Why Do Stars Twinkle

Why Do Stars Twinkle?

Twinkle, twinkle little star...I sang this little nursery song to my children a thousand times without ever stopping to consider why stars twinkle, so I decided investigate this topic for my research paper. While the answer to the initial question is fairly straightforward, interesting related questions such as: why DON'T planets twinkle or why do the colors seem to change as the stars twinkle, presented themselves and are explored. Now, when my grandchildren ask me this question, I will have an answer!

Look into the night sky on any given night and almost any star you look at appears to fluctuate in its brightness and intensity, giving it a "twinkling" appearance. The scientific name for this twinkling is stellar scintillation (or astronomical scintillation).

Stars, with the exception of our Sun, are very far away. So far away, in fact, that they appear to us as single points of light in the sky. The point of light that we see has traveled millions of light years through space, and then passes through the Earth's atmosphere and into our view.

The Earth's atmosphere is a relatively thin layer of gases that surrounds the Earth. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, with trace amounts of other gases. As we have learned this semester, this thin gaseous layer insulates the Earth from the sun's extreme temperatures, keeping heat inside the atmosphere at the same time that it blocks much of the sun's incoming ultraviolet radiation. Earth's atmosphere is approximately 300 miles thick, but the densest part (about 80%) is within 10 miles of the surface of the Earth. There is no exact place where the atmosphere ends; it simply gets thinner and thinner (less dense) until it merges with outer space. However, the air in the atmosphere is not uniformly distributed but organized in "pockets", with some of pockets being denser or more humid or of a different temperature than other pockets.

We are viewing the stars from the bottom of this sea of air. As the pockets of air move across our vision they act like lenses, bending the light from the stars in random ways as it passes through them. This bending is called refraction. A more familiar example of refraction is the phenomenon you experience when driving on a hot day: the hot air just above the road surface bends light more than the cooler air slightly above it. This refraction of the light makes the road surfaces appear to shimmer or to appear covered in water.

The point of light that we see, the light from the star, gets bent around these moving pockets of air as it passes through the atmosphere. Our eye can't really detect the motion, because it's too small; what we do see is the light from the star flickering or 'twinkling'.

Why do stars appear to change color? The degree of refraction (or bending) that the light from the star undergoes depends on its frequency. Different colors of light take slightly different paths as shown in the illustration above. Red light gets bent less than green or blue light.

The pockets of turbulent air refract the different frequencies of light along different paths so that as the star 'twinkles', it also appears to change colors. The oversimplified drawing below shows the path that different frequencies of light from a star might take. And the more the air is moving around, the more the path of the light will move around. The different paths of light will have slightly different lengths and will take different times to travel the path. This makes the light from the star appear to change colors. The more turbulent the air, the more apparent these changes will be. Also, stars that appear to be on the horizon will 'twinkle' and change colors more than stars directly overhead at the zenith because the light must take a longer path through more atmosphere