Chasing Arcturus: Why Do You Twinkle, Little Star?

Right now stargazers have the perfect chance to find an unusual speeding star by remembering this phrase: Follow the arc to Arcturus.

The key is the Big Dipper, a familiar grouping of stars that is currently in the northeastern sky in mid-evening.

If you draw an imaginary line from the Dipper’s handle, your eye should settle on a bright, orange star—congratulations, you found Arcturus!

This star, some 37 light-years from Earth, is noteworthy because—unlike most of its stellar kin—it is cutting perpendicularly across the relatively flat disk of the Milky Way. That means, millions of years from now, the star will have moved out of Earth’s line of sight.

Interested in observing Arcturus this spring? Consider the following:

by Robert J. Vanderbei

One of the first things we notice when we look at stars in the night sky is that they twinkle. This twinkling is caused by turbulence in our atmosphere.

Anyone who has gone snorkeling, scuba diving, or even just swimming in a pool has probably noticed the interesting lighting effects on the bottom of the water body—these effects are essentially the same as those caused by atmospheric turbulence.

Meteorologists (who, btw, don’t study meteors) have become good at predicting how good or bad the sky’s turbulence will be. One can use their forecasts to plan when to go outside and observe the night sky—and when to stay indoors and watch a movie.

There is even a website,, that will tell you how good the “seeing” will be where you live over the next few nights.

But here’s a puzzle for anyone who’s looked through a telescope: In a telescope’s view, a star might have a swimmy appearance as it moves around and changes shape a little bit, but it doesn’t twinkle.

That is, the starlight never blinks off, not even for an instant. Why is the telescopic view qualitatively different than the “naked-eye” view?

The answer derives from the fact that the telescope’s light-collecting area, aka its aperture, is much larger than the light-collecting area of the human eye’s pupil.

Atmospheric turbulence does cause the light to fade over parts of the telescope’s aperture, but at the same time it brightens over another part. That means that, on average, the intensity of the light remains virtually unchanged.

The pupil of the human eye, on the other hand, is small enough that there can be noticeable changes in the amount of light that passes through. Hence, the brightness of the star appears to change, and it twinkles.

Recently, I did a little experiment to illustrate the difference between observing stars with and without a telescope.

Shown below are two five-second, looping animated gifs of Arcturus.

[Note that Arcturus is not a “little” star. It is a red giant, which is why it appears orange to us (go figure!), and it really is a giant. Its diameter is about 26 times bigger than our sun’s. Hence, its volume is about 6,500 times greater.]

This animation shows Arcturus significantly defocused in my four-inch refractor. A defocused image is just a reduced-size picture of the light intensity as it passes through the telescope’s aperture.

In this animation, we can clearly see intensity variations across the pupil of the telescope, but the star isn’t twinkling, per se.

The next animation, meanwhile, shows Arcturus in focus. But here I made one more important change. I put a cardboard mask over the front of my refractor.

The cardboard mask has a small section cut out. Covering that small section, I put a piece of aluminum foil from which I cut out an even smaller circular opening about the size of a human’s pupil.

As it would to the naked eye, the star now twinkles through the telescope.

I did a frame-by-frame analysis of the focused image and found that the difference in Arcturus’ intensity between the brightest and the dimmest frames was a little bit less than a factor of two.

So really the star doesn’t completely wink out as it twinkles, it just dims.

It is well known that stars close to the horizon twinkle more than stars high overhead. The reason is that their light passes through more atmosphere and so the effect of the turbulence is greater.

On the night of the experiment, the blue giant star Spica was much closer to the horizon than Arcturus. For comparison, I made a defocused, full-aperture animation of Spica, too. As expected, the intensity variations across the aperture were much more pronounced.

Robert J. Vanderbei is chair of the Operations Research and Financial Engineering department at Princeton University and co-author of the National Geographic book Sizing Up the Universe. Vanderbei has been an astrophotographer since 1999, and he regularly posts new images on his astro gallery website.

Robert Vanderbei

Human Journey