The concept of stars twinkling in the night sky is so well-known that one of the most prominent children’s poems is based entirely around it. We have all heard it as children and remember it as adults—Twinkle, twinkle little star.

To understand why stars twinkle, it is important to understand what a star is and how its light reaches us. Stars are large astronomical bodies that emit energy by the process of thermonuclear fusion converting hydrogen into helium. In this process, part of the energy that is emitted is in the form of gamma ray photons which in other words means that stars emit light waves. This light must travel many light-years before they reach the Earth’s atmosphere. Then it must travel through the atmosphere before our eyes can perceive the star’s light.

On a clear night, a person can see as many as 6000 stars in the sky. Anyone who has observed the night sky would notice that starlight tends to appear and disappear or change in intensity or position, i.e. it twinkles. Scientifically this phenomenon is known as astronomical scintillation and is defined as variation in the level of illuminance. The term astronomical scintillation has two words—astronomical referring to light waves emerging from a source outside the Earth’s atmosphere and scintillation refers to the variation in position or brightness of a light source. The word illuminance refers to a measure of brightness. In simple terms twinkling of stars or astronomical scintillation is a change in the level of brightness of the star’s light as perceived by the human eye.

Earth’s atmosphere is not uniform and can be rather turbulent. As the light waves from a star traverses through it, it passes through many air pockets with varying qualities. Starlight passes through several atmospheric pockets varying in temperature and hence varying in air density. This variation in air density causes the refraction pattern of the light to change which means that the light waves bend in a different direction. All light that passes through the atmosphere experience this fluctuation is refraction. However, stars are at such great distances from the Earth that their light is represented by a single pinpoint of light in the atmosphere. A good analogy for this is a single pixel on your computer screen. Due to the pinpoint nature of the source of light waves coming from a star they become highly vulnerable to any interference in the atmosphere. Some of the light from the star reaches our eyes directly but some bends away when it passes through different air densities. This variation is what causes us to perceive the light differently in terms of intensity and appears to be twinkling. This refraction may also cause a slight change in the perception of position of the star’s light which also gives the appearance of a twinkling star.

For laymen, it is understood that the difference between the appearance of a star and a planet in the sky is that stars twinkle and planets don’t. The question arises that if all light must go through the turbulent atmosphere, why is that planets don’t twinkle? This difference is primarily attributed to the differences in distance between stars and the earth, and the planets and Earth. Since planets are closer to the Earth, they appear as disks or several points of light rather than pinpoints of light like stars. This means that subtle refractory changes in a single point of light within the disc do not affect the perception of the disk or the planet in its entirety.

The twinkling of stars can be attributed to the two main features—the large distances that cause a point source of light and the variation of the atmospheric density that the light wave must travel through.