What is the universe ?

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All the planets, stars and their moons form what is called the universe. The earth was the center of universe and the universe was what the early people thought they could see with their eyes. But with the discovery of telescope our knowledge of universe greatly advanced. In this vast universe, the earth with all the plants, animals, human beings, mountains, the oceans, is just a tiny negligible spectacle.

It also includes the other stars in the sky and also all the empty space around the earth. In all there are about 100,000 million galaxies in the universe and 200,000 million stars in the Milky Way.

What is the solar system ?

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Solar system comprises the sun, the nine planets and the moons that circle the planets. Mercury, Venus, Earth and Mars are the four planets which are rocky and close to the sun. Jupiter, Saturn, Uranus and Neptune are gas giants, farther from the sun. The last and the smallest is Pluto, which is made up of rocks and ice. It is gravity that holds the solar system together.

The sun's force of gravity keeps other bodies and the planets in the orbits around it. The forces of gravity of the planets keep their moons or satellites in their orbits. This solar system was once regarded as the largest and central part of the universe. Today we know it is just a tiny spectacle even in the galaxy - the Milky Way, which is one among the millions of galaxies in the universe.

How was the earth formed ?

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The earth was formed about 4,600 million years ago. It was a cloud of gas and dust in the beginning which slowly cooled to the solid earth. But it remained very hot and molten body for millions of years. There were constant volcanic eruptions that released gases and water vapour from the rocks and these substances formed an atmosphere. During great storms the water vapour accumulated in the air finally falls down as rain.

The water collected in hollows and formed the oceans. The powerful forces inside the earth kept changing its surface structure even after the hard crust was formed. The crust split into large blocks, called plates and these plates moved slowly around because of movement in the partly molten upper mantle. Rocks were squeezed up to form new landmasses and mountains, when the plates pushed against each other. Plate movements continue today which sometimes causes earthquakes.

How was moon formed ?

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The moon was formed about 4,600 million years ago, the same as the earth. Among the entire moon present in the solar system, this moon is the largest of all. But it is not yet certain where the moon came from, but it may have formed along with the solar system from the materials around the young earth or from the dust clouds. It is also believed that it might have been captured by the earth's gravitational force. A mar's sized body collided with the earth according to the big splash theory.

Moon has sulphur core and a small iron surrounded by the asthenosphere - a layer of partially melted rock. The lithosphere - a layer of solid rock is above this which is covered by a crust of aluminium and calcium rich rocks. Due to the collision of debris from the earth, formed the moon and started orbiting it because of its gravitational pull.

Why is the Sun so hot ?

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In our solar system the Sun is the largest body, which is made up of hydrogen and some helium. The Sun is 3,00,000 times as heavy as the earth. The Sun has enormous pressure at the centre and an extremely large gravitational pull because of its size. This pressure caused so much heat when the Sun was born that the hydrogen started burning. The burning of hydrogen is a nuclear reaction. It is thus called as fusion reaction because hydrogen nuclei are fused in this reaction. The formation of hydrogen nuclei results in the formation of helium.

A great deal of energy in the form of heat, light and radiation is released in this reaction which makes the Sun shine. In this reaction, in every second 600 tons of hydrogen is burnt. Even at this rate of burning, there is enough hydrogen in the Sun for it to shine for another 5,000 million years. And it is because of the heat released by burning hydrogen that makes the Sun so hot.

How was the Sun formed ?

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The Sun was born 5,000 million years ago from a cloud of gas and dust and was created in a group of stars. These stars broke up so that now the Sun is alone. A young protostar was formed from the cloud of gas and dust, called nebula. Under the gravity, parts of the clouds collapsed and each became dense in the centre. Because of the pressure, heat was trapped in the centre when it became hot enough, nuclear reaction started and energy was released. Gravity pulled hydrogen atoms towards the centre whereby joining with each other they form helium atoms. The early Sun also expanded due to the pressure in the centre. This is the stable period of star's life and is called main sequence star.

What is the Milky Way ?

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In the night sky the concentration of stars that we see as a milky path, is the Milky Way. It looks like a path of milk in the sky at night and so is it called as the Milky Way. Long before in the ancient Greece people believed that the Milky Way originated from the milk as infant Hercules drank from the goddess Juno's breast. The science of astronomy developed and revealed the myths of skies with the invention of sophisticated telescope in the last century

The earth and our solar system lie in one of the arms of the Milky Way with all the facets of life. Milky Way moves like any other object in the space. As our earth moves around the Sun so also the stars in the Milky Way move around its centre.

What are Galaxies ?

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In the space where million of stars live together are called galaxies. Galaxies are so vast that from one end to the other, starlight travels in hundreds of thousands of years. A galaxy has distinct shape as the stars are arranged in many different ways. Milky Way was thought to be the only galaxy in the universe earlier. There are approximately 100,000 million galaxies known to us. Galaxies are of 4 types spirals, ellipticals, barred spirals and irregulars. Galaxies in certain part of space are grouped to form clusters.