Question Corner

Twinkling stars

Question: Why only stars blink and not planets?

K. Balaraj, Nandayal

Answer 1: The stars seem to twinkle, because we see the stars through the ocean of air, the atmosphere. The twinkling is caused by differences in temperature in the air. Some layers of air are hotter than others, and one layer is always swirling and moving through another. These different layers of air bend the star light in different ways, and at different angles. It is this passing through layers of air of different temperature that makes the light of the stars unsteady.

The stars near the horizon seem to twinkle much more than those high in the sky. This is because the light of these stars has to travel a longer path through a thicker layer of atmosphere, and thus has more chance to become disturb. Some times the stars twinkle much more than they do at other times. This is true because at some times the atmosphere is not so still as it is at other times, or because there is not such a variation of temperature within its different layers.

Planets do not twinkle, ordinarily, but seem to shine with a steady, unwavering light. Even through large telescopes, the biggest stars appear simply as tiny points of light, while the planets show very definite discs and surfaces. Hence, more rays come to us from the surface of a planet than from the surface of a star. The light from the planets does not waver as much as that from the stars the wavering of one ray of light is counteracted by the wavering of another ray in another direction.

If one could climb up above the atmosphere surrounding the earth and then look at the stars, he would see them shining with a clear and steady light, with no suspicion of twinkling.

M. Leo Christoper, Sirumugai, T.N

Answer 2: When a ray of light travels from one optical medium to another, there is a deviation from its original path. This phenomnon is called refraction. If a light ray travels from an optically rarer medium to an optically denser medium, the light ray always bends towards the normal. The normal is nothing, but an imaginary line drawn at the point of incidence.

The atmosphere of earth consists of a number of parallel layers of air with varying densities. Such that, the most dense layer of air is near the surface of the earth. Layers with decreasing order of densities occupy the successive layers, and the top most layer is the least dense layer.

Light rays originating from a star (say x), pass through the atmosphere, before reaching the observer. In doing so, the light bends towards the normal, thereby deviating from its path slightly. This deviation takes place each time the light ray travels from a less dense layer to a more dense layer. Finally when the refracted rays reach the observer, it traces a straight line path. To the observer, it appears to come from a pointy, which is higher in horizon. The pointy only gives an apparent position of the star.

The parallel layers of air are not stationery, but constantly intermingle with one another, thereby rapidly changing their densities. These changes give rise to the change in the apparent position of the star. As long as the star is within the line of sight of the observer, it is visible, but when the image falls outside the line of sight it is no longer visible. These changes in the apparent position of the star give rise to the ``blinking'' or ``twinkling'' effect.

Planets on the other hand are close to the earth, as compared to the stars. Their apparent position also changes with the changes in the densities of the air layers. But, the size of their apparent image being fairly large, seldom falls outside the line of sight of the observer. Hence they do not blink.

R.Ashok Ram, Madurai

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