Most of us are familiar with the childhood song “Twinkle Twinkle Little Star.” Some of us have even been potty trained with a rendition that leaves out the “w.” And even if some people, who must have lived under a rock for their childhood, do not know the song, they probably have at least witnessed a twinkling star on their own. So we all know about this phenomenon, but what creates the light of stars to fade in and out like icicle lights around Christmas time?
The truth is that stars do not twinkle. They shine in a solid beam of light, just like our sun, a much closer star. They only appear to twinkle because of the effect of Earth’s atmosphere. When a star is shining down on Earth, we are seeing it through layer upon layer of moving air that is known as our atmosphere. The light travels through the moving air, which often causes it to bend, or refract, from its direct beam. As the beam of light hits pockets of air with different densities, such as hot air or cold air, the beam bends. It bends many times on its journey to our eyes, and our brain interprets this bending as a twinkle. An interesting fact is that stars closer to the horizon are going to twinkle more than those that are directly above us. Why? Because those that lie on the horizon send light through more air, which means that they get bent more than other stars.
It’s simple physics if you take the time to think about it. Yes, the words “simple” and “physics” can be juxtaposed in a sentence! When heat warms the air around it, it forces the air to move in all different directions. This is known as convection. Think about when you’re grilling on a nice summer day, and you see the heat rising off the grill in little waves; it’s the same idea. So when the light from a star hits a warm air pocket when traveling through our atmosphere, it will also bend like the waves above a hot grill.
The question then rises about planets and why they don’t twinkle. After all, we also view them through the same atmosphere, and their light must also travel through the different densities that the stars’ light do. Why don’t planets twinkle? The simple answer is that the planets are much closer to our atmosphere than the stars. Since the stars are so far away, they are basically just a little point of light against a blackened sky. This pinpoint of light is easily bended from the distance that we view it. It’s the same reason that we don’t see our sun twinkle; it’s so close that is gives off a much broader ray of light, so it is not bent as easily.
So the simple answer is that stars twinkle because we see them through moving air, which bends their light. In outer space, there is no atmosphere, so the stars do not twinkle. Even on our moon, they appear like the straight beam of light that they are. This stellar scintillation, the scientific name for twinkling stars, will only appear through out atmosphere of moving air.