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The Stars- Physics Guide for Class 8

The Stars Class 8 Science Guide

Information about The Stars

Title

The Stars

Class

Class 8

Subject

Class 8 Physics

Topics Covered

  • Galaxy
  • The Stars
  • The Pole Star


If we look up into the sky on a clear dark night, far from the glow of streetlights, we can see a faint band of light running across the heavens. This is the Milky Way, our own Galaxy. It contains about two hundred billion stars and countless other objects.

The stars of this galaxy spread out from its centre like a spiral. It is, therefore, known as a spiral galaxy
  • All the members of this galaxy, (including the sun (and its 'solar family') and billions of other stars), revolve around its centre in much the same way as the earth revolves round its own axis.
  • The Milky Way Galaxy is, thus, indeed a very huge 'object'. However, we now know that our universe contains billions of such galaxies.
  • The Milky Way Galaxy is, thus, but a very very tiny part of the universe as a whole.
  • On a clear night, we can see many stars with the unaided eye. These appear to twinkle when viewed from the earth.
  • Apart from the stars and the moon, we can also sometimes see some other 'bright objects' in the night sky. These, however, do not twinkle. They are the planets.
  • The word 'planet' is of Greek origin and means 'wanderers'. Unlike the stars (which appear almost fixed), the planets keep on changing their positions.
  • We now know that planets do not have any light of their own. Their observed brightness is because of the light of the sun reflected by them.
  • We also sometimes observe 'bright lines of light', flashing across the sky for a very short duration; we call these as shooting stars.
  • The stars, the sun, the moon, the planets and the shooting stars are the main celestial objects that make up the universe. 

The Stars

Stars are very hot and huge heavenly objects made up of very hot gases. They appear like 'dots' only because they are very very far away from us. Their light takes million of years to reach the earth. Distances of stars are, therefore, expressed in terms of a unit called, the light year. One light year is the distance covered by light in one year.

  • The star, nearest to the earth, is the Sun itself.
  • The next nearest star is the Alpha Centuari. This is at a distance of 4.3 light years from the earth. This means that we get the light from this star only 4.3 years after it gets emitted from it.
  • In comparison, it takes about 8 minutes only for the light of the sun to reach the earth. The sun's distance from the earth is, therefore, (nearly) 8×60×3×108 m = 1.44×1011 m or 14.4×107 km. This is 14.4 crore kilometres.
  • We must remember that there are countless other stars whose light reaches us in a much longer time ranging from thousands of years to even millions and billions of years!

The majority of the stars are, compared to the distance of the sun, millions of times farther away from us. It is for this reason that, our nearest star, the sun, appears like a big spherical ball to us but the other stars appear like 'dots' or 'points' only.

When we look at stars on a clear dark night, what do we observe? Are all the stars equally bright? Do they appear to have the same colour?
  • The colour of a star is determined by its surface temperature.
  • Stars, which have a lower temperature, appear red.
  • Those, with a high temperature, appear white and those, with a very high temperature, are blue.
  • The size and colour of stars change with brightness and temperature.
  • After the sun, it is the star Sirius, which is the brightest star in the night sky.
  • The Alpha Centauri is the third brightest star in the night sky.
  • All the stars, including the sun, move around some celestial 'point' with huge speeds,
  • However, since the stars are so far away from us, the distance between any two stars does not appear to change inspite of their great speeds. 

It is easy to observe that even though the stars do not appear to move with respect to each other in the sky; there is a shift in their positions with respect to the earth. The stars, like the sun, appear to move from east to west. This is due to the rotation of earth on its axis from west to east. Hence a star, which appears to rise in the east in the evening, appears to set in the west in the early morning.
 
  • We also know that earth revolves around the sun. This revolution of earth around the sun, and its rotation about its own axis, is the cause of the same star rising four minutes earlier after each day.
  • This implies that the overall 'set-up' of the sky changes every night.
  • This change may not be noticeable in a day or week, but one can observe the difference over about a month. 

Polar Star (Dhruva Tara)


The Pole Star, known as the Dhruva Tara in Hindi, is a special star present in the northern hemisphere. This star, unlike the other stars, appears to be fixed at one place. In fact all the stars appear to revolve around the pole star. 
  • This is so, because the pole star lies on a line close to the axis of rotation of the earth.
  • Because of this special feature, this star has been very useful to travellers. It helps them to find directions at night. 
  • The pole stare defines the north direction.
  • The other directions can then be defined with respect to this direction. 

Some Important Points

  • The Alpha Centuari consists of three stars — the double stars Alpha Centauri A and Alpha Centauri B together with a small faint red dwarf called Proxima Centuari. 
  • We often use the astronomical unit, (one A.U.) for I measuring distances and sizes of celestial objects. One A.U. is equal to the mean distance between the Earth and the Sun; it equals 1.5×1011m.
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