My Science School

          Our earth is surrounded by a blanket of air, which is called the atmosphere. It is mainly composed of nitrogen, oxygen, carbon dioxide, dust particles, water vapours and other gases. The light that comes to us from the sun has to travel through this atmosphere.

           Sun is the main source of light for earth. When the sunlight passes through the atmosphere, it is scattered by the dust particles, water and air molecules in all the directions. This is why, there is light all around after the sunrise.

          We know that the sunlight is composed of seven colours: violet, indigo, blue, green, yellow, orange and red. When the sunrays pass though the atmospheric molecules, the violet, indigo and blue colours get reflected most and the red colour least of them. Therefore, when we look at the sky the light that enters our eyes mainly consists of violet, indigo and blue colours. The mixture of these three colours is almost blue. That is why the sky appears blue.

          If the earth had no atmosphere, the sky would have just appeared dark. This fact becomes very clear when we look at the sky from the surface of the moon. It appears dark since there is no atmosphere in the moon. Similarly, the sky appears dark when we look from a spacecraft, because there are no light scattering particles in the higher space.

 

A nebula is a large cloud of gases and dust in outer space. There are two types of nebulae - diffuse nebulae and planetary nebulae. Diffuse nebulae are much larger than planetary nebulae. Some times a diffuse nebula is close enough to a star so that its dust reflects the light. A planetary nebula is an expanding cloud of glowing gas that surrounds a star.

If we look at the sky at night through a telescope, we can see many big black and white spots. These spots are called nebulae. The word ‘nebula’ comes from Latin which means ‘mist’ or cloud as they give the appearance of mist to us.

Black spots are called ‘black nebulae’, whereas white ones are called ‘bright nebulae’. In reality these nebulae consist of dust, gas and a cluster of stars. Many nebulae are situated in our Milky Way, but many more are outside it. Lot of information has been obtained about them with the help of powerful telescopes.

There are some 2000 nebulae in our Milky Way. It is estimated that there are millions of them existing outside it. Some of them are elliptical, some others are spiral shaped and some do not have any regular shape at all.

The temperature of the nebulae has been found to be varying from 6,000°C to 12,000°C. The temperature of the black nebulae is much less than that of the bright ones. Therefore, they appear black. They contain mostly hydrogen and helium gases. The nearest nebula is 300 light years away from us. And those situated outside the Milky Way are at least 150,000 light years away from us.

Scientists have photographed some of the nebulae. The photograph of the Crab nebula indicates that it is some 6000 light years away from earth. The photograph of one nebula resembles the head of a horse. It is, therefore, named the ‘Horse’ nebula. Another appears like a ring and thus it is called ‘Ring’ nebula. Nebulae are classified depending on whether they emit, reflect or absorb light. Scientists have already studied many nebulae. 

          During the rainy season we often see lightning in the sky followed by thunder. Do you know what this lightning is and how thunder follows it?

          In ancient times whenever man saw lightning in the sky and heard thunder he used to believe that gods were angry and punishing him for some sin. Benjamin Franklin was the first person who, in 1872, scientifically explained the occurrence of lightning. In fact, whenever the sky gets overcast with clouds, the small particles of water present in them get charged due to air friction. In the process, some clouds become positively-charged, while some others negatively. When a positively-charged cloud approaches a negatively-charged one, there develops a potential difference of millions of volts between them. Because of this high voltage, there is a sudden electric discharge through the air between the two clouds and a streak of light is seen. This is called ‘lightning’. The electric discharge through the air produces a large amount of heat due to which the atmospheric air suddenly expands. With this sudden expansion, the innumerable molecules of the air collide with one another and produce sound. This is called ‘thunder’. In other words, the thunder is the acoustic shock waves, which may be a sudden clap depending upon the lightning path. Although lightning and thunder are produced simultaneously, yet we see the flash of lightning first. It is so because the speed of light is very high i.e., 300,000 kms per second. On the other hand speed of sound is only 332 metres per second. Thus, because of high velocity, light immediately reaches our eyes, but the sound takes some time to reach our ears.

          Whenever a charged cloud passes by some tall tree or high building, by induction, it produces the opposite charge on that tree or building. When the amount of charge so produced is very high, a giant electric spark travels between the cloud and the ground. It is then said that lightning has struck such a tree or building.

          To protect high buildings from such mishaps, pointed rods of copper or some other metal are fixed on the top of buildings which run to the bottom and are buried deep in the earth. These are called ‘lightning conductors’. Whenever some charged cloud passes by such a building and produces opposite charge on it, the charge goes to earth through the metal rod and does not damage the building. This how buildings are protected from the lightning.

 

          The stars become visible in the sky as soon as there is darkness after the sunset. As the darkness increases, the number as well as brightness of the stars also increase. The number of visible stars dwindles with the approach of dawn. Only very bright stars remain visible. At sunrise all the stars disappear. Do you know why stars are not visible during the day?

          Some people think that as the sun disappears during the night, the stars too vanish during the day. But this is not correct. The stars never disappear. They simply become invisible during the day because of the brightness of the sun’s rays. In fact, the dust, gas and water vapours present in the atmosphere scatter the sun’s rays in all the directions. Due to this, the whole atmosphere starts shining. The light of stars is very weak as compared to the sunlight and as such it fails to make any impact on our eyes. We, therefore, can not see the stars during the day. This fact becomes clear by just observing a burning lamp. During night a lamp’s flame is visible from a long distance, but during the day the same is not visible to us even from a short distance. This implies that even a feeble light can reach our eyes during the night and as such stars become visible to us at night. Had there been no atmosphere, the stars would have been visible even during the day. Stars are visible in the space even during the day because in space you are beyond the atmosphere. 

                We all know that in our solar system there are nine planets which always revolve round the sun in elliptical orbits. In addition to these planets, there are many small bodies or irregular lumps of rocks which orbit the sun. These small heavenly bodies are called ‘asteroids’ or ‘planetoids’.

               There are about 100,000 asteroids but majority of them are two small to be seen from the earth. Each asteroid has its own orbit in which it revolves round the sun. The biggest asteroid is ‘Ceres’. It measures 1/800th of the size of the earth. The Italian astronomer Piazzi discovered this asteroid in January 1801. The ‘Vestal’ is the only asteroid visible to the naked eye though it was discovered after “Ceres”. The diameter of these asteroids range from 1 km to 800 kms. The total mass of these asteroids is 1/3000th of that of the earth. Two-thirds of the asteroids revolve in orbits lying between the orbits of Mars and Jupiter. Some asteroids revolve between Mercury and Mars also. The asteroid ‘Incarus’ is one of them. The orbit of the asteroid named ‘Hidalgo’ lies between Mars and Saturn. The asteroids ‘Hermes’ and ‘Eros’ are within a range of a few million kms from the earth.

               Eros is a small asteroid that wanders from the asteroid belt and comes within a range of 25.6 million kms from the earth in every seven years, i.e. closer than any other body in the solar system except the moon. It was discovered by G.Witt, in 1898. A second asteroid belt beyond the orbit of “Pluto” has been assumed by the scientists in their recent studies.

               These bodies appear like saucers under powerful telescopes. Some of them are very bright, while some others are faint. Their sizes have been determined on the basis of their brightness.

               Most of the asteroids are constituted of the same ingredients of which the rocks on the earth are made. Their surface temperatures however differ. Scientists believe that asteroids might have originated when some big planets exploded in space and its small pieces then started revolving round the sun. However, nothing can be said about their origin correctly. 

           The earth is round, and so are the sun and the moon. Even the planets, their satellites and the stars are also spherical. Do you know why all these heavenly bodies are spherical in shape?

           According to a law of science, every physical system in the universe tries to attain a state of minimum energy for maximum stability. It is an established fact that if a sphere is made from a given volume of any material it will have the least surface area compared to any other form. Consequently the spherical bodies have the minimum surface energy. Substances try to acquire a round form like a football in order to achieve the state of minimum energy, that is, the state of maximum stability. This is why the sun, the moon, the stars, the earth and all other heavenly bodies have a spherical shape.

           The rain drops are also spherical. It is so because the surface of the spherical drops has the minimum energy and maximum stability. Whatever be the process of producing the water drops, they immediately try to acquire a spherical shape.

           Atoms and molecules are also spherical in shape. Even the electrons, protons and neutrons present in the atoms are also spherical.

          You might have observed that every man tries to earn the most with the least effort. Every student tries to secure maximum marks after putting in the minimum labour. Thus we see that the tendency to come to the minimum energy state is found in all the living and non-living bodies. As soon as a substance acquires the minimum energy state, its shape becomes spherical.