Twinkling Stars and Static Planets

Twinkle, Twinkle Little Star
How I wonder what you are.




Everyone remembers this nursery rhyme. The question is why do stars twinkle? And how come planets do not?




The short answer is that it has to do with the apparent diameter of the star and the planet. Stars are generally so far away that they have no apparent diameter, so that when light from the star enters our atmosphere, the light is easily diffracted due to different pockets of air. Since the width of the light is so small, these diffractions make it seem as if the star's light is constantly blinking in and out as the light is diffracted away from our eye as the light travels through the atmosphere.




For planets, however, they do have an apparent diameter, albeit much tinier than that of the Sun or the Moon. But this size is enough that when light from the planet travels through different pockets of air in the atmosphere, only a small portion of the light is diffracted away from our light of sight, so the brightness of the planet does not waver, and therefore does not twinkle.

Refraction

Refraction of light is the bending of light as it travels from one medium to another. You have seen this without realizing it when you look at an object under water in a pool while you are on the surface. Another example is when you look at a straw in a glass of water, the straw looks bent. However, it is refraction that makes the straw look bent.

When light travels from one medium to another (for example air into glass or plastic) the light will bend because of the difference in density. A material like glass or plastic will have more atoms per unit volume than air, so the light will bend as they hit the atoms. A material's ability to cause light to "bend" is defined by its index of refraction, n.

 

The equation explaining the bend is given by n1sinΘ1= n2sinΘ2  where n is the index of refraction of the material (either material 1 or material 2) and Θ is the angle of refraction measured from the normal. If the light comes in normal (i.e. perpendicular to the interface of the two materials), the light does not bend because Θ is 0 and sin Θ is 0.


 


Another consequence of light refraction is that the speed of light changes as it transfers from one medium to another. This is given by v1/n1=v2/n2 where v is the speed in medium. So the speed of light really is not a constant if we go from one medium to another. In fact, the speed of light you know and love is measured in a vacuum with n = 1. All other material (including air) has an index of refraction greater than 1, which means that the speed of light is slower in every material in the universe, since by definition, a vacuum is a lack of material.


Refraction is what causes lenses to work and why telescopes with lenses are called refractors. The lenses at the objective end of the telescope and at the eyepiece end bend the light towards your eye (or the detector).


One last consequence of refraction is that the bending is also dependent on the wavelength of the light. The longer the wavelength, the less the light bends since the longer wavelengths have an "easier" time avoiding the atoms in the material. This is how a prism works. Red light is bent less than blue light and when you use a prism on white light (defined as the combination of all colors of light), you get a rainbow of color. This is also how astronomers get spectra of stars and galaxies by using prisms on their light and measuring the emission and absorption lines in the light profile.