Stars as viewed from Earth
The twinkling of stars is caused by the passing of light through different layers of the atmosphere. Stars as viewed by Hubble telescopeAlthough not the first space telescope, Hubble is one of the largest and most versatile, and is well-known as both a vital research tool and a public relations boon for astronomy.
Air temperature varies a great deal. It typically decreases by 6.5°C for every kilometer as we go upward, away from the surface of the Earth. This is the reason for the chillness we feel on the top of mountains. The atmosphere is made up of several “layers”. Each and every layer has got a different temperature and density. Light ray bends more in cold air than in warm air as the cold air has got greater refractive index compared to warm air. Hence the air molecules of warm air are far apart from each other, to produce less scattering.
Our atmosphere is very turbulent, with streams and eddies forming, churning around and dispersing all the time. These disturbances along with temperature variations in the atmosphere act like lenses and prisms that shift the incoming light from a star from side to side by minute amounts several times a second. This causes the star's image to change slightly in brightness and position, hence “twinkle”. Because of this effect observatories for studying the stars are situated on mountain tops. The reason for this is because the higher up you go, the air becomes thinner and it is less likely to cause the twinkling effect.
Our scientists are experimenting on how to compensate for the twinkling effect by adapting the optics of our large telescopes as fast as it occurs. As a result, we should soon be able to produce much sharper images from here on the ground.
You will notice that stars closer to the horizon twinkle more; this is because there is a lot more atmosphere between you and a star near the horizon than between you and a star, which is directly overhead.
The Hubble Space Telescope(HST) is a NASA space telescope that was carried into orbit by a space shuttle in 1990 (Refer figure). It is so successful in outer space because there is no atmosphere to make the stars twinkle, allowing a much better image to be obtained. This is why the Hubble Space Telescope can produce the brilliant and crisp images of the universe that we have come to know.