Most of us are familiar with the song “Twinkle, twinkle, little star”. So, why do stars twinkle?
First of all, though, lets define what me mean by twinkle. When astronomers go out and look at the night sky, we like to see the stars as tiny and very steady pinpoints of light. However, all too often they seem to sort of blink in and out and to dance around slightly. Sometimes, they seem to flash different colors and sparkle. The stars do not appear completely steady and unchanging. This is what amateur astronomers and most people in the general public would call twinkling. Naturally, professional astronomers can’t use such a simple term as twinkling, though! Instead, we call this twinkling effect scintillation.
So, what causes twinkling or scintillation? Well, as it turns out, the stars don’t really twinkle. Rather, this is an effect of our atmosphere. In space, the stars would appear as steady pinpoints. Turbulence in the atmosphere is largely responsible for the twinkling effect, so the steadier the atmosphere, the less the stars seem to twinkle and sparkle, and the more unsteady the atmosphere, the more the stars seem to twinkle and sparkle. Of course, that very turbulence also distorts the view for professional astronomers or for amateur astrophotographers, so while the twinkling might be pretty, serious astronomers hate to see twinkling stars.
But, how does atmospheric turbulence make the stars twinkle (or scintillate)? The stars are very far away. In fact, they are so far away that they basically are like point sources as far as we are concerned. That means that the light appears to come from a single point in the sky (unlike the Moon in which light comes from a disk about half a degree across, or a planet in which light appears to come from a very small disk in the sky). As this light passes through Earth’s atmosphere, it is bent a bit whenever it passes through a region of slightly denser air. Different colors of light, in principle, take slightly different paths, since the degree of refraction (bending) that light undergoes is frequency dependent. So, the path of the light through the air from the star to your eye takes a sort of convoluted path schematically shown in the following drawing. Note that this drawing is greatly exaggerated in terms of the effect of the atmosphere.
Now, if this were all that was going on, then the star would simply appear slightly out of position in the sky rather than twinkling. If the different colors were bent far enough apart, then the star might appear as a tiny little splotch of colors (like a tiny rainbow) rather than a pinpoint. That would be if the air were completely unmoving. But, that isn’t what happens. The air moves around. Solar heating causes convection. Winds blow. Different parts of the air are compressed and rarefied as the air moves. These different regions refract light differently (which is part of the reason that the light takes a convoluted path through the atmosphere, anyway). But, these regions of thicker and thinner air, more and less humid air, moving air, etc., are always moving around. As the move, the convoluted path changes. In my drawing above, you can think of the different colored lines as being the path of the starlight at different times. The more the air is moving around, the more the path of the light dances around.
As the path of light moves around, the apparent position of the star changes slightly. This is part of the twinkling. Normally, though, the motion back and forth in the sky is too small for the human eye to observe. If you ever go outside and you can see the stars dancing around, then you know to just give up and not even bother trying to set up a telescope! But, the different paths are slightly different lengths. The paths do not all take the exact same length of time for light to traverse. So, as the paths shift from one to another sometimes the shift takes less time than the difference in time that the light takes to travel the different paths. When that happens, the star can sometimes appear to wink in and out very quickly. This tiny flicker is what most of what you perceive as twinkling. But, as I said, different colors of light are generally refracted differently, so the different colors also shift back and forth distance (and thus in time of travel). Red light tends to be bent less than the green light which is bent less than blue light. This means that sometimes some colors can shift in time differently than others. The result is that the blue light and the green light flicker in and out at different times than the red light. That can make the star appear to flash different colors. Again, the more unsteady the air, the bigger effect that this will be. Sometimes the air is fairly steady, and the effect happens in too small of time intervals to notice. At other times, however, the time interval is longer (though still a fraction of a second) and you can see the star flash quickly between colors. Sometimes the effect is so pronounced that novice observers might be tricked into thinking that the flashing lights are the navigation lights for a passing aircraft. And, of course, if you look up and see almost all the stars in the sky flashing different colors, then that is not a good night to be observing!
Look at my picture above. I drew it for a star high in the sky as seen by the observer. But, think about a star near the upper left or right corners of the drawing. The light would take a longer path through the atmosphere to the observer. That would mean that the light would pass through more disturbed air, making all the twinkling effects even more pronounced. So, stars near the horizon twinkle far more than stars that are nearly overhead.
When I was growing up, I remember being told, “Stars twinkle, planets don’t.” I wasn’t the only one told this, though, and often at my public talks people ask me why stars twinkle and planets do not. This has to do a bit with what I said earlier about the stars appearing as pinpoints while the planets are tiny disks. The disk of the planet is very small, and you don’t see the disk of a planet (except in certain rare cases with Venus) because the disk is smaller than the resolution limit of your eye. But, it still is a disk. Stars are so far away that they still appear as pinpoints even without the Earth’s atmosphere. Only a tiny handful of stars are close enough that any hint of a disk has ever been detected, and even that was at the detection limit for the best professional instruments available. The light from the star dances around as I have explained, causing twinkling. However, for a planet light comes from a group of points in the sky forming the disk of the planet. The light from each of these points also dances around and flickers and changes colors. However, the flickering of one part of the planet is masked by that of other parts of the planet. The tiny instant that one part of the planet flickers out, another part might flicker into view. When one part of the planet flickers red for a tiny fraction of a second, another part might flicker green and another blue. The end result is that all of these effects tend to average out. So, to the unaided eye, the planet seems to shine steadily while the stars around it twinkle in the sky. But, this is a bit simplistic explanation. You see, the more unsteady the air, the bigger the twinkling effects. If the air is really unsteady, then the light from the whole planet might be taking different paths through the atmosphere. In that case, even a planet can appear to flicker, flash, and twinkle in the sky. But, that means that the old tale that stars twinkle and planets do not twinkle is incorrect. Planets can twinkle if the air is very unsteady. Whenever I go outside and I see Jupiter or Saturn twinkling and flashing, I know that serious work will be difficult because the air is so unsteady. In really bad atmospheric conditions, I have even looked up and seen the edge of the Moon appear to be twinkling. Now, that is really bad air! I didn’t even bother trying to observe on that night.
So, that is part of the reason that stars twinkle. It is all atmospheric.