It is believed that our planet Earth is part of our solar system which began around 4500 million years ago. Scientists believe that life on the earth began about 3500 million years ago.
Stars are huge bodies of glowing gases. They give out huge flames and radiate a tremendous amount of heat and light. So they have heat and light of their own. The distances of stars are expressed in terms of light years. One light year is the distance travelled by light in one year at a speed of light which is about 300,000 km/s. Light year is a unit of distance and is equal to 9,460,000,000,000 km or 9.46×1012 km.
The colour of a star is determined by its surface temperature. Stars which have low temperature appear red. Those with higher temperature appear white and those with very high temperature are blue. All stars including the sun move around some celestial body or a group of bodies with high speeds. However, when viewed from the earth the distance between any two stars does not seem to change in spite of their great speeds. This is because the stars are so far away from us that any changes in distance between them do not become perceptible in a few years.
The stars appear to move from east to west. This is because the earth rotates from west to east about an imaginary axis that passes through its centre. The earth completes one revolution in 24 hours, so the stars. A star, therefore, travel an angle of about one degree in four minutes. Since the star rises four minutes earlier every day, they are one degree higher in the sky than the previous day. In a month, they rise 30 degrees higher. In 6 months they move 180 degrees and will be close to the western horizon. Thus the constellations also move westwards.
However, there is one star, which appears stationary to us known as Pole Star or Dhruv Tara. The North Star or Polaris indicates the north because it is seen directly above the North Pole. It lies close to the axis of rotation of the earth. So it does not appear to move like all other stars. In fact, all the stars appear to revolve around the Pole Star. The pole star is not visible from the southern hemisphere. Some of the northern constellations like Ursa Major may also not be visible from some points in the southern hemisphere.
The ages of the stars range from millions to billions of years. Most stars produce their energy by fusion reactions. When a star has used all hydrogen at its centre, it starts fusing helium into carbon. If the mass of the star is only a few times the solar mass, the fusion process does not go beyond this. As the energy production inside the star stops, its core contracts under its own weight. Its density becomes very high, and it becomes a white dwarf star.
The size of a white dwarf star is comparable to that of a planet. The mass of such stars is always less than about 1.44 times the mass of the sun. This result was proved by S. Chandrasekhar. The maximum mass is, therefore, called the Chandrasekhar Limiting Mass. A white dwarf is a dead star because it does not produce its own energy by fusion process. It shines by radiating the heat stored by it during its lifetime.
Bigger white dwarf stars explode as Supernova, and depending on their size, ends up as Neutron stars or Black holes (in which the gravitational pull is so high that not even light can escape out of them).
A group of stars forming various shapes is called constellation (star patterns). Laghu saptarshi or Ursa Minor is one such constellation. Vrihat Saptarshi also known as Ursa Major, a group of seven stars is another constellation. It forms a part of the constellation of the Big Bear. It can be seen during summer time in the early part of the night.
Orion or Mriga is another well-known constellation that can be seen during winter in the late evenings. The star Sirius, which is the brightest star in the sky, is located close to Orion. Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night.
A constellation does not have only 5-10 stars. It has a large number of stars. However we can see only the bright stars in a constellation with our naked eye. All the stars which make up a constellation are not at the same distance. They are just in the same line of sight in the sky.
A galaxy is a system of stars that also contains a large number of gas clouds, some of which are quite huge. It is in these gas clouds that new stars are born. They are bound to one another by gravitational forces. A galaxy has billions of stars very close to one another. There are billions of such galaxies in the Universe. Some galaxies are spiral in shape and others are elliptical. However, there are galaxies which have no regular shape.
Our galaxy appears like a river of light running from one corner of the sky to the other. That is why it is called the Milky Way Galaxy. It is spiral in shape. In India it is known as Akash Ganga. The Milky Way Galaxy is rotating slowly about its centre. The sun too, along with the solar system, rotates about the centre of the galaxy. It takes about 250 million years to complete one revolution. Our sun is just one of the hundred thousand million (1011) stars in the Milky Way Galaxy. The sun is located about 30,000 light years away from the centre of the galaxy.
In the earlier part of the twentieth century, Edwin Hubble (1889-1953), an American astronomer, showed that all other galaxies are rushing away from our galaxy. The velocity, with which they are rushing away from us, increases as their distances increase from us. The motion of other galaxies away from us gives rise to the idea of the expanding universe. If all the galaxies are moving away from one another at this moment, there must have been a time in the past when all the galaxies were together at one place.
At that time, the whole universe occupied a small volume. The event which occurred at that time and ultimately led to the present movement of galaxies away from one another is called the Big Bang. It is believed that a huge explosion took place at that time, and since then, the universe is expanding. It has been estimated that this event occurred about 15 billion (15×109) years ago. This means, the universe is about 15 billion years old. There are a few scientists who believe that the universe appears the same at all times and at all points. According to them the universe is not changing, it did not have a beginning and will not have an end. This theory is called the Steady State.