Why Do Stars Twinkle, and How Do Telescopes Deal With It?

1. Moving Air Keeps Bending the Light From a Star

Scientific American, in an answer by an astronomer at the Carnegie Institution, compares it to a coin at the bottom of a swimming pool. The coin seems to wobble because the water bends the light on its way to your eyes. Starlight has to cross several miles of air, and the atmosphere is full of streams and eddies that keep churning. These act a bit like tiny lenses and prisms, nudging the light sideways by small amounts several times a second[1].

A star is so far away that it is effectively a single point of light. When the beam from a point is nudged around, the spot seems to jitter, and the amount of light reaching you can rise and fall. That is twinkling[1]. Astronomers call it scintillation[2]. Japan's National Astronomical Observatory describes the root cause the same way: fluctuations in the air's refractive index, meaning how strongly the air bends light, disturb the starlight[3].

The same bending explains the shimmer above a hot road in summer, and the sparkle of distant city lights at night. The star is not flickering. The air between you and it is.

2. Why Planets Usually Twinkle Less Than Stars

Planets like Venus, Mars, and Jupiter can look like bright stars, yet they tend to shine more steadily. They are much closer to us, so through a telescope they show up as small discs rather than points. Each patch of the disc is bent in its own way, and the flickers from neighboring patches average out, so the total light changes only a little[1].

It is a matter of degree, not a hard rule. One astronomy writer points out that planets, and even the Sun and Moon, twinkle too, only by amounts we rarely notice[2]. So "stars twinkle and planets don't" is a handy rule of thumb, not a law.

3. An Old Complaint: Newton Said Telescopes Are Limited by Air

People noticed the problem long before they could fix it. In his book Opticks, Isaac Newton wrote that the air we look through to see the stars is in constant tremor. He pointed to two signs of it: the trembling of shadows cast from high towers, and the twinkling of the fixed stars[4]. He added that telescopes had limits because of this, and that the only remedy was an extremely calm, clear air, such as might be found on the tops of the highest mountains, above the thicker clouds[5].

He was right about the remedy. Many large telescopes today stand on high mountains with thin, steady air. The Subaru Telescope, for example, sits at 4,200 meters on Maunakea in Hawaiʻi[3].

The most thorough escape is to leave the air behind. Where there is no atmosphere, there is no twinkling caused by air, which is why the Hubble Space Telescope can take such sharp pictures[1].

4. Adaptive Optics: Measure the Wobble, Then Cancel It

Even on Maunakea, the air is not perfectly still. Blur from the atmosphere spreads a star's image to about 0.5 arcseconds, which is roughly ten times larger than the telescope could achieve on its own[3]. (An arcsecond is 1/3600 of a degree.) In a 2006 announcement, the Subaru team gave similar numbers for near-infrared light: a theoretical sharpness of 0.06 arcseconds, against an actual average of about 0.6[6].

The fix is called adaptive optics. A sensor watches the light from a bright "guide star" near the target (a natural star, or an artificial one made with a laser, as described below) to measure how the air is distorting it. A deformable mirror, one whose surface shape can be changed, then adjusts to cancel the distortion. Looking through that mirror, a ground telescope can get images almost as sharp as a space telescope's[3]. On October 9, 2006, Subaru tested a new 188-element system and reported that it brought the telescope's sharpness close to its theoretical limit, about ten times better[6][7].

But what if there is no bright star next to the thing you want to study? Before lasers, adaptive optics could be used only in the roughly 1 percent of the sky that happens to have a suitable guide star nearby[6]. The solution is to make one. At 90 to 100 kilometers up, there is a thin layer of sodium atoms. Shine a laser of wavelength 589 nanometers into it, and the atoms glow, creating an artificial guide star[3]. With that, the Subaru team expected that adaptive optics could work over more than half the sky[6].

Newton wanted to climb above the air. Modern telescopes climb the mountain and then measure, and cancel, whatever wobble is still left.

5. Can Twinkling Be Measured? A High School Team Tried

You do not need a giant telescope to put a number on twinkling. In 2014, three students at Kokugakuin University Tochigi High School in Japan recorded the North Star with a 15-centimeter refracting telescope and a cooled CCD camera. They filmed the changing brightness readings for 60 seconds at a time, and took the standard deviation, a measure of how widely the readings scatter, as their "twinkle score." They also measured a light bulb 899 meters from the telescope for comparison[8].

In their data, the star twinkled more on windy winter nights. Between summer and winter, higher humidity went with less twinkling. On one night, December 5, the twinkle of the North Star and the twinkle of the distant bulb moved together, with a correlation of 0.64, which suggests that air near the ground matters too[8].

Treat this as a hint, not a rule. It is a student poster summary from one place and a limited number of nights, and a correlation does not show what causes what. Even so, it shows a nice idea: a vague feeling that stars twinkle more on some nights can be turned into numbers that anyone could check.

6. Watch Distant Lights Twinkle on Different Nights, Safely

With an adult, from a safe spot such as a yard or balcony, look at distant lights: a faraway street lamp or a lit window works. On different evenings, notice whether the lights sparkle a lot or stay fairly steady, and write down the date and how windy it was (the weather forecast's wind speed is fine). After some nights you can see whether windy evenings really look different. Your records come from one place, so they cannot predict the weather, and they may not match the students' results.

Stay with an adult, and do not go onto dark paths to find a viewing spot. At a science museum or observatory, ask: "Why are many large telescopes built on mountains?" The sources below, especially the Subaru Telescope pages, are the best next step.

Two questions are still open in the sources we read: why the color of a star seems to flicker too, and exactly which layer of air matters most. They would be good ones to chase.

Sources

Revision History