Make use of a telescope to observe the sky on a clear night without clouds. (Suggestion for teachers – Arrange such an observation session for all the students of the class. Carry with you some books on astronomy to consult during the session)
If you observe the clear sky on a dark night, you will see a smoky white band full of stars spreading north-south in the sky. This is the Milky Way. It is also known as ‘Mandakini’.
A group of innumerable stars and their planetary systems are together known as a ‘galaxy’. The Milky Way is the galaxy in which our solar system is located. The Milky Way is a part of the ‘Local Group’ of galaxies. There are many such galaxies in the universe.
The Milky Way includes many stars smaller than our Sun as well as many others that are thousands of times bigger than the Sun. It also includes many other celestial bodies such as clusters of stars, nebulae, clouds of gases, clouds of dust, dead stars, newly born stars, etc. The galaxy that is closest to our Milky Way is called Andromeda.
The scientist Edwin Hubble showed that there exist many galaxies beyond our Milky Way. In 1990, NASA, the American space agency launched the ‘Hubble Telescope’ in the orbit of the earth. The Hubble telescope has made it easier to look for stars, to take photographs and to obtain spectrums
The thousands of twinkling stars that we observe in the clear night sky are part of our Milky Way. Some of the stars that we see are bright whereas others are faint. Stars radiating different colours such as blue, white, yellow and reddish can be seen in the sky. We also see stars with varying brightness (luminance). The birth place of stars are the huge nebulae, made of dust particles and gases. Generally, the surface temperature of stars ranges from 3500° C to 50000° C. The colour of stars changes according to their temperature.
Some types of stars
l Sun-like stars : These stars can be slightly smaller or bigger than the sun. But there is a lot of difference in their temperatures. Examples : stars like Sirius, Alpha Centauri.
l Red Giants : The temperature of these stars ranges between 3000° C and 4000° C. But their luminance can be 100 times that of the sun. Their diameter is 10 to 100 times that of the sun and they are red in colour.
l Super Nova : These are even brighter and larger than the red giant stars. Their temperature is between 3000° C to 4000° C but their diameter can be more than a hundred times greater than that of the Sun.
l Binary or Twin Stars : More than half of the stars in sky are binary stars. They consist of two stars that revolve around each other. At times, three or four stars that revolve around each other have also been located.
l Variable Stars : The luminance and shape of these stars is not stable. They are constantly contracting or expanding. When a star expands, it emits less energy and at such times its brightness decreases. As against this, when a star contracts, its surface temperature increases and the star emits greater energy and appears brighter. For example, Polaris (Pole Star).
The solar system
The solar system consists of the sun, the planets, asteroids, comets and meteors. The planets Mercury, Venus, Mars, Jupiter and Saturn can be easily seen.
The sun which is at the centre of the solar system is a yellow coloured star. Its surface temperature is around 6000° C. The size of the sun is so huge that around 13 lakh planets of the size of the earth can be easily placed within it. Due to the gravitational force of the sun, the celestial bodies in the solar system revolve around it. The diameter of the sun is approximately 13,92,000 km. The sun rotates around its axis and while doing so, it revolves around the centre of the Milky Way taking the solar system along with it.
Mercury : This planet is closest to the sun. It is visible in the morning and the evening if it is away from the sun. A number of depressions, which look like volcanic craters, but are actually caused by meteoric falls can be seen on the surface of Mercury. Mercury is the fastest moving planet.
Venus: This planet is closest to the sun. It is visible in the morning and the evening if it is away from the sun. A number of depressions, which look like volcanic craters, but are actually caused by meteoric falls can be seen on the surface of Mercury. Mercury is the fastest moving planet.
Earth: This planet is closest to the sun. It is visible in the morning and the evening if it is away from the sun. A number of depressions, which look like volcanic craters, but are actually caused by meteoric falls can be seen on the surface of Mercury. Mercury is the fastest moving planet.
Mars : It is the fourth planet in the solar system. As the soil on Mars contains iron, its colour is reddish. Hence Mars is also called the Red Planet. The highest and longest mountain in the solar system ‘Olympus Mons’ is located on Mars.
Jupiter : This is the largest planet of the solar system. It is so huge that as many as 1397 planets of the size of the earth can get accommodated in it. Even though the planet is so huge, it revolves around itself with a great speed. As huge storms occur frequently on it, it is also called the ‘Stormy Planet’.
Saturn : It is the sixth planet of the solar system and next only to Jupiter in size. It is considered to be a peculiar planet because of the rings around it. Though its mass is 95 times that of the earth, its density is very low. If it were dropped into a sea large enough to hold it; it would actually float in it!
Uranus : It is the seventh planet in the solar system. It cannot be seen without a telescope. Its axis is so greatly inclined that it appears as if it is rolling along on its orbit.
Neptune : It is the eighth planet in the solar system. A season on Neptune lasts for about 41 years. On this planet winds blow with extremely high speed.
Satellite : The celestial bodies that revolve around a planet without independently revolving around the sun are called satellites. Like planets, satellites rotate around their respective axes. The Moon is the satellite of the earth. It does not have an atmosphere. Its periods of rotation and revolution are both of 27.3 days. Except for Mercury and Venus all other planets have satellites but in varying numbers.
Asteroid : A great number of small sized bodies could not turn into planets when the solar system was formed, but continued to revolve around the sun. These bodies are known as asteroids. A belt of such celestial bodies has formed between the planets Mars and Jupiter.
Dwarf planet : A small sized celestial body that revolves independently around the sun is called a dwarf planet. A celestial body like Pluto can be classified as a dwarf planet. Pluto takes around 248 years to complete its revolution around the sun whereas it takes around 6.38 days for one rotation.
A comet is a celestial body that revolves around the sun. Comets are formed out of ice and dust particles. They are part of the solar system. Since olden times, the appearance of a comet has been considered to be an inauspicious event. Comets appear like points when they are far away from the sun. But when they are close to the sun, they become easily visible to us because of the shorter distance and the heat of the sun.
Comets are made up of frozen matter and dust particles. When they are close to the sun, this frozen matter gets converted into gas due to the solar heat. These gases get thrown in a direction away from the sun. As a result, certain comets appear to have a long feathery tail. Due to their long elliptical orbits, their appearance in the sky is very rare. They reappear in the sky after very long periods of time.
At times, we see a falling star. This event is called a meteor fall. Mostly these meteors are rocky pieces originating from the asteroid belt. Smaller rocky pieces get completely burnt due to friction with air after they enter the earth’s atmosphere. Sometimes the meteors do not burn completely and fall to the surface of the earth. These are called meteorites. It is believed that the Lonar lake in Maharashtra has been formed by the impact of such a meteorite. Meteors or meteorite falls occur on other celestial bodies, too.