Why Do Stars Twinkle?

Often, you might have wondered why do stars twinkle? Stars appear to twinkle when they are seen from the surface of the earth. So, does that mean that they don’t actually twinkle? Yes, because of the turbulence in the atmosphere, they appear to be twinkling. The light from the stars gets refracted as they pass through different mediums when they come in contact with the wind and different temperatures and densities in the atmosphere. In the next section, let us more about it.

Why do stars twinkle?

When a ray of light travels from one medium to another it ‘bends’. This phenomenon is referred to as refraction. If it travels from a rare medium to a dense medium, it bends towards the normal and if it travels from a dense medium to a rarer medium, it bends away from the normal. The speed through which it travels changes depending on the medium and therefore this bending occurs.

Stars Twinkle

Watch this video to learn more about refraction and get a clear picture of this phenomenon:


This effect can be observed when light passes through a prism or a glass slab and even when light passes through water. The light ray actually travels from air to a medium of different density here.

So how are refraction and twinkling connected? The atmosphere of the earth is made of different layers. It is affected by winds, varying temperatures, and different densities as well. When light from a distant source (a star) passes through our turbulent (moving air) atmosphere, it undergoes refraction many times. When we finally perceive this light from a star, it appears to be twinkling! This is because some of the light rays reach us directly and some bends away from and toward us. It happens so fast that it gives a twinkling effect.

If you are in a large empty field such that the horizon is visible to you, you will notice that stars in this region appear to twinkle a lot more than stars right overhead. Why do you think this happens? Why do stars twinkle more when viewed from such places? It is because there are more layers of the atmosphere between you and the star in this direction. Therefore, more series of refractions!

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Quick Summary

Why do the stars twinkle?

As a star’s light penetrates the Earth’s atmosphere, every individual stream of starlight is refracted – caused to change direction, slightly – by the various temperature and density layers in Earth’s atmosphere. You might think of it as the light travelling a zig-zag path to our eyes, instead of the straight path the light would travel if Earth didn’t have an atmosphere.

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