. At that time, clouds of hot gases and particles revolved around the sun. Over time, many particles got stuck together to form large bodies. These bodies pulled in smaller objects near them by gravitational force. This made them larger still. These bodies finally became the planets.
Stars:
Away from the lights of the city, you can see thousands of stars in the night sky. You can also see some planets and their moons, either with the naked eye or with the help of a telescope. These celestial bodies are different from the stars in one important way. Stars are celestial bodies that produce their own heat and light. Planets and their moons shine by reflecting the light of a star such as our sun.
All stars are huge balls of hydrogen and helium gases. In a star, hydrogen gets converted into helium. In this reaction, a large amount of energy is liberated. This is the source of the heat and light of a star. Stars vary in brightness and size. Some are medium-sized, like our sun. Some are so huge that if they were to be placed in our sun's position, they would fill the entire solar system!
Galaxies:
There are trillions of stars in the universe. They occur in groups called galaxies. The gravitational force between stars keeps the stars of a galaxy together. Apart from stars, a galaxy may have other celestial bodies like planets and moons. So you can say that a galaxy is a group of stars and other celestial bodies bound together by gravitational force.
The distribution of the stars in a galaxy can give it a shape such as spiral, ring or elliptical. Our sun is a part of a spiral galaxy called the Milky Way Galaxy. This galaxy is named after the Milky Way. The Milky Way is a band of stars that we can see on a clear night. These stars are a part of our galaxy. The ancient Romans called this band of stars Via Galactica, or 'road of milk'. That is how our galaxy got its name.
Constellations:
As the earth moves round the sun, we see different stars at different times of the year. In the past, people found many uses for this. For example, they would get ready for sowing when particular stars appeared in the sky. Obviously, it was not possible for them to identify each and every star. So, they looked for groups of stars which seem to form patterns in the sky. A group of stars which seem to form a pattern is called a constellation.
Orion:
Orion (the Hunter) and Scorpius are two other prominent constellations. There are different stories linking them. According to one, the mighty hunter Orion vowed to kill all the animals of the world. Alarmed at this, the Earth Goddess sent a scorpion to kill Orion. He ran away, and continues to do so even now. This story takes into account the fact that Orion goes below the horizon when Scorpius rises. Orion rises again only when Scorpius sets.
The Solar System:
The sun is the brightest object in the sky. It is huge. It is about 333,000 times heavier than the earth, and you could fit more than a million earths inside it! Its great mass causes a large gravitational force. This keeps the sun, the planets, their moons and some other smaller bodies together as the sun's family. The sun and all the bodies moving around it are together called the solar system. All the members of the solar system revolve around the sun in almost circular paths, or orbits.