Stars flicker (or twinkle) because they are so far away that they are essentially tiny points of light. As the light passes through the atmosphere, it is refracted back and forth making the light fluctuate.
Planets are closer, and generally light from different parts of the planet surface are refracted differently. These then average out in a way that you don't see the twinkle effect.
Some stars (especially close giants, like Betelgeuse) are so large that their twinkling is often not visible.
Oh, and when stars are nearer the horizon, the appear to twinkle more than stars high in the sky.
Planets are closer, and generally light from different parts of the planet surface are refracted differently. These then average out in a way that you don't see the twinkle effect.
Some stars (especially close giants, like Betelgeuse) are so large that their twinkling is often not visible.
Oh, and when stars are nearer the horizon, the appear to twinkle more than stars high in the sky.
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It is an effect of the atmosphere. The refractive index of air varies with temperature. If you look along a straight road on a hot day, you can usually see a shimmering effect or a mirage. The same variations in the air at night cause the smaller, fainter, stars to "twinkle".
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They don't, their light passing through our turbulent atmosphere causes this effect.
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Because object that are either closer or larger than the star pass by it
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those are probably satellites
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space junk between the star and you