What makes stars twinkle?


Tom was up all last night obsessively watching the weather channel on TV and praying for clear skies, so he’s too tired to write his column this week. He’s asked the amazing Captain Astro, intrepid defender of goodness and niceness everywhere, to answer the email that poured in this week.

Hey Captain Astro,

What makes the stars twinkle?

Timmy Johnson, age 9

A multimillion dollar movie contract?

Hey Captain Astro,

Not that kind of star. Come on now. Really.

Timmy Johnson, age 9

Ha, ha, old Captain Astro was just pulling your leg. Scientists, who have a penchant for big words, call this effect scintillation. When stars …

Hey Captain Astro,

Huh? Penchant? Scintillation? What?

Timmy Johnson, age 9

Never mind, Timmy. What’s happening is that the light from the star is moved around by the blanket of air, called the atmosphere, surrounding planet Earth.

The atmosphere is very turbulent. As layers of the upper atmosphere slosh around, they grab the light from the star and make it seem to flicker.

Astronomers call this effect “bad seeing.” On nights when the seeing is really terrible, the effect can be so drastic that through a telescope a star won’t look like a star at all. It will look like a fuzzy blob because the light from the star is spread out by the movement of the atmosphere.

The atmosphere also “refracts” the light from stars. It acts like a huge lens and bends the light slightly downward, making the stars look slightly higher in the sky than they really are.

To illustrate the refraction of stars, try putting a pencil into a bowl of water. Notice how the pencil seems to bend at the surface of the water? Same deal.

Next, try changing the angle of the pencil. When the pencil is straight up and down, it doesn’t seem to bend much at all. When the pencil is almost parallel to the water’s surface, it seems to bend a lot.

The same thing happens in the sky. When the star is straight overhead, it’s very near to its apparent position. When it’s near the horizon, it looks like it’s as much as 1/2 degree from its real position.

Hey, Captain Astro,

What’s a degree?

Timmy Johnson, age 9

Never mind. 1/2 of a degree is about the width of the full moon.

Hey, Captain Astro,

I heard somewhere that the planets don’t twinkle. Is that true?

Timmy Johnson, age 9

Yes and no, Timmy. Bright planets like Jupiter don’t appear to twinkle like the stars because they are so close to us compared to the stars.

They form a tiny disk, even though it takes a telescope to see the disk.

The stars are so far away that they are not visible as a disk even in the largest and most powerful telescopes we have on Earth. They look like a single point, so the light from them appears to jump around in the sky. That’s all twinkling is really.

If you look at a planet with a telescope, you will see the effects of atmospheric turbulence on the disk. The image will be fuzzy, but there will be moments of clarity as the atmosphere momentarily steadies in the direction of the planet.

The reason they don’t appear to twinkle to the naked eye is that the effects of the turbulence are averaged out over the total disk of the planet.

Hey, Captain Astro,

Yikes! I don’t get any of that. So what’s the bright star I’ve been seeing twinkle like crazy near the horizon?

Timmy Johnson, age 9

Right, Timmy. The biggest, baddest twinkler of them all is up in the early evening right now. Look directly south around 10 p.m. and you’ll see the brightest nighttime star in the sky, Sirius, the Dog Star.

It never gets very far above the horizon. It rises in the southeast, scoots low on the southern horizon and then sets in the southwest.

Because it remains so low, it is always seen through thicker layers of the Earth’s atmosphere than stars that travel directly overhead. Thus, Sirius can put on a show that rivals those laser light demonstrations you young people seem to enjoy so much.

An old, dead guy named Alfred Lord Tennyson wrote these lines of poetry about Sirius:

… the fiery Sirius alters hue

And bickers into red and emerald.

Hey Captain Astro,

Bickers? Red and emerald? Huh?

Timmy Johnson, age 9

Exactly, Timmy. Old Alfred was noticing yet another effect of atmospheric refraction. The blanket of air near the horizon acts like a gigantic prism that breaks up the light from Sirius into some of its component colors. As the air moves around, sometimes you’ll see the red hues and sometimes the blue-green. On a night of poor “seeing,” this effect can be pretty spectacular.

Unfortunately, it can be a pain in the old posterior for astronomers. Bad seeing can blur the image of an astronomical object so badly that it is ruined for scientific research. That’s one of the reasons astronomers like to look at stars and planets when they are high overhead and not close to the horizon.

Hey, Captain Astro,

What are the effects of atmospheric refraction on radio waves 10 centimeters and longer?

Timmy Johnson, age 9

Er, uh. Oops, look at the time. Old Captain Astro has an appointment with his psychiatrist, er, orthodontist.

Remember, Timmy, always obey your parents and make sure to look both ways before you cross the street. Gotta go. Bye.


By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

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