Twinkling of Stars

Twinkling of Stars

Twinkle, twinkle little star' may have been the first rhyme you learnt, but may still be unaware of what makes stars twinkle. When we look at stars, quite often they do not appear to shine steadily. They disappear for a fraction of a second before reappearing (the intensity of light from them fluctuates), or their positions seem to shift slightly in random directions. We call this the twinkling of stars. Let us see why this happens.

Since the temperature of air and its humidity are not the same everywhere in the atmosphere the refractive index of air varies between different masses (or pockets) of air, even at the same altitude. Thus, there exist air pockets whose refractive index is different from that of the air surrounding it.

When a ray of light passes through such a pocket of air, it bends due to refraction as in Figure. Light from a star may pass through one or more air pockets before reaching the eye. But the air pockets keep moving So, the direction of the ray reaching the eye keeps changing, causing the image of the star to shift in random directions or even disappear for an instant. The amount of star light reaching the eye also keeps changing due to the shift in the direction of the light. So, the brightness of the star seems to change.

The stars near the horizon twinkle more than those that are overhead. This is because ligh from a star near the horizon has to cover much more distance in the atmosphere than that from stars which are overhead. Therefore, it has greater chances of encountering air pockets whose density is different from the surrounding air.

Why planets do not twinkle Planets do not seem to twinkle like stars, although light coming from them also passes through the air pockets in the atmosphere. This is because stars appear a point objects to us due to their enormous distance from the earth. On the other hand, planets a comparatively nearer, and therefore, for us they are more like extended object. Different parts of the planet form an extended image at the eye, and we are unable to detect the random shifts in the smaller portions of this image.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

In the atmosphere the refractive index of air varies between different masses (or pockets) of air, even at the same altitude because of ________________.
A. Difference in temperature of air at same altitude.		B. Difference in humidity of air at same altitude.
C. Both A and B		D. None of these
Right Option : C
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Question : 2

Which of the following statement are not correct for stars: 1. They do not appear to shine steadily. 2. They disappear for a fraction of a second before reappearing (the intensity of light from them fluctuates), 3. Their positions seem to shift slightly in random directions.
A. Only 1		B. Both 1 and 2
C. 1, 2 and 3		D. None of these
Right Option : D
View Explanation

Question : 3

Planets do not seem to twinkle like stars, although light coming from them also passes through the air pockets in the atmosphere because __________________
A. Large size as compared to stars		B. Large distance as compared to stars
C. Both A and B		D. None of these
Right Option : B
View Explanation