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Sally Sonderson, the author of this article, works at Homework-Desk.com (http://www.homework-desk.com) and wants students to know as much as they can.
In and of itself, a star does not flicker. A person gets this impression on the Earth, when he/she perceives the star light after its coming through the atmosphere. It is essential condition of the flickering. If you look at even really far star from the space, it won't flicker. Astronauts, when observing stars from the moon, where there is no atmosphere, saw the sky sown with stars that were shining with even non-blinking light. But coming to the Earth, which is covered with a thick "blanket" of the atmosphere, the light rays, before reaching at the surface, are refracted in many directions for many times.
The star light becomes flickering when it comes from an atmosphere layer with high density into the one with lower. Why? The air bulks around us are not fixed. They are constantly moving relatively each other. Warm air goes upwards, cold – downwards. The air refracts light in different ways, depending on its temperature. When the light goes to the layer with lower density, from the one with higher, it starts flickering. Simultaneously, the star shape becomes blurred, and its image becomes bigger. Intensity of the star light, that is their brightness, changes. In some cases, the star can be seen really easily, in others, it pales. And in a moment it is again very clear. In the scientific world, this process is called scintillation.
Not all the heavenly bodies flicker. Planets, for example, gleam with the reflected sun light and do not flicker. Venus and Mars look like the big bright stars on the sky, but differs from the ones by the absence of flickering. Why? Because these planets are closer to the Earth, and we perceive them as small disks, but not as small dots. Light is reflected from different parts of the disks. It is refracted, though, but in a different way. From some parts of the disk, the refracted light is bright, from others, it is more faint. In a moment, they shuffle. The average intensity of emission from the whole surface of the disk stays stable. Therefore, the planet disk gleams with even non-flickering light.
Usually, you can set a planet apart from a star by nature of the emission: stars flicker and planets do not. But, if there is a turbulence in the Earth atmosphere, for instance, hurricane, planets can flicker, as well. Our sun is also a star. But it is much closer to Earth, than those stars that we see at night. Sun is not a spot in the sky. We perceive sun as a huge evenly shining disk. If the sun had become further from the Earth for millions of kilometres, it would have hidden among other stars and would have flickered in the same way as they do.
Flickering stars are really beautiful and can inspire a poet, but for an astronomer, it is a "headache". Even if the sky is clear, there are immense moves of air bulks in the atmosphere, so called atmospheric turbulence, that makes it hard to observe and take pictures of the stars. The best time for the astronomical observations is when the sky is clear and the atmosphere is without atmosphere turbulence. When the atmosphere above the telescope is calm, astronomers work with a good visibility and almost without flickering. With the development of the space era, powerful telescopes were launched into orbit above the atmosphere: they are not disturbed with flickering at all.
Learn more about refraction.
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