Why Do Stars Twinkle?

Twinkling of stars is due to the large distance between them and Earth. Even when viewed with the help of a large telescope, they appear as pinpoints easily disturbed by the Earth's atmosphere. 

Once the light from a star enters the Earth's atmosphere, each of its rays get refracted, causing a slight change in the direction due to various temperature and density layers of the atmosphere. You can visualize that light moves in a zig- zag path instead of a straight path to your eyes in case the Earth didn't have an atmosphere.

Explain the Phenomenon of Twinkling of Stars

The atmospheric refraction causes the twinkling of the stars. In other words, the air above us is like a big ocean and is not very dense fluid, which has a small bending effect on light that passes through it. If it were to be perfectly still, the light from the star would follow a straight path, and the star's image would have appeared stable.

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But in reality, the atmosphere is always moving, and winds at every level will cause slight differences to shift around, and different densities bend light at different amounts. This will result in the light from the star as a zig-zag movement as they come close to our eyes making it appear to twinkle.

A little bit of more apparent disk will take a lot more bending of the incoming light for the rays from all parts of the disk to get bent away from your direction so that large objects would appear to blink momentarily and so appear to twinkle. To prevent twinkling, it doesn’t take much of an apparent disk.

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It also depends upon the condition that happens in the air above you at any given time. During clear nights, there will be a lot of rolling in the atmosphere, and the stars will appear to twinkle more. On other nights, when the upper air is calm, twinkling will not happen as it ought to be.

The whole effect can be compared to what you see when you are at the bottom of a swimming pool during bright sunlight. If the water surface is wavy, there will be a lot of moving light and dark patterns on the pool floor. If there is no disturbance and the surface of the water is smooth, and there is no wind, then the moving pattern will disappear.

Why do Stars Twinkle and Planets do not?

When the light from the stars enters the atmosphere, it collides with gas molecules and scatters (reflection and interference). Planets are a collection of a large number of point sources of light, whereas stars are considered to have a point source.

We are able to see a tiny beam of light from it as the star is so far away from it. This light will get scattered from our eyes and then back into them, almost like it is blinking on and off. The process is so fast it appears like twinkling.

As compared to stars, planets are closer to us and send more light; even if some light beams get scattered away, others get to us. So, planets do not usually twinkle.

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Explain why Planets do not Twinkle

Planets do not twinkle because of the following reasons:

a. Planets reflect the low-intensity light reaching them; they are not the source of light.

b. Compared to stars, the planets are much closer to Earth, due to which the shift in atmospheric refraction is smaller.

c. Since the planets are closer, they appear larger than stars, so the shift is not enough for planets to twinkle.

The planets should twinkle if they are spotted low in the sky. This is because you are looking through more atmospheres in the direction of any horizon than when you look overhead.

If you are able to see stars and planets from outer space, both will shine steadily since there will be no atmosphere to disturb the steady streaming of their light.

Based on twinklers and non-twinklers, can one determine which object is a star and which the planet? The observers who have an experience they can, but for the first time to recognize a planet, would be noticing the steadiness of its light by contrasting it to a nearby star.

Do you know?

Stars don’t usually twinkle, but when seen from the surface of the Earth, they just appear to twinkle. Due to the effects of Earth's atmosphere, the stars twinkle in the night sky. When starlight enters the atmosphere, it is affected by winds in the atmosphere and by areas with different densities and temperatures. These are the causes due to which light from the star appears to twinkle when seen from the ground.

FAQ (Frequently Asked Questions)

1. Why do Planets Shine More Steadily?

Ans. As the distance between the Earth and planets is small; they don't appear as pinpoints but tiny disks in our sky. One can see planets as a disk when observed through a telescope while stars remain pinpoints. The lights from these little disks are also refracted from the Earth's atmosphere, as it travels towards our eyes.  

But while light from one edge of a planet disk might be forced to zig one direction; light from the opposite edge of disk might be zagging in an opposite way. The planet appears to shine steadily since the zigs and zags of light from a planetary disk cancel each other out.

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2. Why are Some Stars Bright and Others Dim?

Ans. All stars are not equidistant from us, some are close, and some are farther. The star will appear brighter the closer they are to us. The stars also come in various sizes and brightness. Compared to smaller stars, larger stars shine brightly. So, the brightness of the star depends upon the size and how far it is from us.

3. What are Constellations?

Ans. A group of objects that make an imaginary shape in the night sky is known as the constellation. This is usually named after mythological characters, people, animals, and objects. Different shapes from the same group of bright stars have been made by different people in different parts of the world. It is similar to the game of connecting dots. In the past, it was useful for navigation and keeping track of the seasons. Since all the stars are at a different distance, the constellations would appear totally different to inhabitants of another planet orbiting another star.