Why do stars twinkle?
Stars appear to twinkle because they are pinpoint sources of light. In reality, of course, the stars are far from tiny, but their enormous distances from earth reduce them to dimensions so miniscule that even the largest telescopes are unable to reveal them as disks. They are mere point sources. Thus a beam of light from a star entering your eye is a fragile thread that is easily rippled by the ever present turbulence in the earth’s atmosphere, which causes the twinkling.
Stars twinkle because their light must pass through pockets of Earth’s atmosphere that vary in temperature and density, and it’s all very turbulent. On rough nights, a star appears to shift position constantly as its light is refracted this way and that. It’s much like watching a coin appear to dance at the bottom of a pool. Astronomers try to overcome the twinkling by using adaptive optics, in which many small mirrors on the scope adjust constantly to allow for the atmospheric disturbances.
The stars aren’t actually doing anything to create this effect. This appearance is due, instead, to the motion of the air layers of our own atmosphere. It is like looking at air just above a hot, summer road: everything is all wiggly. Well, if our atmosphere is ‘wiggling’ between the star and your eyes, then the light coming through it is disturbed and we see the star as ‘twinkling’. Astronomers love non-twinkling skies, because that means the air is stable and the images of planets and stars will be very clear through their binoculars and telescopes.
Twinkle, twinkle, little star, How I wonder … The scientific name for the twinkling of stars is stellar scintillation (or astronomical scintillation). Stars twinkle when we see them from the Earth’s surface because we are viewing them through thick layers of turbulent (moving) air in the Earth’s atmosphere. Stars (except for the Sun) appear as tiny dots in the sky; as their light travels through the many layers of the Earth’s atmosphere, the light of the star is bent (refracted) many times and in random directions (light is bent when it hits a change in density – like a pocket of cold air or hot air). This random refraction results in the star winking out (it looks as though the star moves a bit, and our eye interprets this as twinkling). Stars closer to the horizon appear to twinkle more than stars that are overhead – this is because the light of stars near the horizon has to travel through more air than the light of stars overhead and so is subject to more refraction. Also, planets
The songline goes “Twinkle twinkle little star”. What is the cause of the “twinkling” of stars? Does light from planets “twinkle” as does light from stars? A young person of my acquaintance asked me this question, and I didn’t have a good answer. Stars twinkle because of turbulence in the atmosphere of the Earth. As the atmosphere churns, the light from the star is refracted in different directions. This causes the star’s image to change slightly in brightness and position, hence “twinkle.” This is one of the reasons the Hubble telescope is so successful: in space, there is no atmosphere to make the stars twinkle, allowing a much better image to be obtained. Planets do not twinkle the way stars do. In fact, this is a good way of figuring out if a particular object you see in the sky is a planet or a star. The reason is that stars are so far away that they are essentially points of light on the sky, while planets actually have finite size. The size of a planet on the sky in a sense “ave
