Textbook: The Universe | General Science Class 6 (Maharashtra Board) PDF Download

 Page 1


112
??.? ? ???? ????????
16.3 : The Andromeda galaxy 
16.2 : The Milky Way
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.
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)
16.1: Observation of the sky
16. The Universe
Observe and discuss.
The universe includes innumerable galaxies, 
the space between them and also energy. 
Location of 
the sun
Page 2


112
??.? ? ???? ????????
16.3 : The Andromeda galaxy 
16.2 : The Milky Way
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.
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)
16.1: Observation of the sky
16. The Universe
Observe and discuss.
The universe includes innumerable galaxies, 
the space between them and also energy. 
Location of 
the sun
113
Types of galaxies : Various types identified according to their shapes
Spiral Elliptical Barred spiral Irregular
16.4 : Various galaxies
16.6 : A nebula
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 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.
16.5 : Hubble Telescope Stars
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.
Page 3


112
??.? ? ???? ????????
16.3 : The Andromeda galaxy 
16.2 : The Milky Way
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.
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)
16.1: Observation of the sky
16. The Universe
Observe and discuss.
The universe includes innumerable galaxies, 
the space between them and also energy. 
Location of 
the sun
113
Types of galaxies : Various types identified according to their shapes
Spiral Elliptical Barred spiral Irregular
16.4 : Various galaxies
16.6 : A nebula
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 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.
16.5 : Hubble Telescope Stars
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.
114
1. Which celestial bodies form the solar system ?
2. What is the difference between stars and planets ?
3. How many planets are there in our solar system ?
4. What is to be found between Mars and Jupiter ?
The solar system
The solar system consists of the sun, the planets, 
asteroids, comets and meteors. The planets Mercury, V enus, 
Mars, Jupiter and Saturn can be easily seen. 
The Mercury, Venus, 
Earth and Mars are the 
inner planets whereas the 
Jupiter, Saturn, Uranus and 
Neptune are outer planets. 
Outer planets have rings 
around them. The crust 
of all the inner planets is 
hard. The outer planets 
have gaseous outer cover.
 
The sun
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. 
The sun
Can you tell ?
Find out.
Visit the sites of the institutes ISRO (www.isro.gov) and NASA (www.nasa.
gov) and collect information about the various celestial bodies in the solar 
system and universe, and discuss the same in the class. 
Name of 
the  
planet
Number 
of known 
satellites
Inclination 
of the axis 
(in  
degrees)
Period of 
rotation
*
Period of 
revolution
*
Magnetism Atmosphere Rings 
Mercury 0 0.01 58.65 days 88 days No No None
Venus 0 177.2 243.00 days 225 days No Yes None
Earth 1 23.5 24 hours 1 year  
(365 days)
Yes Yes None
Mars 
2 25.2 24 hrs 37 mn 1.88 years No Yes None
Jupiter 64 3.1 9 hrs 56 mn 11.87 years Yes Yes Yes
Saturn 33 26.7 10 hrs 40 mn 29 years Yes Yes Yes
Uranus 27 97.9 17 hrs 24 mn 84 years Yes Yes Yes
Neptune 13 28.8 16 hrs 11 mn 164 years Yes Yes Yes
Planets of the solary system - facts and figures
Page 4


112
??.? ? ???? ????????
16.3 : The Andromeda galaxy 
16.2 : The Milky Way
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.
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)
16.1: Observation of the sky
16. The Universe
Observe and discuss.
The universe includes innumerable galaxies, 
the space between them and also energy. 
Location of 
the sun
113
Types of galaxies : Various types identified according to their shapes
Spiral Elliptical Barred spiral Irregular
16.4 : Various galaxies
16.6 : A nebula
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 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.
16.5 : Hubble Telescope Stars
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.
114
1. Which celestial bodies form the solar system ?
2. What is the difference between stars and planets ?
3. How many planets are there in our solar system ?
4. What is to be found between Mars and Jupiter ?
The solar system
The solar system consists of the sun, the planets, 
asteroids, comets and meteors. The planets Mercury, V enus, 
Mars, Jupiter and Saturn can be easily seen. 
The Mercury, Venus, 
Earth and Mars are the 
inner planets whereas the 
Jupiter, Saturn, Uranus and 
Neptune are outer planets. 
Outer planets have rings 
around them. The crust 
of all the inner planets is 
hard. The outer planets 
have gaseous outer cover.
 
The sun
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. 
The sun
Can you tell ?
Find out.
Visit the sites of the institutes ISRO (www.isro.gov) and NASA (www.nasa.
gov) and collect information about the various celestial bodies in the solar 
system and universe, and discuss the same in the class. 
Name of 
the  
planet
Number 
of known 
satellites
Inclination 
of the axis 
(in  
degrees)
Period of 
rotation
*
Period of 
revolution
*
Magnetism Atmosphere Rings 
Mercury 0 0.01 58.65 days 88 days No No None
Venus 0 177.2 243.00 days 225 days No Yes None
Earth 1 23.5 24 hours 1 year  
(365 days)
Yes Yes None
Mars 
2 25.2 24 hrs 37 mn 1.88 years No Yes None
Jupiter 64 3.1 9 hrs 56 mn 11.87 years Yes Yes Yes
Saturn 33 26.7 10 hrs 40 mn 29 years Yes Yes Yes
Uranus 27 97.9 17 hrs 24 mn 84 years Yes Yes Yes
Neptune 13 28.8 16 hrs 11 mn 164 years Yes Yes Yes
Planets of the solary system - facts and figures
115
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 : It is the 
brightest planet in 
the solar system. It 
is seen in the sky 
in the east before 
the sunrise and in 
the west after the 
sunset. It rotates around itself from 
east to west. It is the hottest planet.
Earth : It is the 
third planet of the 
solar system. No 
other planet other 
than the earth has 
life on it. As the 
earth is a magnet, 
there is a magnetic field around the earth. 
It diverts the harmful rays from the sun 
towards the polar regions of the earth.
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.
* The periods of rotation and revolution of the planets are expressed relative to those periods on 
the earth (Page 114).
Page 5


112
??.? ? ???? ????????
16.3 : The Andromeda galaxy 
16.2 : The Milky Way
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.
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)
16.1: Observation of the sky
16. The Universe
Observe and discuss.
The universe includes innumerable galaxies, 
the space between them and also energy. 
Location of 
the sun
113
Types of galaxies : Various types identified according to their shapes
Spiral Elliptical Barred spiral Irregular
16.4 : Various galaxies
16.6 : A nebula
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 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.
16.5 : Hubble Telescope Stars
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.
114
1. Which celestial bodies form the solar system ?
2. What is the difference between stars and planets ?
3. How many planets are there in our solar system ?
4. What is to be found between Mars and Jupiter ?
The solar system
The solar system consists of the sun, the planets, 
asteroids, comets and meteors. The planets Mercury, V enus, 
Mars, Jupiter and Saturn can be easily seen. 
The Mercury, Venus, 
Earth and Mars are the 
inner planets whereas the 
Jupiter, Saturn, Uranus and 
Neptune are outer planets. 
Outer planets have rings 
around them. The crust 
of all the inner planets is 
hard. The outer planets 
have gaseous outer cover.
 
The sun
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. 
The sun
Can you tell ?
Find out.
Visit the sites of the institutes ISRO (www.isro.gov) and NASA (www.nasa.
gov) and collect information about the various celestial bodies in the solar 
system and universe, and discuss the same in the class. 
Name of 
the  
planet
Number 
of known 
satellites
Inclination 
of the axis 
(in  
degrees)
Period of 
rotation
*
Period of 
revolution
*
Magnetism Atmosphere Rings 
Mercury 0 0.01 58.65 days 88 days No No None
Venus 0 177.2 243.00 days 225 days No Yes None
Earth 1 23.5 24 hours 1 year  
(365 days)
Yes Yes None
Mars 
2 25.2 24 hrs 37 mn 1.88 years No Yes None
Jupiter 64 3.1 9 hrs 56 mn 11.87 years Yes Yes Yes
Saturn 33 26.7 10 hrs 40 mn 29 years Yes Yes Yes
Uranus 27 97.9 17 hrs 24 mn 84 years Yes Yes Yes
Neptune 13 28.8 16 hrs 11 mn 164 years Yes Yes Yes
Planets of the solary system - facts and figures
115
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 : It is the 
brightest planet in 
the solar system. It 
is seen in the sky 
in the east before 
the sunrise and in 
the west after the 
sunset. It rotates around itself from 
east to west. It is the hottest planet.
Earth : It is the 
third planet of the 
solar system. No 
other planet other 
than the earth has 
life on it. As the 
earth is a magnet, 
there is a magnetic field around the earth. 
It diverts the harmful rays from the sun 
towards the polar regions of the earth.
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.
* The periods of rotation and revolution of the planets are expressed relative to those periods on 
the earth (Page 114).
116
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.
Satellite
Asteroid
Find out.
Obtain information about the various 
asteroids and dwarf planets in the solar 
system and discuss it in the class.
A comet 
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. 
Can you tell ?
Use your brain power !
Comets are classified 
in two main groups.
Long period comets : 
These comets take 
more than 200 years to 
complete one revolution 
around the sun. 
Short period comets : 
These comets take 
less than 200 years to 
complete one revolution 
around the sun.
Dwarf planet
1. Why do we see only one side of 
the  moon?
2. Which planet has a day longer 
than its year?
Have you ever seen in the evening or in the predawn hours  
a large celestial body with a long tail? What is it called?