Stars twinkle and twinkle because their light is refracted and reflected as it travels through the atmosphere.
First, let's take a look at the structure of the atmosphere. The Earth's atmosphere is composed of multiple layers, from the ground to the upper altitude, which can be roughly divided into troposphere, stratosphere, and upper atmosphere. The air density, temperature, and pressure of these layers all change with increasing altitude, thus affecting the propagation characteristics of light in these layers.
When starlight passes through the troposphere, the path of light travels slightly due to changes in air density and temperature, creating a phenomenon known as "flickering". The temperature and density changes in the troposphere are caused by a variety of factors such as wind, air pressure, temperature gradient, etc. These factors cause the refractive index of light to change constantly, causing the light to travel along a path that makes the stars appear to be twinkling.
In addition, water vapor, aerosols, and other particles in the atmosphere scatter and absorb starlight. As starlight passes through these particles, a portion of the light is scattered or absorbed, causing the starlight to diminish in intensity or change color. These scattering and absorption can also cause changes in the brightness of the star, making it appear to be flickering.
In conclusion, the reason why stars twinkle and twinkle is mainly due to a variety of factors such as refraction, reflection, and scattering of light as it travels through the Earth's atmosphere. These factors cause the light to travel in a constant way, which in turn changes the brightness of the stars. By understanding these reasons, we can better understand the propagation properties of light in the atmosphere, while also being able to better appreciate the beautiful starry sky.