I was sitting at the observatory, waiting. It was 1990, and I was trying to make some observations as part of my master’s degree work. The problem was the rain. It had poured that afternoon (not unusual for September in the mountains of Virginia), and I was waiting for the sky to clear up enough to actually get some good images.
After a few hours my luck changed, and the clouds broke up. Working quickly, I found a bright star and aimed the telescope there to focus on it. But try as I might, the image of the star on the computer screen never sharpened. I would move the focus in and out, trying everything, but no matter what I did the star image was hugely fuzzy.
So, I did what any astronomer locked in a small, dark room for three hours would do. I went outside and looked up.
The bright star I had chosen was high in the sky and twinkling madly. As I watched, it flashed spastically, sometimes even changing colors. I knew immediately why I couldn’t get the star image sharp and crisp. The telescope wasn’t to blame, our atmosphere was. I waited a couple of more hours, but the star refused to focus. I went home, resigned to try again the next night.
Who hasn’t sat underneath the velvet canopy of a nighttime sky and admired the stars? So far away, so brilliant, so… antsy?
Stars twinkle. It’s very pretty. As you watch a star, it shimmers, it dances, it flickers. Sometimes it even changes color for a fraction of a second, going from white to green to red and back to white again.
But there, look at
Planets can and do twinkle, as much as stars. It’s just that twinkling rarely affects the way they look.
Having an atmosphere here on Earth has definite advantages like letting us breathe, fly paper airplanes, spin pinwheels on our bikes, and so on. But as much as we all like air, sometimes astronomers wish it didn’t exist. Air can be a drag.
If the atmosphere were steady, calm, and motionless, then things would be fine. But it isn’t. The air is turbulent. It has different layers, with different temperatures. It blows this way and that. And that turbulence is the root of twinkling.
One annoying property of air is that it can bend a light ray. This is called refraction, and you’ve seen it countless times. Light bends when it goes from one medium to another, like from air to water or vice versa. When you put a spoon in a glass of water, the spoon looks bent where the air meets the water. But, really, it’s just the light coming out of the water and into the air that bends. If you’ve ever gone fishing in a stream armed with just a net, you’ve experienced the practical side of this, too. If you don’t compensate for refraction, you’re more likely to get a netful of nothing than tonight’s dinner.
Light will bend when it goes from one part of the atmosphere to a slightly less dense part. For example, hot air is less dense than cooler air. A layer of air just over the black tar of a highway is hotter than the air just above it, and light going through these layers gets bent. That’s what causes the blacktop ahead of you to shimmer on a summer’s day; the air is refracting the light, making the highway’s surface look like a liquid. Sometimes you can even see cars reflected in the layer.
Here on the ground, the air can be fairly steady. But, high over our heads things are different. A few kilometers up, the air is constantly whipping around. Little packets of air, called cells, blow to and fro up there. Each cell is a few dozen centimeters across and is constantly in motion. Light passing in and out of the cells gets bent a little bit as they blow through the path of that light.
That’s the cause of twinkling. Starlight shines steady and true across all those light years to the Earth. If we had no atmosphere, the starlight would head straight from the star into our eyes.
But we do have air. When the starlight goes through our atmosphere, it must pass in and out of those cells. Each cell bends the light slightly, usually in a random direction. Hundreds of cells blow through the path of the starlight every second, and each one makes the light from the star jump around. From the ground, the size of the star is very small, much smaller than the cell of air. The image of the star, therefore, appears to jump around a lot, so what we see on the ground is the star appearing to dance as the light bends randomly. The star twinkles!
Astronomers usually don’t call this twinkling, they call it seeing, a confusing holdover from centuries past, but like most jargon, it’s stuck in the language. Astronomers determine how bad the seeing is on a given night by measuring the apparent size of a star. A star’s image dances around so quickly that our eyes see this as a blurring into a disk of light. The worse the seeing is, the bigger the star looks. On a typical night, the seeing is a couple of arcseconds. For comparison, the Moon is nearly 2,000 arcseconds across, and the naked eye can just resolve a disk that is about 100 arcseconds across. The best seeing on the planet is usually a half an arcsecond, but it can be much larger, depending on how turbulent the air is.
Seeing also changes with time. Sometimes the air will suddenly grow calm for a few seconds, and the disk of a star will shrink dramatically. Since the light of the star gets concentrated into a smaller area, this lets you see fainter stars. I remember once sitting at the eyepiece of telescope for several minutes, looking for the very faint central star in a nebula. The star was just at the visibility limit of the telescope. Suddenly the seeing steadied up for a moment and the ghostly, pale-blue star snapped into my sight. Just as suddenly, the seeing went sour and the star disappeared. It was the faintest star I have ever seen with my own eyes, and it was amazing.
So why don’t planets twinkle? Planets are big. Well, in reality they’re a lot smaller than stars, but they are also a
Jupiter is affected by seeing just as much as a star. But, since the disk of the planet is big, it doesn’t appear to jump around. The disk does move, but it moves much less relative to its apparent size, so it doesn’t appear to dance around like a tiny star does. Small features on the planet are blurred out, but the overall planet just sits there, more or less impervious to turbulence.
Another way to increase twinkling is to observe near the horizon. When a star is just rising or setting, we are looking at it through more air because our atmosphere is curved. This means there are more cells between us and the star, and it can twinkle madly. Ironically, if you happen to be looking over a city, the air can be more stable. There are commonly smog layers over cities which stabilize the seeing, perhaps their only beneficial effect.
As it happens, different colors of light are refracted more easily than others. Blue and green, for example, bend much more than red. Sometimes, in really bad seeing, you can see stars change colors as first one color and then another is refracted toward you. Sirius is the brightest nighttime star, and it usually appears to be a steadily white color to the eye. But sometimes, when Sirius is low, it can flicker very dramatically and change colors rapidly. I have seen this myself many times; it’s mesmerizing.
It can also lead to trouble. Imagine: You are driving along a lonely road at night and notice a bright object that appears to follow you. As you watch it flickers violently, going from bright to dim, and then you notice it’s changing colors, from orange to green to red to blue! Could it be a spaceship? Are you about to be abducted by aliens?
No, you are a victim of bad astronomy. But the story sounds familiar, doesn’t it? A lot of UFO stories sound like this. Stars appear to follow you as you drive because they are so far away. The twinkling of the star changes the brightness and the color, and imagination does the rest. I always smile when I hear a UFO tale like this one, and think that although it may not have been a UFO, it was
Twinkling stars may inspire songs and poetry, but astronomers consider them an inconvenience. One of the reasons we build big telescopes is that they help increase our resolution of objects. Imagine
