Why do stars twinkle?

by Stephanie Chasteen on February 15, 2013

It’s been a long time since I posted a simple “how things work” post.  These used to be my bread and butter, and then, well, I got tired!  Learning new things is exhausting.

But digging through my draft post archives, I found this nice little question that merits a bit of exploration.

Why do stars twinkle?

Or, to be more scientifically jargonesque, Wherefore dost stellar scintillation arise?

Light from stars reaches us through a fluid — namely, the Earth’s atmosphere.  The atmosphere is more than just “there” — it’s moving and turbulent.  It’s also got layers — the density of the atmosphere increases as you get closer to Earth, due to the weight of the air above it.  But, the primary cause of the twinkling is the fact that the atmosphere has little local variations in density because it varies in temperature from point to point.  Higher temperatures = lower density, since those molecules are far apart.

Light traveling through any material bends as it passes through, and it bends more the more dense the medium.  (This is because light slows down more in a denser medium).  So, starlight gets randomly bent away from what would be a straight line-of-sight path if it were traveling through a vacuum.  Yes, that means the stars don’t twinkle from space.

So, the result is that the light dances around on your retina as you look at the star.  This movement is the “twinkling” you see.  Interestingly, planets don’t twinkle because of their size.  Stars look like points of light, so when the light from that point is bent slightly, the whole point appears to move.  Planets, however, have a discernible diameter, and don’t appear as points of light.  So while any one point of light coming from the planet might wobble, the planet as a whole looks quite stable in the sky.

This is analogous to when you look at something through the heat above a fire, or above a patch of highway on a hot day.  Things viewed through these hot areas, which have varying density, look wavy and distorted.

And, for teachers, you can do demonstrations in class with dry ice  and a laser (I’m told).  Shine the laser through the cold gas as it spills out of the container and you should see it “twinkle”.   The same should be able to be done with a bunsen burner.  Has anyone tried this?

(Tip of the hat to Al Sefl, who posts such interesting answers to scientific questions)

(Image from Acerview54 on Wikimedia)


Susan Cahalane March 8, 2013 at 10:29 pm

Thanks for a great explanation! I’m your newest follower!


Marci Koski March 13, 2013 at 7:06 pm

Thanks for your explanation! And, I love your blog. You’ve got a lot of great stuff here! It will be a great resource for my own website; thanks so much! :o)

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