STARS AND THE SOLAR SYSTEM Class 8 Chapter 17

Introduction-

We get mesmerized to see the twinkling stars, moon and planets in the clear night sky. But, all these will not be visible during day due to bright sunlight. They get hide in the brightness of sunlight. The sun, the moon, stars, planets and other objects that we see in the sky are called heavenly bodies or celestial objects. Those celestial bodies which emit heat and light of their own are called stars. The stars appear to move from east to west due to the rotation of the earth from west to east.

These heavenly bodies are clearly visible by a device called telescope. The branch of science in which we study about various celestial bodies is called astronomy. No one knows the size of universe. As per scientific research the universe is regularly expanding.

In large cities the watching of stars may be fainted because of bright lights, smoke and dust.

Galaxies– The group of billions of stars is called as galaxy. e. g.- Milky way (Akash Ganga)- it is a spiral galaxy that looks like a white band across the sky. The sun, the earth, other planets of solar system and many stars are part of milky way.

How to measure the distances in the universe?

The distances among celestial bodies are so large that it cannot be measured in kilometer. We use an astronomical distance measuring unit called light year. The speed of light is 3 X 10⁸ m/s. So, light year is the distance travelled by the light in one year.

1light year = 3 X 10⁸ X365 X 24 X 60 X 60s m/s

                   = 9.5 X 10¹² km

The parsec (Symbol-PC) is an astronomical length used to measure very large distances between celestial objects outside solar system.

1 Parsec= 3.26 light Year

The nearest star to the earth is the sun. The distance between the earth and sun is 15 X 10⁷ Km. The sunlight takes 500 seconds or 8 minute 20 second to reach to the earth.  The second nearest star to the earth after the sun is ‘Proxima Centauri’. It is at a distance of 4.2 light years. It means the light that we can see from this star is 4.2 years old.

The Moon- The celestial body that revolves around the planets is called as moon or natural satellite.  There are different numbers of moons to different planets of the solar system. The earth has only one moon. It revolves around the earth in about 27.3 days. It also rotates around its axis in the same period of time.

   Fig-Phases of Moon (Source-http://www.astronomy.ohio-state.edu/)

The solar system and planets-The sun is a medium size closest star to the planet earth.  It is made up of hydrogen and helium like gases. The nuclear fusion reaction in the sun is the cause of heat produced by the sun.  The temperature of the core is 14 million degree celsius. At the surface the temperature is 6400 degree celsius. The diameter of the sun is about 1.4 million Km. It is 110 times more than the earth. The sun is the ultimate source of energy on the earth.

There are eight planets in the solar system. The planets and their moons constitute the solar system. ‘Pluto’ was too small to be called as planet. So, it was removed from the planets of solar system in 2006.  

Those celestial objects which revolve around the sun are called planets. They do not have their own light and reflect the light of sun. They are close to earth, therefore, their reflected light does not go under much refraction and they do not twinkle. They are smaller than stars. They are made up of rocks and gases. The time taken by the planet to complete one revolution is called ‘period of revolution’. The planets rotate on their own axis also along with revolution around the sun. The time taken to complete one rotation on its axis is called ‘period of rotation’. The planets revolve around the sun in a definite pathway called ‘orbit’.

 The four planets close to the sun are called inner planets. e.g.- Mercury, Venus,  Earth and Mars. The four far away from the sun are called outer planets. e.g.- Jupiter, Saturn, Uranus and Neptune.

1. Mercury (Budh)-It is the closest planet to the sun. It is the smallest planet of solar system. It is difficult to observe it during daytime because of glare of the sun. It can be seen just before the sunrise and after sunset near the horizon. Mercury has no natural satellite. Its gravity is low so it cannot hold the atmosphere.  There is no atmosphere on the mercury. So, the temperature varies between -200 ˚ C to 400˚C.

2. Venus (Shukra)-It is closest planet to the earth and almost of same size to the earth. There is atmosphere on the Venus having carbon dioxide gas. So, the temperature rises upto 480 ˚C. It reflects most of the sunlight. So, it is the brightest star of solar system. It can be seen before the sunrise and after sunset. It is also called as ‘morning star’ (for 292 days in eastern horizon) and ‘evening star’ (for 292 days in western horizon) although it’s a planet. Unlike other planets Venus rotates on its axis from east to west.

3. Earth (Prithvi)- Earth is a moderately hot planet where life exists. The presence of gases like oxygen and carbon dioxide helped in origin and existence of life on earth. There is presence of water and ozone layer in the atmosphere that stops absorbs harmful UV (Ultraviolet) rays coming on earth from the sun. It appears bluish-green from the outer space. The earth rotates around an imaginary axis that is inclined at an angle of 23 ½ ˚.

The equator divides the earth into two hemispheres-northern and southern hemispheres. When northern hemisphere is inclined towards the sun then there is summer season in northern hemisphere and winter in southern hemisphere. Similarly, when southern hemisphere is inclined towards the sun then there is summer season in southern hemisphere and winter in northern hemisphere.

On 21^st March, the length of day and night is same. It is also called ‘Vernal equinox’ or ‘March equinox’. During this time, there will be spring season in northern hemisphere while autumn season in southern hemisphere. After this, the day starts to become longer and night starts to become shorter in northern hemisphere. It continues till 21^st June or ‘Summer solstice’. It will be longest day of year. After this, days start to become shorter and nights start to become longer in northern hemisphere.

On 23^rd September, length of day and night become same. It is also called ‘Autumnal equinox’ or ‘September equinox’. During this time there will be autumn season in the northern hemisphere and spring season in the southern hemisphere. After this, the day starts to become shorter and night starts to become longer in northern hemisphere. It continues till 22^nd December or ‘Winter solstice’. It will be shortest day of year. After this, days start to become shorter and nights start to become longer in northern hemisphere.

On two days in a year, 21^st March and 23^rd September the sun rises exactly in the east direction. On all other days, the sun rises either north of east or south of east.

The term solstice means either most sunlight of the year (summer solstice) or least sunlight of the year (winter solstice) except equator.

4. Mars (Mangal)- It is about half in size of earth and called as red planet also. It’s very thin atmosphere has mostly carbon dioxide gas and water vapour. It temperature ranges from -120 ˚C to -25 ˚C.  It has a rocky surface of creators and mountains. The largest volcano and longest canyon are found on the mars. There are two moons of mars.

5. Jupiter (Brihaspati)-It is the largest planet of the solar system. It’s volume is 1300 times to the earth and its weight is about 318 times to the earth. Its diameter is about eleven times to that of the earth. It has a gigantic rotating storm called ‘Great Red Spot’. Jupiter rotates (spins) on its axis faster than any other planet. Jupiter has 72 natural satellites.

6. Saturn (Shani)- It I the second largest planet of the solar system. It has three beautiful rings made up of ice and dust. Its surface is made up of liquid and gases.  It has 53 moons.  E.g.- Titan, Hyperion , Phoebe etc.

7. Uranus( Indra) – It is made up of hydrogen , helium and methane like gases. It appears blue-green because of very low temperature that condenses methane gas. Like Venus, it also rotates from east to west direction. It is third largest planet. It has 27 moons.

8. Neptune (Varun)-It is the fourth largest planet of solar system. It atmosphere is made up of hydrogen, helium and methane. It appears blue in colour because the red light is absorbed by methane and blue light is mainly scattered. It is also called as ‘Blue Giant’.  Its diameter is about four times to the earth. It has 14 moons.

Constellations- A group of stars that has a recognizable shape or pattern is called a constellation.  Ancient people started identifying the shapes of constellation. The shapes of constellations resemble the objects familiar to ancient people like animals and mythological characters. There are 88 known constellations. e. g.-Great Bear, Orion, Cassiopeia, Leo major, Capricorn etc.

1. Ursa Major/ Big Dipper/ Great Bear/ Saptarshi/ Seven sages– There are seven prominent stars in this constellation. Its shape is like a big ladle or a question mark. There are 4 stars in the bowl and 3 stars in the ladle. It can be seen in northern sky during summer season in a clear moonless night. In north direction to the two last stars of bowl, there is a bright star called pole star (Dhruvtara). It is about five times the distance between the distance of the two stars of bowl.

Due to the rotation of the earth, the Ursa Major appears to move around the pole star from east to west.

2. Orion/Hunter-This is a very magnificent constellation in southern sky. It can be seen during winter season in late evening. It has 7-8 bright stars. The 3 middle stars represent a belt of a hunter.

The four other bright stars form a shape of a quadrilateral. In the east direction of the belt of Orion, there is a bright star called Sirius.

3. Cassiopeia– It is a prominent constellation in northern sky. It is visible during winter in late evening. Its shape is like distorted ‘W’ or ‘M’.

Artificial Satellites-They are manmade objects that revolve around the earth much closer to the moon. The speed of revolution of artificial satellites is as same as the speed rotation the earth. The artificial satellites are used for telecommunications, radio and television broadcasting, weather forecasting, marine study, defence, education etc.

e.g.- Aryabhat, EDUSAT, Kalpana-1, IRS, INSAT etc.

EXERCISE QUESTIONS

Q.1 Which of the following is NOT a member of the solar system?

(a) An asteroid                             (b) A satellite

(c) A constellation                       (d) A comet

Ans- (c) A constellation

Q.2 Which of the following is NOT a planet of the sun?

(a) Sirius                                       (b) Mercury

(c) Saturn                                     (d) Earth

Ans-(a) Sirius

Q.3 Phases of the moon occur because

(a) we can see only that part of the moon which reflects light towards us.

(b) our distance from the moon keeps changing.

(c) the shadow of the Earth covers only a part of moon’s surface.

(d) the thickness of the moon’s atmosphere is not constant.

Ans-(a) we can see only that part of the moon which reflects light towards us.

Q.4 Fill in the blanks:

(a) The planet which is farthest from the Sun is …………………

(b) The planet which appears reddish in colour is ………………

(c) A group of stars that appears to form a pattern in the sky is known as a …………….

(d) A celestial body that revolves around a planet is known as ………………….

(e) Shooting stars are actually not …………………..

(f) Asteroids are found between the orbits of ……………. and …………………..

Ans- (a) Neptune     (b)Mars    (c) constellation    (d) natural satellite    (e) stars   (f) Mars, Jupiter

Q.5 Mark the following statements as true (T) or false (F):

     (a) Pole Star is a member of the solar system. ( )

     (b) Mercury is the smallest planet in the planet in the solar system. ( )

     (c) Uranus is the farthest planet in the solar system. ( )

     (d) INST is an artificial satellite. ( )

     (e) There are nine planets in the solar system. ( )

     (f) Constellation Orion can be seen only with a telescope. ( )

   Ans. (a) F , (b) T , (c) F , (d) T , (e) F , (f) F

  Q.6 Match item in column A with one or more item in Column B:

             Column A                              Column B

       (i) Inner Planet                          (a) Saturn 

       (ii) Outer Planet                        (b) Pole Star

       (iii)Constellation                       (c) Great Bear

       (iv) Satellite of the earth         (d) Moon

                                                            (e) Earth

                                                            (f) Orion

                                                             (g) Mars

Ans- (i)- (e) Earth,(g) Mars       (ii)-(a) Saturn        (iii)- (c) Great Bear, (f) Orion      (iv)- (d) Moon

Q.7 In which part of the sky can you find Venus if it is visible as an evening star?

Ans- It appears as the evening star in the western sky just after the sunset.

Q.8 Name the largest planet of the solar system.

Ans- Jupiter

Q.9 What is a constellation? Name any two constellations.

Ans-A constellation is a group of stars that make an imaginary shape in the night sky. Name of two constellations are- Ursa Major and Orion

Q.10 Draw sketches to show the relative positions of prominent stars in

         (a) Ursa major   and      (b) Orion

Ans-

Q.11 Name two objects other than planets which are members of the solar system.

Ans- The Moon and Asteroids

Q.12 Explain how you can locate the pole star with the help of Ursa Major.

Ans- First of all one has to locate the Ursa Major in northern sky during summer season in a clear moonless night. There are 4 stars in the bowl and 3 stars in the ladle. In north direction to the two last stars of bowl, there is a bright star called pole star (Dhruvtara). It is about five times the distance between the distance of the two stars of bowl of Ursa Major. It appears that Ursa Major is rotating around the Pole Star.

Q.13 Do all the stars in the sky move? Explain.

Ans- No, Pole star does not appear to move. It is because the Pole Star is situated close to the direction of the axis of rotation of the earth. So, it appears stationary.

Q.14 Why is the distance between stars expressed in light years? What do you understand by the statement that a star is eight light years away from the earth?

Ans- The distance between stars is very large. It is inconvenient to express such a large distance in kilometers (km.).So, light year is used to measure the distance between celestial bodies.

A star is eight light years away from the earth means the distance between the star and the earth is eight light years away.

The distance between the earth and that star is = 8 X 9.5 X 10¹² km =7.6 X 10¹³ km.

Q.15 The radius of Jupiter is 11 times the radius of the Earth. Calculate the ratio of the volumes of Jupiter and the Earth. How many Earths can Jupiter accommodate?

Ans- Suppose, Radius of the Earth = r

          So, radius of the Jupiter= 11 r

The volume of Earth= 4/3 π (r)³   and  The volumes of Jupiter =4/3 π (11r)³

The ratio of the volumes of Jupiter and the Earth=4/3 π (11r)³/4/3 π (r)³   

                                                                                         =1331:1

Q.16 Boojho made the following sketch of the solar system. Is the sketch correct? If not, correct it.  

Ans- No, the sketch in the question is not correct. The positions of Mars, Venus, Uranus and Neptune are incorrect. The position of asteroids is also incorrect. The correct sketch is as like-

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