why do stars twinkle on a clear night

At night when you look up to the sky, if you try to focus on a star you might see it twinkling or flashing. This is called Бstellar scintillationБ. We can see up to 6,000 stars at night with the naked eye. Stars twinkle because we are viewing them through EarthБs atmosphere, which has lots of thick, moving air in it. The light is refracted (bent) through the atmosphere, in lots of different directions, which causes the appearance of twinkling. This light refraction also makes the twinkling appear as if the colours of the star light are changing. Light is composed of a spectrum of colours (red, orange, yellow, green, blue, indigo, violet), and this includes starlight. Light travels in БwavesБ, and different frequencies of the waves produce different colours.

Because the light is being bent at so many different angles, and travelling at different speeds when it passes through EarthБs atmosphere, it causes many different wave patterns in the light. This causes the twinkling of the star to be multicoloured! You can see this refraction at home by shining light through a prism. The light slows down or speeds up as it travels through the prism at different angles, and produces a rainbow! Figure 1: A prism and different light frequencies. Image credit: NASA (public domain). Planets do not usually appear to twinkle, because they are much closer to Earth than the stars.

The stars are very far away points of light (like our sun), that are travelling very far through space. The planets have a more stable image because the light that we can see coming off them is reflected from our own sun, and is much closer! Stars that are closer to the horizon appear to twinkle more than stars that are high up in the sky. This is because there is more atmosphere between you and a star on the horizon, than you and a star in the sky. If we were in outer space, stars would not twinkle, because the light coming off them would not be warped by any atmosphere. This is one of the reasons why the Hubble Space Telescope is so effective - it is already in space, so it can get much clearer pictures of the stars, and can see galaxies that are billions of light-years away.

Some other telescopes here on Earth can see the stars without the twinkling, by using complex mirrors that are constantly moving. The mirrors try to reflect the light from the star into a consistent beam, and combat the effect of the turbulence in EarthБs atmosphere. This is called adaptive optics, and is very useful for astronomers to more clearly view stars from Earth. The Hubble Space Telescope is much better Б but very expensive!
The stars appear to twinkle at night. Even the star closest to earth (sun) is about 150 million kilometers from the earth. There are more than about 6000 stars visible to us on a clear night sky.

These stars emit light. While some shine with steady light others appear to have varying brightness. This is what we perceive as twinkling. The twinkling of stars is scientifically termed as stellar scintillation. Between us and the stars is lots of air which is our atmosphere. The light from the star travels to earth via our atmosphere. The earth s atmosphere has lot of turbulence due to the moving warm and cold wind. Thus the light that passes through this air turbulence is at different angles at different moments. The refractive index of the different layers of the earth s atmosphere changes continuously and so the position of the image of a start changes with time.

This or bending of light coming from the stars makes the star to appear with varying brightness which the human eye perceives as twinkling. As the atmosphere above earth changes and blocks some of the light that comes from space, the largest space telescope Hubble telescope is put up in outer space above atmosphere. This telescope takes picture of planets, stars and galaxies that are billions of light years away. Planets do not scintillate. Stars are extremely distant from earth compared to the planets. Because they are nearer to earth, we receive a greater amount of light and therefore minor variations in the intensity are not noticeable.