Why Stars Twinkle And Sputter In Color

Three stars - Antares in Scorpius, Arcturus in Bootes and Capella in Auriga - and one bright planet are all near the horizon in early September during evening hours. The stars sometimes twinkle furiously, while Jupiter remains placid. Learn why below. Created with Stellarium

Twinkling stars always catch our attention. People will often report a bright object flashing or pulsating in the different colors and swear it’s a UFO. When I ask the time and location of their sighting, it almost always turns out to be a bright star like Sirius, Capella or Arcturus seen near the horizon.

Dim stars twinkle too, but we don’t usually notice it, because our vision isn’t sensitive enough to detect the bouncing about of a star’s image that creates the twinkling effect. Bright stars can scintillate like crazy especially when low in the sky. To my eye they look like nonstop shimmers and flashes in every color of the spectrum.

Twinkling is caused by moving air pockets in our atmosphere. Twinkling is much stronger and more noticeable near the horizon, because our line of sight passes through a much greater thickness of air. Illustration: Bob King

Our atmosphere’s to blame for all this frenetic activity. Starlight passes through air pockets of different density and humidity on its way to our eyes. Each pocket act like an individual lens that focuses its own image of the star. As the air churns and the winds blow, the number and positions of all those individual images are constantly changing.

We perceive these nonstop, tiny shifts as sputtering light or twinkling. The scientific description of the cause is atmospheric refraction. Refraction by moving air causes star images to constantly jump about and change brightness.

Planets generally don’t twinkle, because they appear much bigger than stars (they’re close) while stars are essentially point sources. The air bundles are too small to “knock” planets around and create multiple, separate images, so they appear steady to the eye. Most of the time. During high turbulence, I’ve seen planets jump about, too. On the next clear night, compare the bright stars shown above with Jupiter and you’ll see the difference.

The colors come from the air, too. Just as white light is composed of a rainbow or spectrum of individual colors from indigo to green to red, so is starlight. When a star is near the horizon, refraction is strong enough to create images of the star in every color of the rainbow and cast them about in different directions. To our eye, the star looks like a continuous sparkle of varying colored light as split-second variations in moving air pockets make it dance about. Wonderful!

Pretty to the eye, twinkling drives astronomers crazy, since images won’t sit still long enough to get a sharp view or picture. To compensate, many professional observatory telescopes use a recently developed technology called adaptive optics. Here, a bit of a star’s quivering light is analyzed live by a computer, which calculates how the telescope’s deformable mirror must be shaped to compensate for the distortion. Machinery attached to the mirror tips, tilts and otherwise slightly changes the mirror’s shape every few milliseconds to keep the star images “calm”.

Amateur astronomers aren’t so lucky. Most nights, the stars jump about or look blurry in our eyepieces at medium and high magnifications. We persevere in hopes of striking gold on those occasional nights of little turbulence and fine air when star images are pinpoint and still – something called ‘steady seeing’. Ah, they’re to die for!

52 Responses

  1. erick

    Hi Bob,
    I had a conversation with a co-worker and he said he posed the question “do you know we see stars twinkling?” My goto answer was that light refracts in a non-continuous fashion through our atmosphere depending on the conditions of the atmosphere along with a dependency on the distance of the star. He replied that it actually had something to do with the way our eyes process light. He referenced the analogy of night-blindness when one walks in a dark movie theater and that we had to adjust our vision. He went on further that our eyes had certain receptors that cause this. He went into even more detail; however, I could not follow due him missing some of the key names of these receptors that caused this (my skepticism was picked at this point). Have you ever heard anything like this as I tried to research and found no correlaions to what he was speaking about on this subject. I am a layman, but like to discuss these things analytically and with very detailed info, so please be as scientific as you would like. Thanks for any help!
    Erick

    1. astrobob

      Hi Erick,
      You are correct. Twinkling has to do with the atmosphere, not our eyes getting used to the darkness or with special receptors. Matter of fact, twinkling is more obvious the more dark-adapted our eyes, so it’s quite different from walking into a darkened movie theater from outside. Maybe your friend is confusing the issue with the two different receptors we use for color and detail vision (cones) and for night vision (rods). Next time you see him, ask him if that’s what he meant.

  2. Steve W.

    Hi Astro Bob,

    Recording something in the nightsky tonight ( January 18th, 2013 ), and from reading the info you posted along with other intel I am still very curious. The footage I have is of an object that seems to be moving like anything we record with the rotation of the earth.the thing is that the color changes are extremely rapid and appear to be all the colors of the spectrum. Not just red or blue…

    1. astrobob

      Steve,
      It sounds like it may be the star Sirius especially if it’s moving slowly. Is it in the south-southeastern sky and not too high up?

  3. Gav

    Sirius has been really beautiful in the skies around the Eastern UK lately (on the odd occasion we’ve been able to see through the clouds). It *appears* to be rotating through the spectrum in very pure blues, whites and reds. Really very noticeable.

    1. astrobob

      Gav,
      You probably get to see more color than those living in more southerly latitudes since Sirius, being lower in the sky, is more strongly affected by air currents.

  4. Travis

    Hello Mr.Astrobob
    Quick question, what do the different colors of the “twinkle” tell us about our hemisphere at the time?

    1. astrobob

      Hi Travis,
      Twinkling tells you about the local atmospheric conditions. The different colors come from pockets/layers of air of different densities and temperatures that act like lenses or prisms. They refract the light into every color of the rainbow the way a prism spreads white light out into a band of rainbow colors. Each color gets refracted by the air slightly differently and takes a slightly different path through the turbulent atmosphere until arriving at your eye. We see it as twinkling and changing colors.

  5. Tim W1959

    I have been looking with my 5 1/2″ Celestron telescope as well as binoculars and other things.Every so often I see a bright object in the sky that quiet literally looks like police lights strobing.How can this be?Red,White and Blue?

    1. astrobob

      Tim,
      Sure sounds like you’re seeing Sirius or another one of the brighter stars. They flash or “strobe” in multiple colors as their light is distorted by the atmosphere.

  6. alright i looked in my teleascope and there was something bright i looked closer and it was a rainbow looking star what does that mean?????????????????????????????????????????????????????????????????????????

    1. astrobob

      Tamara,
      You’re seeing exactly what I wrote about in that blog – the air acting like a prism and refracting the starlight into colors. Remember that white light is made of all the colors of the rainbow.

  7. Janitza

    Hi,
    I was driving and I saw like a star vanished just 10 feet in front of me. just like the same way you can see it in the sky. It was incredible and very bright. How can I be sure it was a star?

    1. astrobob

      Hi Janitza,
      If it looked like a star but vanished it was either a satellite that entered Earth’s shadow, a flash of reflected light from a satellite or a meteor. Sounds wonderful!

  8. Christopher

    Mr.Bob, last night me and a friend was looking up into the sky looking at the beautiful star’s. Something i never seen befor happen before our own eyes. They would change from white, to blue purple and green. Can you please tell me we weren’t seeing stuff?

    1. astrobob

      Christopher,
      Indeed you weren’t seeing stuff. You were paying close attention to ‘twinkling’ caused by moving air currents overhead. Not only do the currents’bounce’ the light around, which we see as twinkling, but also refract or bend it to shimmer in every color of the rainbow.

  9. Ryan

    Hi Astrobob,
    I spent this morning searching the web trying to find an explanation of what my friend and I have seen this night in the sky above the horizon… This could be the best explanation…

    We saw a bright star sitting alone above the sea horizon changing colours between red green white blue… To see a star changing these colours wasn’t a surprise for me but this star seemed moving from side to side and up and down… It looked like a small plane fighting through strong wind…

    I remembered that I had googleskymaps app on my phone and when I pointed it at that direction it showed that the most brightest star that way was the Arcturus… Never heard of that before but now reading these articles makes sense…

    Read articles about moving Sirius star but not about Arcturus… How do you explain that movement please?

    Thanks,
    Ryan

    1. astrobob

      Ryan,
      You are an excellent observer. You saw exactly what was happening. The air currents shift the apparent position of the star in the sky causing it to jerk this way and that. That in essence is what’s happening when a star ‘twinkles’. Most people wouldn’t notice the shifting of position unless it was a very bright star like Arcturus, the star was very low in the sky and they paid very close attention. Great observation Ryan – thanks for sharing it with us!

  10. Sandy

    I read that you said dim stars twinkle as well, but our eyes don’t really detect it. Then what are those bright stars that appear not to be twinkling at all? In Chicago I see one star in particular that’s straight above my head that’s quite solid, bright and calm. Am I just not catching the twinkling or is this even a star?? And thanks, Astrobob, that was a wonderfully entertaining article! Can’t wait to enlighten others with my new twinkling knowledge (;

    1. astrobob

      Hi Sandy,
      A star that’s overhead twinkles the very least because its light passes through the minimum amount of air possible. The further you look from overhead, the more air you look through and the greater the twinkling. I’m guessing you’re seeing the star Vega these nights. Keep a close eye on it – it should still twinkle some on nights when the air aloft is turbulent.

  11. Joel

    Thanks for this post! My wife and I have spotted a few furiously twinkling and color-changing stars near the horizon. I new the explanation behind the twinkling, but not about the color shifting. We call them “Disco Stars” and think about the amazing dance party going on around those distant suns. It’s a little sad to know that it is simply refraction. But, still fun to see them at night.

  12. tom tulinsky

    I saw a very twinkly star low in the west tonight. It was almost shooting out red and green sparks. Maybe it happens all the time, but I have seldom seen it so twinkly and I’ve been looking at the sky for 50 years. Thanks to you and the Current Planets page at Sky and Telescope for helping me figure it out.

  13. Chris G

    Astrobob, thanks for the wonderful and very informative blog. We currently have a telescope on loan from our local library because my preschooler is very interested in space and in particular the moon. Unfortunately we haven’t been able to get a good look given the fact that it has been a very narrow crescent that is too low on the horizon by the time the sun goes down. Tonight we were searching for some interesting stars and Capella caught my eye because of its brightness and the fact that I could see it twinkling with my naked eye. When observed through the telescope you could see the prismatic effect you describe in this blog post very clearly and it was an excellent viewing experience for all of us. I searched for an explanation thinking it perhaps had something to do with this particular star and came across this blog post which I read to my daughter so she could also understand the effect. Thanks!

    1. astrobob

      You’re welcome Chris! By the way, you won’t have to wait long for the moon to get in a better position. Starting tomorrow evening it should be up just fine in the southwest and it just gets better.

  14. Sean

    Seen 2 big ones flashing and changing colors tonight, i thought i was seeing something rare. Great explanation astrobob, thanks.

  15. brittaney

    Im seeing one right now I always see it in the same spot I can see it from my bed.
    It looks really cool it looks like it turns blue and green I didn’t now what it was that’s why I Googled it I was curious to know what it was. But I don’t know if it supposed to move because every time I see it is in the same spot.

  16. Monica

    Hey Bob, i have a question. tonight while my father was gazing at the sky he seemed to noticed what he says is a star but i think its a comet changing colors. he started freaking out and everything and made us me and my mom go look at it. while my mother was looking at it closer she says that one star is atop another and they were basically dancing and changing colors. my parents seemed to have found several tonight and i was just wondering if it meant anything and if you can look and see if you see what they described.

    1. astrobob

      Hi Monica,
      I’m guessing your dad saw Sirius which flashes in many colors due to turbulence in the atmosphere. Other bright stars do it too, so I wouldn’t be surprised they saw more.

  17. Alexandra

    Hello, I read your blog and comments and I noticed that a few people have seen what I have been seeing for a while now. For the past 2 months I have been seeing one, sometimes 2 twinkling stars that not only flash different colors extremely fast, but as I stare at them they almost appear to be moving. When I say moving, I mean sometimes I see one move slowly from side to side or up and down and other Times it looks to be moving to one side rather rapidly. My window is facing South…or Southeast, so is this Sirius? I hope so, haha. Kind of freaks me out seeing these moving stars, makes me think it’s something unidentified.

    1. astrobob

      Alexandra,
      It’s an interesting things about bright stars that as you keep looking at them closely they can appear to move. It’s probably the same reason a satellite seems to move jerkily when you follow it closely – a reflection of our own eye movements. I think you’re right on the star being Sirius. It’s been in the south the past couple months. It’s now in the southwestern sky. Vega and Arcturus will do the same; they come up in the northeast. Thanks for sharing your observation.

  18. mike B

    Hi Astrobob,

    After reading most of your replies here, it seems (after much hilarious discussion and googling) we have seen a flashing star on he horizon.

    18/4/2015 looling North off the North West coast of Bali. We got a friend to look through a telescope and it was definitely flashing all sorts of colours.

    What was strange is that it appeared to be spinning rapidly in a clockwise motion…any pointers on this observation?

    Its movement was limited but I certainly saw it jump around and possibly at one point disappear entirely.

    Many thanks.

    Mike

    1. astrobob

      Mike,
      I’m not surprised it might have looked like it was spinning. I’ve seen that on occasion through my telescope. I’ve always assumed it’s directional airflow / movement of the atmosphere. The lower you look, the wilder the motion due to looking through a much greater thickness of air cells than when looking high in the sky.

    1. astrobob

      Hi Jusef,
      It looks like a ring because it’s out of focus. Cameras have a very hard time autofocusing at night. Out of focus star images often have darker centers.

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