Stars and the Solar System


  • We the human beings live on the surface of the earth.
  • When we look up we see the sky.
  • The sky appears blue in day because of sunlight and appears us dark at night.
  • Actually above us is a vast empty space to which we call it sky.

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Now what is sun?

  • During day time ,we see a bright ball in the sky which seems to move from east to west .
  • we call it a sun.


  • During night we see thousands of starts twinkling
  • If we observe them properly or through a telescope we can see a million of stars , some twinkling brightly
  • Few stars in the sky appear as such they are in a group. and we call that cluster/group as constellation
  • You Know our sun is also a big brightening star

What is planet

  • There are star like objects which do not twinkle .
  • they are actually planets which revolve around the sun.
  • Actually we had 9 planets previously but now we have only 8 planets as Pluto is not considered as a planet

Along with this there are moons meteors comets and lot more terms . Stay with us and we will have a beautiful visit of our beatiful universe in coming lessons.

As we have used this word Universe, So lets find out what a Universe is .

Universe- The Vast unimaginable space which encompasses most distant stars ,planets and anything else,which exist is called universe.

Basically Universe means everything including planets stars Galaxies an everything inside it .

Astronomy- The branch of science which deals with the study of universe is called astronomy.
it involves the methods instruments used for the study of the universe

Why stars not visible during day time?

  • During Day time, the light from the sun is so strong that it suppresses the light coming from the stars,and hence ,they are not visible to us.

How do the stars emit light

  • All stars are actually Giant ball of hydrogen gas.
  • Even our sun has Hydrogen gas inside it
  • at the center of this giant cloud of hydrogen the temperature is very high and about 2 million to 5 million degree Celsius
  • At such high temperature the hydrogen gas fuses (joins or melt) to form a heavier gs called helium,with the liberation of a huge amount of heat and light energy.
  • Thus this fusion of hydrogen gas which emits light energy.

The Sun

  • The Sun is the nearest star to the earth
  • in terms of mass and brightness it is a medium term star.but because of its closeness to earth it appears more brighter,Some how bigger and gives more heat.


So the question is how far is sun from our mother earth?

  • The distance between sun and earth is around 150 million kilometer
  • where as the distance of the nearest star after the sun from the earth is 40678000 million kilometer
  • Because of this huge and huge distance they appear as very small and give us very less heat and light as compared to the sun
  • The diameter of the sun  is approximately 109 times the diameter of the earth
  • The sun is around 3lakh times heavier than earth


Units for measuring the distance of stars from the earth.

Light Year- 

  • The Distance traveled by the light at a speed of 300000 km/s in one year (365 days) is called light year.

So one light year = Speed of the light * 365 Days 

300000 km/s*365*24*60*60 s= 9.46*1012 Km

Light Minute

  • The distance traveled by light at a speed of 300000 km/s in one minute is called a light minute

1 Light Minute is  = 300000 km/s * 60 Sec = 18*106 km.

How far is the sun from the earth if light is reaching the earth from the sun in 8.3 light minutes

  • 1 light minutes = 18*106 km.
  • 8.3 light Minutes= 18*106 km * 8.3 = 150000000 Km (15 crore km)

Why Does the distance between any two stars not change from viewed from the earth

  • Starts are moving away from each other at very high speed.
  • When looked from earth the distance between them does not seem to change as they are very very far away from the earth.
  • Hence to observe the change of their position it takes thousands of years

Why do the stars appear to move from east to west 

  • Earth rotates about its north-south axis from west to east . thus due to relative motion ,all heavenly bodies appear to move from east to west .

Compare this thing just like you are moving in a train. as you move in a direction(Lets say towards front) then other objects appear to move backwards or in opposite direction.

Why does the pole star not change its position in the sky.

  • The pole star is situated in the direction, which is directly above the geographic north-pole of the earth’s axis.
  • Thus its position relative to the earth doesn’t change and appears us stationary

How does the pole star help travelers and sailors at night.

  • The pole star always points in the north direction.
  • Thus looking at the position of the Pole Star during the night ,one can easily find the geographic directions ,hence the correct route.

Remember that Pole Star is not visible in the southern hemisphere ,so sailors cannot find direction in south hemisphere.

Now question is why Pole star is not visible in the southern hemisphere .


  • When you are in the Southern Hemisphere, the Earth is blocking your line of sight to the Pole star.
  • In the Southern Hemisphere there is a star at  the southern pole (Sigma Octantis), but is very hard to see with the naked eye.


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