Why do stars twinkle? Explain.

Hint: The stars are seen from the earth via the earth’s atmosphere, which acts as a medium for the light traveling from the star. Additionally, the stars are not at rest, they are in constant motion, so the light rays coming from the star vary with time.

Complete step-by-step answer:
Twinkling of stars occurs when the brightness or position of a distant luminous star varies when it is viewed through a medium, which in the case of stars is different layers of the turbulent atmosphere. This phenomenon of the twinkling of stars or other astronomical objects is termed as astronomical scintillation.
Stars are very far from the Earth due to which they appear as a point source of light. The Earth’s atmospheric turbulence caused by fluctuation in air density, temperature and speed at different altitudes, acts as prisms or lenses for that source of light, also the velocity of light decreases with increased air density causing atmospheric refraction of light.
Atmospheric Refraction makes the point source of starlight deviate from its original path and refracts the light as per the changing atmospheric turbulence due to which the star brightens or dims with a change in the density, velocity, and temperature of the air.
The atmospheric refraction of a star’s light is inversely proportional to the altitude of Earth which means as the altitude decreases the atmospheric refraction increases.

Additional Information:
i. Atmospheric Refraction increases as the altitude of Earth decreases.
ii. Mirage happens due to atmospheric refraction near the ground level.
iii. Humans can see 6000 or more stars in a clear night sky.
iv. Refraction also applies to electromagnetic radiation, sounds, and other astronomical bodies.

Note: Taken at $10^\circ {\text{ }}C$ and 1013.25 kPa, the atmospheric refraction of light from a star at the zenith or the highest point of Earth taken as a celestial sphere is zero. At $45^\circ $ altitude it is calculated to be 1ˈ (one-arc-minute), at 10° altitude it is 5.3 arc-minute, at $2^\circ $ altitude it is 18.4 arc-minute and at the horizon, it is 35.4 arc-minute.