If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:7:42

Video transcript

twinkle twinkle little star why do these stars twinkle at us and planets don't twinkle why don't fly let's twinkle the stars only twinkle due to our atmosphere and we know this because if we look at these stars from outside our atmosphere like this astronauts on the space station they don't see the stars twinkling at all so let's explore how this atmosphere makes all these stars twinkle at us so let's say we're standing over here on earth looking at this star now due to our atmosphere there are two things that are gonna happen first as the light enters from space into the atmosphere it's gonna bend a little bit and this happens because the light decreases its speed it slows down as it enters into our atmosphere just a little bit and as it isn't it refracts it bends and because of this bending when we look at this star it appears to us as if these rays of light are coming from somewhere higher in the sky and so when we look at the star we don't see over here we're gonna see it a little bit higher so the first effect of the atmosphere is that it shifts the position of the star this effect is greater when the stars are closer to the horizon because the rays of light pass through a larger part of the atmosphere but the twinkling effect is not caused due to this so what causes the twinkling effect well if we were to zoom in and look at the light as it traveled through our atmosphere then we would see that as it travels down towards the Earth's surface light doesn't bet light doesn't travel in a straight line where it continuously keeps bending this happens because the light is passing through several layers of our atmosphere and as it passes from one layer to another there is changes in the temperature there is changes in the density there is some there's pollution and dust particles and because of this the light continuously keeps changing its speed as it moves from one layer to another and as a result light continuously keeps refracting or bending as it moves towards us and more importantly the temperature and the and the density of the air it's continuously fluctuating with time it changes with time continuously and as a result the part taken by the light also slightly keeps changing so the bending is not the same the very next moment it might take a little different part and this causes the Stars to twinkle but why exactly what happens if the light bends well let's zoom in a little further so let's say this is our eyes as you are looking at that star and currently let's say about five rays of light are reaching our eyes and because of this we are seeing right now the star to be as bright as we can see over here but because the atmospheric conditions are continuously changing with time maybe the very next moment the part taken by this light changes a little bit and as a result let's say that one of these rays slightly bend away missing our eyes so maybe the very next moment instead of five only four rays reach our eyes now because of four rays reaching our eyes the star is gonna look a little bit dim maybe the next moment only two rays reach our eyes so now the star is going to look even more dim and so you see due to the continuous changes in the atmosphere fluctuations in the temperature and density and all the other things because the light is continuously changing its part the Rays that which our eyes continuously changes and as a result the the brightness of the star keeps changing and this is what causes the twinkling effect of the stars so the next question is why don't planets twinkle the answer is the planets actually do twinkle in fact everything outside our atmosphere anything we're looking at outside our atmosphere does twinkle due to the effect that we discussed however we can't notice the jingling effect of the planet because planets appear bigger in size to us let's go to a different screen so here is the star a star pretty much looks like a dot in the sky but if you were to look at a planet a planet actually appears much bigger in the sky that's because the planets are much closer to us now although we can't see that with our naked eyes if you had to use a telescope and look at them then stars would still look like a dot but planets well we can actually make out their shape if you're looking at a star Saturn we could you can actually see the rings of the Saturn and so on so because planets are much bigger in the sky compared to the stars we can assume that planets are made up of many such dots so stars are just single dots but we can assume that planets are made up of many such dots and so each of this dot will twinkle just like a star but because we have many dots when one of these dots become dim some other dot will become bright when that dot becomes dim some other dot becomes bright and as a result when we look at the planet as a whole the overall brightness doesn't change much or doesn't fluctuate much we can now actually see this effect in action I have copied this twinkling dot and pasted it over here several times so each of these dots are going to twinkle the same way pretty much the same way then we'll zoom out and we'll look at them all right so let's play the animation if you see carefully you can actually note that all these dots are twinkling the same are because I've copy pasted them but now let's zoom out until both of this the planet and the star pretty much look like a dot and you can pretty much see that the brightness of the planet remains almost a constant that's because on an average the light from the planet pretty much remains the same and that so we can't make out the twinkling effect now if you find it hard to see this on your mobile screen or something then what we'll do is we'll zoom in a little bit and you can move away from the screen until both of them look like a dot to you so more away until the star the star is barely visible and the planet just looks like a dot and then you can actually see the twinkling effect of the star but not of the planet so to quickly summarize we learned that the twinkling effect of the star is caused due to the fluctuations of the atmospheric conditions which make the star light Bend as it moves towards us and as it is that the brightness of the star keeps changing and this means everything that is outside our atmosphere will twinkle but because planets have a bigger size that twinkling effect cannot be made out with our eyes and for the same reason the Sun and the moon also don't seem to twinkle for us and this string chling effect is pretty annoying for astronomers who use their telescope and try to take the photographs of our planets or the galaxies because the forests don't come out very crisp and it's for that reason we built the Hubble Space Telescope the idea was to build a telescope and make sure that it orbits around the earth outside our atmosphere then it will not be affected by the twinkling effect and as a result the photographs produced by the Hubble Space Telescope is unaffected by the atmospheric fluctuations and that's why these photographs are extremely crisp and extremely useful for scientific research