Twinkle, twinkle, little star. How I wonder what you are.
January 4, 2012 8:21 AM   Subscribe

What makes a star "twinkle"?

Last night I went far out into the Alabama country to get a truly awesome view of the nighttime sky. I saw several shooting stars, but what interested me more was the fact that at least two objects I saw, which were not Jupiter nor Mars, were visibly "twinkling" -- from red to blue to white to red and so on.

What were these objects? Planets? Stars? What made them appear to be twinkling?
posted by fignewton to Science & Nature (13 answers total) 2 users marked this as a favorite
The light passing through atmospheric layers of varying temperature and density. The edges between layers refract the air in a manner similar to a prism.
posted by gauche at 8:26 AM on January 4, 2012 [2 favorites]

Stellar scintillation.
posted by lydhre at 8:26 AM on January 4, 2012 [5 favorites]

Stars appearing to twinkle has little to do with the star and more to do with the effect of the earths atmosphere refracting the light that's reaching your eyes.
posted by h0p3y at 8:27 AM on January 4, 2012

When I was younger I was told turbulence in the atmosphere.
posted by token-ring at 8:28 AM on January 4, 2012

Oh -- read a little closer. It is possible you saw something man-made with lights on it. Did you perceive it to be moving through the sky relative to the backdrop?
posted by gauche at 8:28 AM on January 4, 2012

"The light of the stars is steady and clear
but we see the stars through the atmosphere.
The atmosphere has layers of air,
the layers keep moving from here to there.
Because of their different temperatures
the layers keep moving from here to there...

The air moves in, the air moves out,
and tosses the light of the stars about.
The moving air bends the light,
and that's why the stars twinkle at night."
posted by anaelith at 8:41 AM on January 4, 2012 [14 favorites]

Just to be clear, the objects were NOT moving. They were definitely fixed. The answers help, guys. Thanks for educating me. :-)
posted by fignewton at 8:47 AM on January 4, 2012

Depending on where they were, it's possible that the lights you saw were towers of some sort, which would explain both their fixity and their changing colors.
posted by valkyryn at 9:03 AM on January 4, 2012

The objects that tend to scintillate most noticeably are bright stars nearer the horizon, because light from objects nearer the horizon has to travel through more of the atmosphere. (This diagram shows why.) Strong twinkling can make the object appear to change color, because different wavelengths of light are refracted by different amounts.

Interesting factoid: except in the worst viewing conditions, planets don't twinkle because their size on the sky is larger than the size of the convective cells whose motion causes twinkling.
posted by BrashTech at 9:08 AM on January 4, 2012 [1 favorite]

It's due to turbulence in the atmosphere, as noted above. This is also why objects on the horizon "shimmer" on a hot day: air near the ground gets hotter and therefore less dense, starts to rise, and causes a turbulent flow.

You-didn't-ask-about-it-but-I-think-it's-cool-so-I'll-include-it factoid: Astronomers, being ingenious sorts, have developed tools to make the stars hold still—basically using deformable mirrors to cancel out these effects from the atmosphere. If you could look through such a telescope, the stars wouldn't appear to twinkle.
posted by Johnny Assay at 10:17 AM on January 4, 2012 [2 favorites]

I've definitely seen twinkling stars change colors. It's just turbulence in the atmosphere.

Think of what happens when sunlight passes through a piece of cut glass, or a high-quality diamond: the sunlight gets refracted into lots of different directions, and different colors get refracted into different places. So your nice sparkly makes the sunlight appear in lots of different colored spots on the wall, which are in hard-to-predict places. Now if you look at the sparkly itself, then you'll usually see only one color, which depends on exactly where you're standing relative to the orientation of the sun and the sparkly. (Though if you go to the jewelry store to shop for diamonds, you'll see lots of different colors at once, because jewelry stores have lots of bright lights rather than just one sun.)

You get refraction through big angles between air and diamond because the index of refraction is very different for each. The difference between index of refraction for warm air and cold air is not so large, so the angles of refraction are small, and the starlight appears to come from nearly the same spot in the sky. But you will still see some variations in brightness (which is what people usually think of as twinkling) and color (which is what you saw).

p.s. thanks to you i will be singing "stars fell on alabama" all week, which is supposedly a poem based on a remarkable meteor shower
posted by fantabulous timewaster at 9:02 PM on January 4, 2012

They twinkle because they are so distant that they are 'point sources' of light. The beam of light is so narrow that it's easily displaced slightly and repeatedly by the atmosphere and this displacement causes colour and positional changes which account for the twinkle. Planets, being vastly closer, emit more light which can effectively punch its way through the atmosphere and hence they do not twinkle.
posted by nickji at 11:50 AM on January 9, 2012

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