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.

Constellations and Asterisms

I've discussed constellations before but never really defined what they are. A constellation is a group of stars that are in the same approximate area in the sky. These stars generally are not associated with each other except that they are all in the same location in the sky.


For example, Orion has two really bright stars: Rigel, a blue supergiant that is the left foot of the Hunter, and Betelgeuse, a red supergiant that is the right shoulder. In the sky, they are only separated by 18.29°, but in space Rigel is 260 pc from Earth and Betelgeuse is 197 pc from Earth. Same area of the sky, but really far apart in space.

From APOD

Credit & Copyright: Matthew Spinelli

Betelgeuse is also part of something called an asterism: a smaller group of stars in the same general area in the sky that can either be a part of a constellation, or stars that are in other constellations. The asterism to which Betelgeuse belong is called the Winter Triangle along with Sirius (the Dog Star, the brightest star in Canis Major) and Procyon (the brightest star in Canis Minor).

Winter Triangle

Procyon, Betelgeuse and Sirius form the Winter Triangle.

Credit: StarryNightEducation

Remember the zodiac is made up of thirteen constellations. In all, there are 88 recognized constellations in the nighttime sky. I am not going to list them all here, but they can easily be found by using your favorite search engine to find them listed. Constellation names are used to actually describe stars in terms of what constellation you may find them. For example, α Centauri (one of the closest stars to our Sun) is found in the constellations Centaurus, hence Centauri. Rigel is also known as β Orionis and Betelgeuse is α Orionis. (I'll explain what the Greek letters before a constellation name means - Bayer designation).

Other example of asterisms include the Big Dipper and the Little Dipper, which are smaller portions of the constellations Ursa Major (Big Bear) and Ursa Minor (Little Bear), respectively. How many other asterisms do you know of?