The "sky" we see is actually a vast and boundless space. There are a lot of celestial bodies and planets in this space, some of which can shine like the sun, but we can only see it at night because it is too far away from our earth and looks very small, which is what we call a star.

The stars in the sky are very far away from the earth where we live, and they are all several light years away. For example, the Altair is 0/6 light-years away from the earth/kloc-0, so the light we see from Altair is actually emitted from Altair 16 years ago. Light from stars will pass through the atmosphere that protects the earth. The air in the atmosphere is not static. The hot air rises, the cold air falls and the wind keeps blowing, which makes every part of the atmosphere different in density, density and light transmittance, so it makes the stars look like twinkling in our eyes.