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SciShow explains why stars do that twinkling that all the kids are singing about, and explains how astronomers can get around it to make observations, and why it can be kinda useful.
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Sources:
http://www.universetoday.com/25345/why-do-stars-twinkle/
http://www.scientificamerican.com/article/why-do-stars-twinkle/
http://www.livescience.com/32315-why-do-stars-twinkle.html
http://curious.astro.cornell.edu/question.php?number=114
http://adsabs.harvard.edu/full/1952JRASC..46..191E
Lots of kids grow up singing that tune about little stars twinkling, but even as adults, many folks don’t know why stars seem to flicker and wiggle.  
  
Astronomers describe the twinkling of the stars as stellar scintillation. Technically, it refers to variations in apparent brightness or position of a distant, luminous object as its light travels through a medium.  
 
In other words, that twinkling is not caused by the stars which are super constant in their brightness -- it’s caused by the Earth’s atmosphere. 
  
As the light from a star travels toward you through the atmosphere, it passes through many layers of air of varying temperatures and densities.  
  
Each of the boundaries between those layers of air refract the light, bending it in a slightly different direction from moment to moment. 
  
So, the starlight zigs and zags as it passes through the air to your eye or to a telescope on the ground. 
  
From our perspective, this makes the star appear to shift ever so slightly. 
  
Of course, we can get around this astronomical nuisance by taking advantage of the fact that stars do not twinkle in outer space. 
  
This is why our best images of stuff in outer space have so far have come from the Hubble Space Telescope, which makes its observations outside our atmosphere. 
  
With no medium to refract the light hitting its lens, Hubble is able to take brilliantly beautiful, completely crisp, twinkle-less photos of the stars. 
  
To be fair though, twinkling may frustrate astronomers here on the ground, but it can actually be useful as well.   
  
Thanks to scintillation, when you’re gazing up at the night sky, you can tell the difference between a star and a planet. 
  
That’s because planets don’t twinkle - at least not as much.  
  
Stars are so far away that they’re what we call point sources of light. Like, if I were to carry a flashlight far, far away from you, it would appear as one point of light, shining out of the darkness.  
  
But because planets are so much closer, the light we see bouncing off of them consists of many points of light. So, kind of like me shining a flashlight in your face, planets essentially appear as disks.  
  
Even if one individual point is disrupted by the atmosphere, several others reach you at the same time. This averages out the twinkling effect, making the planet appear relatively stable. 
  
So next time you wish upon a star, make sure it’s twinkling! 
  
Otherwise, you may very well be wishing on Saturn. Which, which honestly is fine; you can wish on whatever you want. It’s up to you. 
  
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