Class -8, Science, Chapter - 17 Stars And The Solar System Notes, Ncert Solutions & Frequently Asked Questions
Science
Class - 8, Chapter - 17
Stars And The Solar System
Notes, Ncert Solutions & Frequently Asked Questions
-- Notes --
⭐Celestial bodies:- The Universe includes all the stars, planets and galaxies and its size is unknown.
• The spherical region of the universe that can be observed from the Earth is called Observable Universe.
• This Observable Universe comprises of different physical objects that occur naturally on their own. These objects are called celestial objects.
• For eg.- the sun, the moon, the stars & the planets.
⭐Astronomy:- Astronomy is the study of the celestial objects and the phenomena associated with them.
• It is said to be one of the oldest Sciences in the world.
• It is associated with the study of the evolution of the universe, the motion of the celestial objects and their formation.
• It also help in predicting the climate of a place and the rainfall patterns.
⭐The Moon:- The moon is a natural satellite of the earth that revolves around the earth in a fixed orbit.
• Each planet of the solar system have their own moons or natural satellites.
• The moon does not emit any light of its own but it reflects the light of the sun that falls on it.
• As the moon revolves around the earth the part of the Moon that faces the sun gets to light up while the other part remains in dark. This results in different shapes of the moon in the sky.
• The different shapes of the lit part of the Moon are called the ‘Phases of the Moon’.
⭐The Phases of the Moon:-
Figure 2.- Phases of the Moon
• The phases of the moon repeat themselves after every 29 days and there are eight major moon phases.
• The day when we can see the whole moon in the sky is called a ‘Full Moon Day’. On this day this sunlight falls all over the moon.
• As the moon revolves and changes its position, the part which receives sunlight starts to decrease and the moon appears as if it is getting thinner.
• On 15th day after a Full Moon, the moon becomes completely invisible as no light of the sun falls over it due to its position behind the Earth. This is called a ‘New Moon Day’.
• On the next day of the New Moon Day, the moon that appears in the sky is called ‘Crescent Moon’.
• From this day, the moon starts growing larger until the 15th day when it reaches its full shape, that is, the Full Moon Day comes.
• Waning phase of the moon:- It is the phase of the Moon in which it decreases in size.
• Waxing phase of the moon:- It is the phase of the moon in which it increases in size.
• Blue moon:- Generally there occurs only one New Moon every month. But, sometimes there can be two new moons in a single month. The second New Moon in a month is called the Blue Moon.
• The phases of the Moon hold an importance in the Indian society and culture as different festivals in India are celebrated depending upon the Phases of the Moon.
• Festivals of India Relation with the phases of the Moon:-
⭐The surface of the Moon:- The surface of the Moon is barren and all covered with dust.
• There are Craters or bowl-shaped cavities present on the surface of the Moon.
• There are steep and high mountains present in huge numbers on the Moon.
• Some of the mountains are as high as the highest mountains found on the earth.
• There is no presence of water and atmosphere on the moon.
Figure 3.- Surface of the Moon
⭐No Propagation of Sound on the moon:- There is no atmosphere on the Moon so no gases are present on the Moon which could act as a medium for the propagation of sound. Hence, the astronauts of the Moon cannot hear each other.
• They generally use signs or microphones to communicate with each other.
⭐First Astronaut to land on the moon:- Neil Armstrong was the first person to land on the moon followed by another Astronaut Edwin Aldrin. Neil Armstrong landed on the moon on July 21 1969.
⭐Lunar eclipse:- The lunar eclipse occurs when the moon, the Earth and the Sun are very closely aligned with each other and the earth is present exactly in the middle of the moon and the sun.
• The shadow of the earth falls upon the moon.
• Therefore during the lunar eclipse, the moon does not reflect light of the sun but the light that it receives from the earth. As a result, the moon appears reddish in colour during a lunar eclipse.
Figure 4.- Lunar Eclipse⭐Stars:- The stars are celestial bodies comprising of hot gases (mainly helium and hydrogen).
• All the stars emit their own light.
• All the stars are not same. They may vary in brightness, size, colours & temperature.
• They are massive in size however they appear as a point-sized object from the Earth’s surface. This is so because they are at a huge distance from the earth.
• Stars have a life period. They take millions of years to born and live for tens of millions of years and eventually come to an end. Due to their long lifespan, the changes in the stars are not very sudden. Hence, they appear as permanent to us.
Figure 5.-There are millions of stars in the universe⭐Why stars twinkle? :- Stars emit light that when enters the earth surface has to travel through the several layers of the earth’s atmosphere.
• Hence, it appears that the light is travelling in a zig-zag motion due to the change in densities of the earth’s atmosphere and the density of the space. Therefore, it appears to us as if the stars are twinkling.
⭐Why Sun is a Star? :- The sun is also a star because the sun can produce heat and light energy on its own.
• The sun has a life period just like every other star.
• The sun is estimated to be born around 5 billion years ago and would live for the next 5 billion years.
⭐Distance between the Sun and the Earth:- The sun does not appear as a point-sized object like other stars because the sun is at a close distance to the earth as compared to other stars.
• The distance between the Earth and the Sun is estimated to be 150,000,000 km.
• The star that is closer to the Earth, after the sun, is called Proxima Centauri. The distance between the Earth and the Proxima Centauri is estimated to be 40,000,000,000,000 km.
⭐Light Year:- The distance between celestial objects is described in light years.
• A light year is the total distance that light travels in 1 year’s time.
• The speed of light = 300,000 km per second
• The distance between the sun and the earth = 8 light minutes
• The distance between Alpha Centauri and the earth = 4.3 light years
⭐Why are stars invisible during the daytime? :- Stars are present in the sky during the day as well as night time. However, during the daytime, the sunlight is so bright that it becomes difficult to locate stars in the sky.
⭐Why stars change their positions? :- If we observe a star or a group of stars for some hours their position appears to change. This is because of the motion of the earth. The earth keeps on revolving around the sun and hence changes its position with respect to the stars.
⭐Why stars appear moving from east to west? :- The rotation of the Earth takes place in a direction from west to east. To us, it appears as if the earth is still and the sky is moving from east to west direction. However, it is not true. Also, this is the same reason why the sun appears to rise from the east and set in the west.
⭐The significance of the Pole Star:- The pole star also called as Polaris is situated close to the earth's axis of rotation. As a result, the position of the pole star appears to be fixed. This is unlike the other stars as their position keeps on changing with respect to the earth.
Figure 6.- Pole Star⭐Constellations:- A constellation is described as a group of stars that form a recognizable shape or pattern.
• Ancient people used constellations as a way to recognise stars. The shape of constellations generally resembles with objects.
• There are not only 5 to 10 stars in a constellation. A constellation comprises of several stars but only some of them which are the brightest can be viewed.
• The stars that make up a constellation are found in the same line of sight; however the distance between these stars is not the same.
Figure 7.- A constellation⭐Major Constellations:-
a) Ursa Major / The Great Bear / Saptarishi / The Big Dipper:- This constellation is comprises of seven major stars.
• These stars form the shape of a big question mark or a ladle.
• The handle of the ladle is made by joining the 3 stars in a row and the remaining four stars are in the shape of the bowl of the ladle.
Figure 8.- Ursa Major• Significance of Saptarishi:- The seven stars in the Ursa Major constellation are believed to signify seven saints according to the Indian mythology. These seven Saints are believed to preserve the knowledge of the Vedas that they propagate to people in every new age.
Figure 9.- Saptarishi⭐ Locating pole star using Ursa Major:-
Figure 10.- Locating Pole Star using Ursa Major• The Ursa Major constellation includes two stars called Merak and Dubhe.
• If we draw an imaginary line from Merak to Dubhe and extended further in the same direction the first bright star that we would come across is the pole star.
• The pole star is located above the northern hemisphere and hence it cannot be seen through the southern hemisphere.
• Similarly, many constellations of the northern hemisphere can't be located from different points of the southern hemisphere.
b) Orion or the Hunter:-
• It comprises of 7 or 8 bright stars.
• There are three stars in the middle that appear as the belt of a hunter.
• The remaining four stars are arranged in the form of a quadrilateral.
⭐Locating Sirius Star using Orion:-
Figure 12.- Locating Sirius Star using Orion• The Sirius Star is the brightest star of the sky.
• It can be located with the help of the Orion constellation.
• If we draw an imaginary line through the three stars in the middle of the Orion and extended further we will reach the brightest star, Sirius.
c) Cassiopeia:-
• It is a constellation found in the Northern hemisphere & can be located in the sky during the winter season at the beginning of the night.
• The shape of this constellation resembles with a W or a distorted M.
• The name of this constellation was given after queen Cassiopeia in Greek mythology.
d) Leo Major:-
Figure 14.- Leo Major• It consists of several bright stars so it can be easily located in the sky.
• It can be viewed from the Northern hemisphere (January to June) & Southern hemispheres (summer and autumn).
• The Leo appears upside down from the southern hemisphere.
• It is one of the oldest constellations that were recognised almost 6000 years ago.
• This constellation signifies the Adventures of Hercules.
⭐ The Solar System:-
Figure 15.- The Solar System• The Solar System comprises the sun, the planets, asteroids, meteors and other celestial bodies.
• There is a gravitational attraction between the sun and the celestial objects that keep them revolving around the sun.
• The sun acts as the main source of heat and light energy for all the planets in the solar system.
⭐ The Planets:-
Figure 16.- the Planets• Our solar system comprises of 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
• The planets do not have their own light. They can reflect the light of the sun that falls upon them.
• The planets revolve around the sun in a fixed path called orbit.
• The period of revolution of a planet is the time that it takes to complete one revolution around the sun. As the distance between the sun and the planets increases the period of revolution of that planet also increases.
• All the planets also revolve around their own axis. The time of rotation of a particular planet is defined as the time it takes to complete one rotation.
• The planets have satellites or celestial objects that revolve around them.
⭐ Why do planets not fall into the sun?:- The sun exerts a gravitational force on all the planets. But they do not fall into the sun because they are revolving around the sun in a fixed path. This revolution prevents them from falling off into the sun.
⭐ Mercury:- It is the planet that lies closest to the sun.
• It is the smallest of all the planets.
• It is difficult to observe Mercury because it hides in the glare of the sun.
• Therefore it can only be observed or viewed in the sky just before the sunset or before the sunrise.
• It has no natural satellite of its own.
⭐ Venus:- It is the brightest planet.
• It can sometimes be viewed in the eastern sky before the time of the Sunrise or in the western sky before the sunset. Hence it is often called morning or evening star.
• It has no natural satellite of its own.
• Venus rotates from east to west, unlike the Earth which rotates from west to east.
• Venus also shows phases just like the moon.
⭐ Earth:- Life exists only on earth.
• The earth has favourable environmental conditions such as the presence of atmosphere and water that enable the existence of life on the earth.
• The temperature conditions of the Earth are also favourable for the existence of life because it is at a perfect distance from the sun.
• The earth possesses tilted axis which results in changing of seasons on the earth.
• The earth is covered with water naturally and therefore appears bluish-green when viewed from the space.
• The earth possesses its own natural satellite - the moon.
• Orbital and Equatorial Plane of the Earth:-
Figure 17.- Earth rotates on tilted axisa) Orbital Plane of the Earth:- Earth revolves around the Sun in an orbit. The plane in which it revolves around the Sun is called its orbital plane.
b) Equatorial Plane of the Earth:- The Equator is an imaginary line drawn from the middle of the earth. The plane of the equator is called the equatorial plane.
• The equatorial plane and the orbital plane of the earth are inclined at an angle of 23.5༠. Hence, the axis of the earth is tilted at an angle of 66.5༠ to the orbital plane.
Figure 18.- Change in Seasons Due to Earth's tilted axis
⭐Mars:- It is called the red planet because it has a reddish colour.
• It has two moons of its own.
• Mangalyaan:- Mangalyaan is an Orbiter launched by ISRO (Indian Space Research Organisation) that orbits around Mars. It was launched in November 2013 and was placed in its orbit around the Mars in September 2014 successfully. India is the first country that could place an orbitar in mars's orbit in the first attempt.
⭐ Jupiter:- It is the largest planet in the solar system. It can comprise of 1300 Earth-like planets.
• It does not have high mass as compared to its size. The mass of the Jupiter is almost 318 times of the mass of the earth.
• The rotation time of Jupiter is small around 9 hours 55 minutes.
• Jupiter has many moons out of which 4 large moons can be viewed from the earth with the help of a telescope.
• Jupiter has thin rings around it.
⭐ Saturn:- Saturn has thick rings around it that can be easily viewed with naked eyes.
• It has 30 natural satellites of its own.
• The density of Saturn is less than that of water making it the least dense Planet of the solar system.
⭐ Uranus:- Uranus rotates from east to west.
• It has 21 natural satellites.
• The rotation axis of Uranus is highly tilted.
• The atmosphere of Uranus comprises hydrogen and methane gas mainly.
⭐ Neptune:- It is the farthest planet from the sun hence it takes the most time to revolve around the sun.
• It has 8 natural satellites of its own.
⭐Period of Rotation, Revolution and Distance of all the Planets:-
⭐ Inner and Outer Planets:-
• The first four planets of the solar system are called inner planets. These planets have few moons of their own.
• The planets after Mars are called outer planets of the solar system. All these planets have a ring system around them and possess a large number of moons.
⭐ Differences between stars and planets:-
⭐ Other Members of the Solar System:-
a) Asteroids:- Asteroids are small rock-like objects that are found orbiting the sun.
Figure 19.- Asteroids in Space
• They are generally found between the orbits of Mars and Jupiter. This region is also called the Asteroid belt. However, asteroids can also be found in other places in the solar system.
Figure 20.- Asteroid Beltb) Comet:- Comets are big pieces of ice and rock that also revolve around the sun in elliptical shaped orbits.
Figure 21.- A comet• Comets are the leftover pieces of rocks that were formed at the time when the solar system was beginning its formation.
• Comets are often observed to have a tail. This tail appears as a comet approaches the sun.
• At that time, dust and gas are discharged from the comet due to the heat of the sun. Since the comet is approaching the sun, the tail of the comet is always in the opposite direction of the sun.
• As the distance between the sun and the comet increases the size of the tail of the comet also increases.
• Since comets revolve around the sun they can be viewed at periodic times from the Earth.
• Halley's Comet appears after every 76 years. It was last viewed in 1986.
c) Meteoroids and Meteors:- The asteroids in the space sometimes collide with each other and form meteoroids. Hence the small pieces of an asteroid can be called as meteoroids
Figure 22.- A meteoroid approaching Earth• From these meteoroids, meteors are formed.
• When a meteoroid enters the atmosphere of the Earth it appears as a streak of light in the sky or a meteor.
• The Meteors generally have high speed as they enter the atmosphere. Thus, they get heated up due to the friction of the atmosphere.
• They get evaporated in the sky before reaching the Earth's surface and this streak of light is therefore visible for a short time span only.
• Meteors are also called shooting stars because of their characteristic feature however they are not stars but just a piece of rock.
Figure 23.- A meteor is the sky
Figure 24.- Meteor Shower⭐ Artificial satellites:- These are the man-made satellites that are sent out into space to revolve around the earth.
• They orbit nearer to the earth as compared to the moon.
• Functions of artificial satellites:-
-- Weather forecasting
-- Transmission of signals of radio and television
-- Remote-sensing - acquiring information about an object from a distance, for example, the artificial satellite can acquire information about the earth from a distance
-- Telecommunication - transmission of information by means of radio signals and electromagnetic systems.
• Some Indian Artificial Satellites of the Earth:-
-- Aryabhata
-- INSAT
-- IRS
-- Kalpana-1
-- EDUSAT
⭐ Difference between artificial satellite and natural satellite:-
Figure 26.- Natural Satellites of the solar system-- NCERT Solutions --
Exercise Questions
Choose the correct answer in Questions 1-3.
Question 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
Solution:- c) A constellation
(A constellation is not a member of the solar system. They are the group of stars that forms a recognisable shape.)
Question 2.- Which of the following is NOT a planet of the sun?
a) Sirius
b) Mercury
c) Saturn
d) Earth
Solution:- a) Sirius.
(Sirius is a star, not a planet of the solar system.)
Question 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 the moon’s surface.
d) the thickness of the moon’s atmosphere is not constant.
Solution:- a) we can see only that part of the moon which reflects light towards us.
(The moon does not have its own light. It is only seen by us when sunlight falls on the moon and gets reflected towards us. So, we can only see the part of the moon on which the sunlight falls and reflects towards us.)
Question 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 appear 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 _________ .
Solution:-
a) Neptune
b) Mars
c) Constellation
d) satellite
e) stars
f) Mars, Jupiter
Question 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 of the solar system.
c) Uranus is the farthest planet in the solar system.
d) INSAT is an artificial satellite.
e) There are nine planets in the solar system.
f) Constellation Orion can be seen only with a telescope.
Solution:-
a) False
b) True
c) False
d) True
e) False
f) False
Question 6.- Match items in column A with one or more items in column B.
Question 7.- In which part of the sky can you find Venus if it is visible as an evening star?
Solution:- The planet Venus is found in the western part of the sky after sunset and is said to be known as an evening star.
Question 8.- Name the largest planet of the solar system.
Solution:- Jupiter is the largest planet of the solar system.
Question 9.- What is a constellation? Name any two constellations.
Solution:- The group of stars that forms a recognisable pattern in the sky is known as a constellation. Two constellations are Ursa Major and Orion.
Question 10.- Draw sketches to show the relative positions of prominent stars in
a) Ursa Major
b) Orion
Solution:-
a) Orion seems to be like a hunter. The three bright stars appear in the belt, while four bright stars are arranged in the form of a quadrilateral.
b) Ursa Major seems to be like a big dipper. There are four stars in the bowl of the dipper and three bright stars in the handle.
Question 11.- Name two objects other than planets which are members of the solar system.
Solution:-
a) Meteors:- Meteors are small celestial objects that are seen as bright streaks of light in the sky. Due to the heat produced by the friction of the Earth’s atmosphere, the Meteors are burnt out while entering in. This causes the bright streaks in the sky. They are not planets.
b) Asteroids:- Asteroids are a collection of a huge number of small objects, gases, and dust revolving around the sun. Between the orbits of Mars and Jupiter, they occupy a large gap. These are not planets; they are celestial bodies.
Question 12.- Explain how you can locate the Pole Star with the help of Ursa Major.
Solution:- For locating the pole stars in the sky, first, the big dipper, or the Ursa Major constellation, must be found. The bowl of the big dipper consists of four bright stars.
Imagine the two stars at the end of this bowl. Then make an imaginary straight line towards the northern direction connecting these two stars.
This imaginary line meets the star called the Pole star. The length of the imaginary line from the bowl is about five times the distance between the two stars of the bowl.
Question 13.- Do all the stars in the sky move? Explain.
Solution:- No, the earth rotates on its axis from west to east. Therefore, all the stars in the sky (except the pole star) seem to move from east to west. With reference to Earth, the pole star in the sky does not seem to move because it is located above the axis of rotation of the Earth in the north direction. It seems to remain stationary at a point in the sky.
Question 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?
Solution:- One light-year is equal to the distance travelled by light in one single year. Light year is used to express the huge distances of planets, stars & the sun. One light-year is equal to 9.46 ᥊ 10¹² km.
• The distance of the star from the Earth is eight light years. This means the distance between them is equal to the distance travelled by the light in eight years.
• The distance of the star from the Earth is 8 ᥊ (9.6 ᥊ 10¹²) = 7.6 ᥊ 10¹³ km away from Earth.
Question 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?
Solution:- Let us consider the radius of Earth is R.
Since, radius of Jupiter is 11 times the radius of the Earth.
So radius of Jupiter = 11R
Volume of the sphere of radius R is given as = 4 ⁄ 3πR³
Therefore, 1331 Earths can Jupiter accommodate.
Question 16.- Boojho made the following sketch (Fig. 17.29) of the solar system. Is the sketch correct? If not, correct it.
Solution:- No, the diagram made by Boojho is not correct because, in the solar system, the sequence of the planets of their distance from the sun is like Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Here. he had changed the positions of the planets Venus, Mars, Neptune and Uranus. Besides this, in the diagram, he showed the Asteroids belt between the orbits of Saturn and Jupiter. This is incorrect. The Asteroids belt is located in the middle of the orbits of Jupiter and Mars. The correct diagram of the solar system is given below.
-- Frequently Asked Questions --
Question 1.- How is the tail of a comet formed ?
Answer:- When a comet comes near the sun, some of its ice turns into gas. Gas and loose dust, freed from ice create a long illuminating tail behind it.
Question 2.- Venus is hotter than mercury, though it is farther away from Sun. Why ?
Answer:- Venus contains a higher percentage of carbon dioxide. So, the greenhouse effect is more by the trapping of the infrared rays of the sun.
Question 3.- Define light year. What is the relation between light year and kilometers ?
Answer:- One light year is the distance travelled by light in one year.
1 light year = 9.46 ᥊ 10¹² km.
Question 4.- Give any two points of difference between comets and meteors.
Answer:-
Question 5.- What is the cause of seasons on the earth ?
Answer:- The axis of rotation of the earth is not perpendicular to the plane of its orbit. This tilt causes a change in seasons.
-- By Durgesh Pandey
(Eklavya Coaching Institute)
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