Why do stars twinkle?
Published on Sep 8, 2007 at 8:47 pm.
53 Comments.
Filed under astronomy.
Most of us are familiar with the song “Twinkle, twinkle, little star”. So, why do stars twinkle?
First of all, though, lets define what me mean by twinkle. When astronomers go out and look at the night sky, we like to see the stars as tiny and very steady pinpoints of light. However, all too often they seem to sort of blink in and out and to dance around slightly. Sometimes, they seem to flash different colors and sparkle. The stars do not appear completely steady and unchanging. This is what amateur astronomers and most people in the general public would call twinkling. Naturally, professional astronomers can’t use such a simple term as twinkling, though! Instead, we call this twinkling effect scintillation.
So, what causes twinkling or scintillation? Well, as it turns out, the stars don’t really twinkle. Rather, this is an effect of our atmosphere. In space, the stars would appear as steady pinpoints. Turbulence in the atmosphere is largely responsible for the twinkling effect, so the steadier the atmosphere, the less the stars seem to twinkle and sparkle, and the more unsteady the atmosphere, the more the stars seem to twinkle and sparkle. Of course, that very turbulence also distorts the view for professional astronomers or for amateur astrophotographers, so while the twinkling might be pretty, serious astronomers hate to see twinkling stars.
But, how does atmospheric turbulence make the stars twinkle (or scintillate)? The stars are very far away. In fact, they are so far away that they basically are like point sources as far as we are concerned. That means that the light appears to come from a single point in the sky (unlike the Moon in which light comes from a disk about half a degree across, or a planet in which light appears to come from a very small disk in the sky). As this light passes through Earth’s atmosphere, it is bent a bit whenever it passes through a region of slightly denser air. Different colors of light, in principle, take slightly different paths, since the degree of refraction (bending) that light undergoes is frequency dependent. So, the path of the light through the air from the star to your eye takes a sort of convoluted path schematically shown in the following drawing. Note that this drawing is greatly exaggerated in terms of the effect of the atmosphere.
Now, if this were all that was going on, then the star would simply appear slightly out of position in the sky rather than twinkling. If the different colors were bent far enough apart, then the star might appear as a tiny little splotch of colors (like a tiny rainbow) rather than a pinpoint. That would be if the air were completely unmoving. But, that isn’t what happens. The air moves around. Solar heating causes convection. Winds blow. Different parts of the air are compressed and rarefied as the air moves. These different regions refract light differently (which is part of the reason that the light takes a convoluted path through the atmosphere, anyway). But, these regions of thicker and thinner air, more and less humid air, moving air, etc., are always moving around. As the move, the convoluted path changes. In my drawing above, you can think of the different colored lines as being the path of the starlight at different times. The more the air is moving around, the more the path of the light dances around.
As the path of light moves around, the apparent position of the star changes slightly. This is part of the twinkling. Normally, though, the motion back and forth in the sky is too small for the human eye to observe. If you ever go outside and you can see the stars dancing around, then you know to just give up and not even bother trying to set up a telescope! But, the different paths are slightly different lengths. The paths do not all take the exact same length of time for light to traverse. So, as the paths shift from one to another sometimes the shift takes less time than the difference in time that the light takes to travel the different paths. When that happens, the star can sometimes appear to wink in and out very quickly. This tiny flicker is what most of what you perceive as twinkling. But, as I said, different colors of light are generally refracted differently, so the different colors also shift back and forth distance (and thus in time of travel). Red light tends to be bent less than the green light which is bent less than blue light. This means that sometimes some colors can shift in time differently than others. The result is that the blue light and the green light flicker in and out at different times than the red light. That can make the star appear to flash different colors. Again, the more unsteady the air, the bigger effect that this will be. Sometimes the air is fairly steady, and the effect happens in too small of time intervals to notice. At other times, however, the time interval is longer (though still a fraction of a second) and you can see the star flash quickly between colors. Sometimes the effect is so pronounced that novice observers might be tricked into thinking that the flashing lights are the navigation lights for a passing aircraft. And, of course, if you look up and see almost all the stars in the sky flashing different colors, then that is not a good night to be observing!
Look at my picture above. I drew it for a star high in the sky as seen by the observer. But, think about a star near the upper left or right corners of the drawing. The light would take a longer path through the atmosphere to the observer. That would mean that the light would pass through more disturbed air, making all the twinkling effects even more pronounced. So, stars near the horizon twinkle far more than stars that are nearly overhead.
When I was growing up, I remember being told, “Stars twinkle, planets don’t.” I wasn’t the only one told this, though, and often at my public talks people ask me why stars twinkle and planets do not. This has to do a bit with what I said earlier about the stars appearing as pinpoints while the planets are tiny disks. The disk of the planet is very small, and you don’t see the disk of a planet (except in certain rare cases with Venus) because the disk is smaller than the resolution limit of your eye. But, it still is a disk. Stars are so far away that they still appear as pinpoints even without the Earth’s atmosphere. Only a tiny handful of stars are close enough that any hint of a disk has ever been detected, and even that was at the detection limit for the best professional instruments available. The light from the star dances around as I have explained, causing twinkling. However, for a planet light comes from a group of points in the sky forming the disk of the planet. The light from each of these points also dances around and flickers and changes colors. However, the flickering of one part of the planet is masked by that of other parts of the planet. The tiny instant that one part of the planet flickers out, another part might flicker into view. When one part of the planet flickers red for a tiny fraction of a second, another part might flicker green and another blue. The end result is that all of these effects tend to average out. So, to the unaided eye, the planet seems to shine steadily while the stars around it twinkle in the sky. But, this is a bit simplistic explanation. You see, the more unsteady the air, the bigger the twinkling effects. If the air is really unsteady, then the light from the whole planet might be taking different paths through the atmosphere. In that case, even a planet can appear to flicker, flash, and twinkle in the sky. But, that means that the old tale that stars twinkle and planets do not twinkle is incorrect. Planets can twinkle if the air is very unsteady. Whenever I go outside and I see Jupiter or Saturn twinkling and flashing, I know that serious work will be difficult because the air is so unsteady. In really bad atmospheric conditions, I have even looked up and seen the edge of the Moon appear to be twinkling. Now, that is really bad air! I didn’t even bother trying to observe on that night.
So, that is part of the reason that stars twinkle. It is all atmospheric.
-Astroprof
Ed Davies on September 9, 2007 at 5:52 am: 1
“…so the steadier the atmosphere, the more the stars seem to twinkle and sparkle.”
Oops, I think you meant the opposite.
Maybe this is your next post but it’s probably worth adding that astronomers call the condition when there is little twinkling good seeing and that often when the seeing is poor (i.e., there’s a lot of twinkling) the transparency is good so it’s a sensible night to look at large dim objects.
Astroprof on September 9, 2007 at 12:16 pm: 2
Thanks Ed.
I corrected my misstatement. It is amazing how when you read back what you just wrote, you see what you meant to write rather than what you wrote!
Yes, transparency and seeing are good topics for a post. I think that I sort of mentioned that a long time ago, but a separate post at this point would be a good idea. Thanks for the suggestion!!!!
A Ler…-- Rastos de Luz on September 10, 2007 at 6:01 am: 3
[…] Why do stars twinkle? no Astroprof’s Page […]
Ed Spencer on September 15, 2007 at 6:29 am: 4
Brilliant explanation of this often infuriating effect. I woke up early this morning to have a look at Venus (which is ludicrously bright at the moment - I thought it was a light on a plane initially), and could see the whole crescent shimmering around like crazy - now I know why!
Joan on February 3, 2008 at 8:33 am: 5
can you predict when the atmosphere will be steady? Is it steadier in winter? Can you see colors everywhere-all parts of the U.S.?
Thanks
Blaze! on February 3, 2008 at 9:19 am: 6
That’s made a lot more sense than what my science book said … stars twinkle and planets don’t because they’re farther away than planets.
Astroprof on February 3, 2008 at 12:35 pm: 7
Joan,
The air is typically, though not always, a bit steadier in the winter. Usually, the air is steadier when there is a high pressure dome sitting on top of you, too.
Laylah on June 2, 2008 at 4:51 am: 8
why don’t they twinkle in space?
Astroprof on June 2, 2008 at 2:40 pm: 9
If there is no air to distort the light, then there is no twinkling.
dario sciammarella on June 9, 2008 at 12:40 am: 10
so why do the planet reflected light does not twinkle if it has to pass through the same atmosfer
Kean on August 3, 2008 at 12:23 pm: 11
Why do some stars twinkle while others right next to them do not? It appears that if they appear next to each other, the light is taking roughly the same path and, presumably, being affected by the same atmospheric disturbances
Frank on September 16, 2008 at 10:35 am: 12
I was told from a young age that stars twinkle because they have died and the last remnants of their light are just reaching the earth. Is this true too?
Astroprof on September 16, 2008 at 2:33 pm: 13
Frank, you were told wrong. It is purely an effect of Earth’s atmosphere.
Steve Pollard on October 31, 2008 at 8:27 pm: 14
Is there absolute proof that stars do not twinkle when viewed outside the earth’s atmosphere, because I believe they do and I think they should, I have my reasons.
Obviously we see twinkling stars on earth with our naked eyes and that would be impossible to replicate in space, all images have to, by neccessity, be captured or viewed through glass whether lense or window, but is there irrefutable evidence in the form of video that they do not twinkle in space?
I think this would be especially evident if the camera in space used a lense the exact size of the lense in the human eye or smaller.
Astroprof on November 3, 2008 at 9:20 am: 15
The astronauts report that they do not see stars twinkling in space. Also, high speed images do not show the twinkling effect that such images do when used from the ground.
Steve Pollard on November 17, 2008 at 12:07 am: 16
Hey Prof…shoot me down if I’m wrong please, but I feel sure that stars should twinkle in space too.
Imagine if you will, the star being a bit like a sea urchin, in that each of the spines of the sea urchin represent an emission of photons emanating radially from a central point. Now, if observations are made close to the sea urchin the chances are that a few of these photon beams will hit home and be seen.
Now move further away and angular displacement will be greater, so less will be seen. Then move immensely far away and accordingly very few photons at all will hit home from our photon emitting sea urchin.
Now put aside the sea urchin prop and use instead a very very distant source of photons, a star, and consider also that the particles emitting the light to begin with are not likely to be static, hence their outward stream will be shifting as they roil and boil within their gravitational confines. Then throw in a camera with the light capturing capacity of our eyes only, and you have it. It seems to me reasonable and likely that from any distant point in space outside of our atmosphere the photon stream will occasionally “blank out” and “return”. Hence twinkle.
Astroprof on November 17, 2008 at 1:56 pm: 17
Well a star is not much like a sea urchin at all. It is a big ball of gas. Deviations from that ball are absolutely tiny on the scale of the star.
As for photons being emitted, the shear quantity is enormous. Even the light from dim stars is composed of vast numbers of photons hitting your eye. The twinkling effect that you see when you look at the star is only the shifting of the star light by atmospheric turbulence. What is why stars don’t twinkle in space, no matter with how fast of a shutter speed that they have observed by orbiting telescopes.
There is a related effect called scintillation, though, that is similar to twinkling due to passage of the light through the interstellar medium, but it is nowhere near as obvious, and can not be observed by the human eye.
Steve Pollard on November 17, 2008 at 5:24 pm: 18
OK! Thanks Prof, I thought it possible, but the photon flow is on a bigger scale than I imagined, not single individuals arriving one by one. Weird things photons. Thanks again.
Gene on December 16, 2008 at 6:17 am: 19
Isn’t a portion of the twinkling effect also created within our eye, due to the light from a distant star falling on only one receptor within our eye? Because we are unable to hold our eye perfectly still the pinpoint of light flicks across multiple receptors giving us a twinkling effect. A similar effect occurs when viewing a (clamped) laser shining on a flat surface. Due to the parallel light rays of the laser and the diffusion of the flat surface, a moire pattern is seen that also appears to twinkle. I suspect that the non-twinkling observed by the astronauts would not be verifiable by a digital imaging device, due to the relative large (nowhere near as small as our eye) pixel size of the photo sensitive surface. I also don’t think a film camera would do any better due to diffusion of light within the emulsion. In either case, Steve’s point that, when viewed through glass, especially of the thickness and composition used on a spacecraft or spacesuit, that the diffusion within the glass might mask the twinkle. However, earth based observations, distorted by “heat waves” would show up as twinkling in either digital or film based cameras due to the much larger refractive distortion due to our atmosphere. A note for Blaze, your science book was, in fact correct, it IS because the planets are much closer, and therefor are not mere pinpoints of light.
John T. on December 28, 2008 at 7:06 pm: 20
Hi there, I always thought that there was another mechanism at work in this phenomenon. Simply that light from a star has a much higher bandwidth than light from a planet, which is predominanently reflected light, and therefore filtered light with a much lower bandwidth. With a wider range of frequencies from a star, we see a much larger range of angular fluctuations from a start due to atmospheric refraction - have I got my physics all wrong after all these years ?
Vinay on January 2, 2009 at 11:09 am: 21
This was a wonderful piece of info and clarity in which Prof has atriculated the informations. Thanks again for enlighting the thoughts !!…
Eddy R. on February 7, 2009 at 8:40 pm: 22
I noticed somebody asked this same question that I have, but it didn’t get an answered. If a star is twinkling and there’s another one right next to it but it’s not twinkling, why is one twinkling and the other isn’t if they are so close? Same goes for a group of stars that are really close to eachother but some twinkle and other don’t, why is this?
Astroprof on February 8, 2009 at 11:32 am: 23
John, that may be a portion of what is going on, but the angular size of the planet is a bigger issue.
Eddy, it could be the colors of the stars involved. Blue and white stars tend to have more of the visual spectrum and they tend to twinkle a bit more. Reddish stars twinkle less noticeably.
Edward Z on February 15, 2009 at 5:31 pm: 24
Why wont you answer Eddy R. and Dean’s question concerning two stars close together in the sky where one will twinkle and the other won’t. It appears there is more going on than here than just the earth’s atmosphere.
Astroprof on February 15, 2009 at 5:39 pm: 25
Edward, I thought that I did answer them. Different color stars may appear to twinkle differently. Also, different brightness stars may appear to twinkle differently due to how the eye perceives light. I have not personally experienced two stars near one another in the sky of similar brightness and similar color twinkling differently (except temporarily when a contrail was blowing past).
acidhead on February 16, 2009 at 1:12 am: 26
last night i swear orion’s family jewels were flickering
rrallnt on March 25, 2009 at 6:25 am: 27
Too funny acidhead! Crazy thing is I saw the exact same thing about 4 years ago but thought I was hallucinating…apparently I wasn’t!
(or that had nothing to do with it…)
ScienceBlogs Channel : Physical Science | blogcable on July 30, 2009 at 12:17 pm: 28
[…] it. The Earth’s atmosphere, and not gas clouds, cause stars to twinkle. You can find a much more detailed explanation at Astroprof’s page, but this is one of the neatest little bits of information to come from manned spaceflight. Hope […]
B.Satish on October 18, 2009 at 12:50 pm: 29
Dear Astroprof,
Had been to a planetarim today and when listening to some theory of stars light reaching earth in about 800 years really astonished me. Logically i am of the opinion that the theories pertaining to light is all wrong. Just imagine that a star has collapsed and is reduced to carbon or whatever , i mean there is no light anymore. Does that mean you still get to see the star for another 800 years or so. Is that possible when the source itself is off , just like putting off your torch? Does that not sound stupid. Am sure that there must be ways of accelerating or deccelerating light waves. May be scientists must try shooting & bombarding photons directly or in oppsite directions to do so. Just curious to know what is the maximm distance a human eye can actually see. Is it the stars we actually see or image captured on a space screen. Hmmm let me know, for the moment am sure the theories need major correction.
Astroprof on October 19, 2009 at 9:43 am: 30
What is not “logical” about the speed of light? Consider the analogy of sound. If something very loud occurs some distance away from you, then you don’t hear the sound for several seconds after the event. Think about lightning. You often hear the thunder several seconds later because sound travels only so fast. So, it would make sense that light, which also has a finite speed that you can measure in the laboratory, would behave the same way. You don’t see an event as it happens. You see it when the light arrives, and it takes a while to arrive.
richie on October 21, 2009 at 12:49 am: 31
so if air causes the twinkle how much do biogenic emissions affect the colour and timing of the “twinkle”? I always thought that the light from distant stars gets refracted so many times off so many different objects that it slows or speeds up light. Which causes the same light photons to take longer or faster time to get to us causing a flutter “twinkle”. Is that in anyway correct?
Astroprof on October 21, 2009 at 3:08 pm: 32
Anything in the air will have an effect on the “twinkling” phenomenon.
richie on October 22, 2009 at 9:09 am: 33
So your saying when light hits objects an matter in space it doesn’t add to the twinkle affect?
Astroprof on October 22, 2009 at 4:12 pm: 34
Yes, any interstellar medium will also effect the light. However, the IM is very diffuse, so the effect is very subtle, and it would be below the threshold of human perception.
daveb on November 4, 2009 at 9:13 pm: 35
I saw loads of these stars flashing tonight, now I know why! Bloody weather!
craig near the famous bonnybridge on November 11, 2009 at 7:59 pm: 36
thanks for the info was looking out my window, thought i was going to get attacked by ufo’s
Fufe on November 13, 2009 at 7:22 pm: 37
OMG that is so great to know. Before I had no clue why it did that. I see them outside my house all the time and now I know what it is caused by. I cant wait to tell my teacher. Thanks a lot!!!!!!!!!!
Erica on November 25, 2009 at 9:48 pm: 38
What about intrinsic anatomy of the eye? Is that contributory to the effect?
Annette on January 6, 2010 at 6:06 pm: 39
I live near London and it is currently very snowy here with icy temperatures and all the stars that are visible from my window are twinkling. One particularly appears like a plane’s lights flashing in the sky but is. Is this likely to be due to low pressure in the atmosphere?
Maria on January 30, 2010 at 8:42 pm: 40
I have observed a year or more ago that one particular star twinkles all colors in a sequence. Sometimes more than others. I have no recall before that if that star was twinkling that much. It is mostly east of the moon, any idea if that is the polar star or any other? It is so vivid, it is amazing! sometimes I had mistaken it, for an airplane.
I was wondering too if the density of the earths atmosphere can change with what you’ve described on the article, can add as a cause to the more vivid sparkling effect?
Thanks for the info!
Lynn on February 21, 2010 at 7:16 pm: 41
Can you explain the analemma?
maricel V. on March 3, 2010 at 4:44 am: 42
why do stars do have colors?where do they get them?
SHAM on March 29, 2010 at 2:02 pm: 43
so we can safely say that the phenomena behind twinkling of stars is refraction of light right?
joe on September 17, 2010 at 10:47 pm: 44
in the case off a newly formed star that has not reached is equilibrium between outward exploding and gravity rebound , would you see a little pulsing or twinkling in space ??
%!ZA on February 11, 2011 at 5:38 am: 45
stars don’t really twinkle it’s our atmosphere
B. K. Goswami on March 20, 2011 at 12:21 am: 46
Dear Astroprof,
Today while helping my daughter’s physics preparation in the secondary exam, I faced this issue (”starts twinkle, planets don’t.”, two-marks question) and its obscure explanation in their textbooks (”Since start are located very far, they are apparently point-sized. The light coming out from them, through a non-stationary atmosphere, comes to our eye, sometimes more and in other times less, giving the effect of twinkling. This effect is negligible in the case of planets, due to its relatively closeness that makes planets look like a disk or collection of point sources.”) I do not extend about the book’s issue anymore as I wanted her to have a little better explanation. I see in your original blog you have extensively dealt many issue, including the dispersion and the original nature of light spectra at its source.
Could you please add from the viewpoint of “image” of the star (planet) as seen by us on the earth. Will I be right if I say, in case of starts (assuming white light source), the location of image flicks due to the convective nature of the atmosphere. Also, since the change in the refractive pattern in the atmospheric layers occurs is a slow process (in the scale of human perception time), we could visibly locate the image of the star flickering around a certain average location.
On the contrary, in the case of our planets, because of proximity, their images are relatively big and the fluctuations in their locations are negligible in comparison to their sizes and therefore appear steady.
In brief, I wanted to emphasize the issue of spatial (angular) fluctuation around the average location of the image due to convection-induced non-stationary refractive index pattern of the atmosphere) issue. It would be nice to have your comments on the same.
With best regards,
Binoy Goswami
Margie on May 18, 2011 at 12:33 pm: 47
Thank you for this explanation. I also thought planets don’t “twinkle”, and used to use this to spot them in the sky sometimes. Then, when I moved to New Zealand, I could no longer spot the planets, because everything “twinkles”! Now I know why.
Curious on July 20, 2011 at 1:14 am: 48
I was wordering would stars twinkle on different Planets as well? or are the atmospheres different than ours that they don’t twinkle as much or not at all
leo on August 16, 2011 at 11:40 pm: 49
hi there i dont know much of science earth cuz im still a little kid but every night i look up into the stars and i see the stars changing into colors i always thout it was couse some typ of gas chemical that couases a refelxeion to the stars and i think every year the moon gos futher away from earth.What i think y the stars go differnt colors is cuz there old and about to blow up k bye. but can one of u people reply to this and tell me were i can buy a cheap telescop that works really good thanks 4 hereing
Jay on January 15, 2012 at 4:08 pm: 50
Thanks for the info, always wondered what it was all about, just never got to actually finding out, very helpful
Ron on February 9, 2012 at 4:19 am: 51
I’m not trained properly in any way in any field of science. I couldn’t help but ask this question as I read thru though:
In regards to “stars twinkle, and planets don’t”. Stars EMIT light and planets REFLECT light.
I know that stars scintillate due to atmospheric conditions, but wouldn’t the properties of the light change our perception if viewed from a reflected source? For all intents and purposes the sun seems to scintillate if viewed thru a welders mask. I have seen the planets and they don’t “twinkle” in the same sense that stars do.
All light viewed at night whether its from the moon, or a star has to travel the same distance in our atmosphere. Viewable passing satellites in orbit don’t twinkle. geosynchronous satellites don’t twinkle.
so therefore the reflected light viewable in the night sky does not seem to act in the same behavior as emitted light.
I guess this is begging a question on a different tangent, the behavior of light.
This is just my humble observation…
anne loves science :P on November 19, 2012 at 5:19 am: 52
i love this website it tells me more about stars
douglas on February 23, 2013 at 3:53 pm: 53
hi astroprof, lil’ Leo, and all, props to you prof, awesome explanation! to all with questions seemingly unanswered, carefully re-read what astroprof wrote. and for Leo, sounds like you are going to become a science guy! go for it and learn about stuff you think is interesting to learn about. and check amazon.com to see a wide range of telescopes that would help you look more closely at the twinkling stars and the non-twinkling planets.(this subject certainly is scintillating..heehee..luv,dhb)