Why do stars twinkle ? 🌟 | Why don't planets twinkle ? 🤔

Why do stars twinkle ? | Why don't planets twinkle ?

              "Twinkle ! Twinkle ! little star, how I wonder what you are?". This was something we all enjoyed singing as kids.

            But today we will see whether stars actually twinkle or not.

         Twinkling of stars is because of the atmosphere - a layer consisting of gases, ions, dust particles surrounding the earth. This atmosphere is responsible for 2 things - one is the twinkling of stars and if you were to look up at sky, point out to a star and say that's the position of the star... you are probably wrong. Let's understand this.

          The presence of various gases, ions and dust particles in the atmosphere, makes the atmosphere more denser than vacuum and this density varies from place to place. If there is more oxygen or nitrogen at one place, it is more dense and if it is less at another place, then it is less dense.

    

           When light from stars enters the atmosphere, due to change in medium, light bends and this bending of light when light travels from one medium to another medium is called refraction.

           These refracted rays reach our eyes and when we trace back those rays to find its origin, it would lead us to a new position where light “appears” to be coming from that point ( point 2 in the  above figure ).

            The actual position of the star will be just below the new position. We call this new position as ‘apparent position’( apparent here means- light is appearing to be but not actually coming from that point)

            Hence due to refraction, you are seeing the apparent position of a star but not its actual position.

 

Now let’s understand twinkling of stars.

           As said before, density is varying from place to place in atmosphere(understand - change in density means change in medium again), it means that light rays are undergoing refraction continuously ( in a zig zag pattern ).

 

            After continuous refraction, these refracted rays reach our eyes. There are numerous rays coming but let us assume that 5 light rays are reaching our eyes (as shown in figure below 1st diagram).

            Now you see that star is shining brightly. Let’s say density changed after some time at that place and out of those 5 rays, 3 rays refracted more and got deviated from the path.( above figure 2nd diagram)

            So only 2 rays are reaching your eyes and the star is appearing dim. The next instant, density changed again and 5 rays are reaching you, star appears bright, after some time density changed again, it’s appearing dim now.

           This process continues and due to this, star appears to be continuously flickering and this is known as “Twinkling of stars”.   

 

 Planets are way nearer to us than stars. Venus is also called the evening star. But why don’t they twinkle ?

            Let’s assume the planet to be a bunch of point sources of light like stars packed in. In case of a single star, their twinkling can be seen easily, but here there are so many stars twinkling at a time, if one is brighter, some other one is dim and at the next instant, former becomes dim, latter shines bright and same thing is happening with other stars too.

           To understand this in a better way, let’s say the planet to be a bunch of 100 stars packed in like a sphere. Say, at an instant, randomly 50 are shining bright and rest 50 are dim. At the next instant stars shining bright become dim and stars which were dim shine brightly and same cycle continues.

           Observe, at both the instants, only the bright portion is falling to your eyes ( since stars which are dim are not visible at all ). So a constant light is being received by your eyes and hence you see planets shining brightly at all time and not ‘twinkling’.

             Next time, when you look up at sky in night, you will be able to differentiate between a star and a planet and you will say –

“STAR, I know you are a wonder, kids enjoy singing your rhymes, but now I know ‘what you are’.”

 

Hope you have enjoyed reading this article.

 

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