Why Do Stars Flicker?

TwinkleLittleStar-1Why Do Stars Twinkle?

The observation of innumerable shining stars in the night sky is an amazing experience. But the question is ‘why do the stars twinkle?’ The answer is not simple because it lies in the principles of physics that govern the passage of light through a medium such as the Earth’s atmosphere. To find the answer it is necessary to understand the characteristic features of stars, the composition of Earth’s atmosphere, the property of light rays, and the phenomenon of light refraction.

Stars in the night sky:
The most of tiny dots of light observed in the night sky are stars. The stars are located at a great distance from the Earth; the star closest to the Earth is the Sun. Like the Sun, all stars are giant celestial objects, and are light emitting source. Some stars are several hundred of kilometers in diameter but are located at such a great distance from the Earth that their light takes several hundred light years to reach the Earth. Before reaching the Earth the light travels through two distinct medium, the outer space above the Earth’s atmosphere and the Earth’s atmosphere. The outer space is devoid of any matter and therefore the light rays move through it undisturbed in a straight path. But as soon as the light rays enter the Earth’s atmosphere their passage is faced with a disturbance in their straight progression.

The Earth’s Atmosphere and the Twinkling Effect:
The Earth is surrounded by a mixture of gasses which form the Earth’s atmosphere. The answer to the observed phenomenon of twinkling of stars is hidden in the composition and properties of the atmosphere. The atmosphere differs in density and pressure at different levels from the surface of the earth (the sea level) to the other extreme where the atmosphere merges with the outer space. The density is a measure of concentration of air particles in a certain volume of the air. Near the surface of the Earth, the atmospheric density is the highest and it exerts a high pressure but moving upwards gradually the density and pressure are reduced until the boundary of atmosphere ends in the space.

It may be noted that while the stars as such are a steady sources of light, the ‘twinkling’ observed by eyes is an aberration in viewing them as an unsteady light source. The scientific term for ‘twinkling’ is scintillation which describes rapid changes in the brightness of a star and also its change of position and color. Scintillation is observed with the celestial objects which are situated at great distance and appear as tiny dots. Objects which are not situated very far off, such as the planets, do not scintillate.

‘Twinkling’ of stars observed by our eyes, in effect, is a perception of light emitted by stars moving in different directions. This ‘twinkling’ aberration is caused because the light from the stars is not able to reach our eyes in a straight line. Instead, while passing through the atmosphere, the passage of light is twisted and turned in random directions several times before reaching the eyes.

Phenomenon of Scintillation and Refraction of Light:
The heat from the Sun constantly affects the density of air in the Earth’s atmosphere. The changes in air density lead to constant push and pull in the atmosphere giving rise to air pockets of different densities which act as lenses and prisms. A light ray passing through a medium of a uniform density travels in a straight line. But, when the light travels from one density to another its direction is changed due to refraction. Therefore, when the light rays emitted by stars pass through the Earth’s atmosphere comprising air pockets of different densities these rays are reflected side to side by minute amounts several times a second giving the scintillation effect.
The reasons why stars scintillate but planets do not also lies in the mechanism of refraction of light rays. The planets being closer to Earth have multiple observable points of light. While the light rays emerging from those points pass through the atmosphere, the deviation in the path of rays from multiple points (refraction) average out to give the viewer an steady image. On the contrary, the light rays emitted from stars appear to emerge from a single point source, and show a greater degree of deviation thus causing the scintillation effect.

For the same reasons, stars closer to the horizon appear to scintillate more than the stars overhead because the thickness of atmosphere is greater at horizon than the sky overhead. Scintillation or twinkling is a challenge in the study of stars because it affects their clear view. Efforts are being made to develop appropriate optics in large telescopes to negate the twinkling effect of stars. To avoid the effect of scintillation on clear observation of stars and other stellar objects, the Hubble Space Telescope is placed in the space.

References:
1. Imaging Through Turbulence. By Michael C. Roggemann & Byron M. Welsh, CRC Press LLC, Florida. 1996. ISBN 0-8493-3787-9 (alk.paper). http://www.crcpress.com/product/isbn/9780849337871
2. P.A.Robinson (2000). Wave propagation in disordered media. Philosophical Magazine Part B, Volume 80, Issue 12, pages 2087-2108. http://www.tandfonline.com/doi/abs/10.1080/13642810008205764?queryID=%24%7BresultBean.queryID%7D#.VPa023yUd3M
3. Why do star twinkle? By John A. Graham, Carnegie Institution of Washington. Scientific American. October 24, 2005. http://www.scientificamerican.com/article/why-do-stars-twinkle/
4. Why do stars twinkle while planets do not? http://math.ucr.edu/home/baez/physics/General/twinkle.html
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