If you look up at the stars on a dark night, you might notice that these bright points of light appear to twinkle. In actual fact, stars do not actually twinkle: they just appear to do so from our perspective on Earth. Show Our atmosphere reaches about 10,000km up from the surface of the Earth, and within the atmosphere air gets blown around, while hot air rises and mixes with cooler air. Stars appear to twinkle because as light from those stars passes through our atmosphere, it is bent and distorted by varying temperatures and densities of air.  There is even a scientific term for stars' twinkling, and that's 'atmospheric scintillation'. This is the astronomical term for those quick changes in the apparent brightness of a star (more on this in our guide to stellar magnitude) or even the colour of a star produced by the aforementioned atmospheric irregularities. Similar effects are seen in the way our view of an object might be distorted by heat rising from a hot radiator or roaring fire, for example. It is possible to capture these effects of atmospheric distortion in an image, by photographing the changing colours of a twinkling star. For more basic stellar astrophotography, read our guide on how to photograph the stars. Astronomical seeingFor most people, the concept of a twinkling star is quite romantic, and conjures up memories of one of the most famous nursery rhymes of all time. But for astronomers, the effect of twinkling stars can be a nuisance as it can cause the image seen through a telescope to shake and jump around. The extent to which astronomical objects appear to shake and jump around is known by astronomers as 'seeing'. You will often hear practical astronomers complain about 'bad seeing' or praising 'good seeing', for example. For more on this, read our guide to astronomical seeing. Why do stars twinkle, but planets don't?Stars twinkle while planets don't because stars are so much further away from Earth. This makes them appear as concentrated points of light, and that light is more easily disturbed by the effects of Earth's atmosphere. Planets, on the other hand, are much closer, and the sunlight reflected off them comes back through Earth's atmosphere in a much thicker beam of light than starlight, so it is not as noticeably affected by the distorting effects of the atmosphere. The fact that planets don't twinkle, and the fact that they are found along the ecliptic, are worth remembering if you are looking up at the night sky and trying to work out whether that bright dot is a star or a planet. For more on this, read our guide on how to find planets in the night sky. Iain Todd is BBC Sky at Night Magazine's Staff Writer.
Planets do not twinkle the way stars do. This is because the planets shine by reflecting light and are also larger in appearance than stars. This causes less refraction than with stars. In fact, this is a good way of figuring out if a particular object you see in the sky is a planet or a star. Stars are so far away that they effectively act as point sources of light in the sky, and the light we see flickers in intensity as the incoming beams bend rapidly from side to side, while planets actually have finite size and are more closer to earth. The size of a planet on the sky in a sense “averages out” the turbulent effects of the atmosphere, presenting a relatively stable image to the eye. (Through a telescope with high magnification, however, we see shimmering images of planets). 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 flickers and changes colors. However, the flickering of one part of the planet is masked by the other part 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. Planets like Mars, Venus and Jupiter, which appear to us as bright stars, are much closer to Earth and look like measurable discs through a telescope. Again, the twinkling from adjacent areas of the disc averages out, and we see little variation in the total light emanating from the planet.
Stars twinkle because they are so far away from Earth that they appear as point sources even through powerful telescopes: the light rays emitted by them are refracted several times so that they look as if they are blinking. However, the sun and other planets are very close to us relative to stars and therefore appear like disks. Twinkle twinkle, little star, how I wonder what you are! Some rhyme, right? Everyone must be familiar with that famous rhyme from Jane Taylor’s poem “The Star,” which was a big part of childhood for many. Well, it’s not just a song for children; it actually refers to a certain scientific phenomenon that has fascinated observers of the Earth for millennia. Growing up, we were taught that stars couldn’t be like diamonds, but they do twinkle, right? They certainly seem to do so, but what about our nearest star? The sun is a star, the heart of our solar system, but we do not see the sun twinkling. Why do all the other stars in the night sky seem to sparkle, but our sun does not? Recommended Video for you: Does Earth come back to the same spot on your birthday? If you wish to buy/license this video, please write to us at . Why do stars twinkle, but the sun and planets do not?Short answer: Stars twinkle because they are so far from Earth that when the light of the stars passes through the atmosphere, it is bent countless times by refraction, making it look as if they are blinking. Earth’s atmosphereAstronomers have given a scientific name to the twinkling of stars: astronomical scintillation. This also explains why astronomers do not write songs for young children; it would have been much more difficult to memorize songs about twinkling stars with words like scintillation thrown in there…” Why the Sun Does Not TwinkleThe other stars we see in the sky are very far away from Earth: The Sun is the closest star to Earth with a distance of about 93 million miles (0.0000158 light-years). However, the nearest star is about 4.22 light-years away from Earth, so you get an impression of how far these other stars are really from Earth. The Sun doesn’t twinkle because it is too close to Earth compared to other stars. Thus, unlike stars, the sun appears much larger than a small dot in the sky and therefore does not seem to twinkle. You see, the Earth’s atmosphere is so turbulent that images of all objects in the sky tend to “swim.” Consequently, a tiny dot in space is mapped to 2 or more points in space, giving the dot source this blinking effect. (Source) In other words, stars appear so small due to their distance from us that they look like they are in more than one place due to atmospheric refraction, making them look as if they are blinking. However, the sun is quite close and therefore looks more like a disk than a tiny dot in the expanse of space. Therefore, atmospheric refraction does not play such a large role in the perception of the earth and therefore does not seem to blink. Why Do Other Stars Twinkle?Light from other stars crosses a great distance to reach us and passes through areas of the Earth’s atmosphere that vary in temperature and density. Our atmosphere is very turbulent, with currents and eddies forming, swirling, and scattering. In each layer of the Earth’s atmosphere, air moves in different directions with varying intensities. As the light from stars passes through the atmosphere, it is bent due to refraction, which is why stars seem to twinkle when we stare at them. This process is similar to the sight of a coin dancing at the bottom of the pool. This optical illusion occurs because the water in the pool bends the path of light emanating from the coin. Why Planets Do Not TwinklePlanets, like the Sun, do not twinkle. Planets are also closer to Earth than distant stars, so planets appear larger in comparison. Due to the planets’ proximity to Earth, the light from these celestial bodies does not bend as strongly due to the Earth’s atmosphere. Therefore, the light coming from the planets of our solar system does not seem to twinkle like stars.
Light from planet (left), Light from star (right) We should be very thankful for the Earth’s atmosphere; apart from protecting us from harmful UV rays that could fry us in an instant, the atmosphere is the reason why this sky full of stars seems to twinkle at night. Fortunately, this means that we can pass on the famous rhyme of Jane Taylor to future generations. It is a good poem… it would be a shame to waste it! Suggested ReadingWas this article helpful? |
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