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why do some stars appear to twinkle

Stars twinkle, while planets (usually) shine steadily. Why? Stars twinkle because Б theyБre so far away from Earth that, even through large telescopes, they appear only as pinpoints. And itБs easy for EarthБs atmosphere to disturb the pinpoint light of a star. As a starБs light pierces our atmosphere, each single stream of starlight is refracted Б caused to change direction, slightly Б by the various temperature and density layers in EarthБs atmosphere. You might think of it as the light traveling a zig-zag path to our eyes, instead of the straight path the light would travel if Earth didnБt have an atmosphere. Planets shine more steadily because Б theyБre closer to Earth and so appear not as pinpoints, but as tiny disks in our sky. YouБd could see planets as disks if you looked through a telescope, while stars would remain pinpoints.

The light from these little disks is also refracted by EarthБs atmosphere, as it travels toward our eyes. But Б while the light from one edge of a planetБs disk might be forced to БzigБ one way Б light from the opposite edge of the disk might be БzaggingБ in an opposite way. The zigs and zags of light from a planetary disk cancel each other out, and thatБs why planets appear to shine steadily. You might see planets twinkling if you spot them low in the sky. ThatБs because, in the direction of any horizon, youБre looking through more atmosphere than when you look overhead. If you could see stars and planets from outer space, both would shine steadily.

ThereБd be no atmosphere to disturb the steady streaming of their light. Can you figure out which objects are stars and which are planets just by looking for the twinklers vs the non-twinklers? Experienced observers often can, but, at first, if you can recognize a planet in some other way, you might notice the steadiness of its light by contrasting it to a nearby star. Bottom line: Stars twinkle because they appear as tiny pinpoints as seen from Earth, even through telescopes. Planets donБt twinkle because they are closer, and thus appear larger in our sky, as tiny disks instead of pinpoints.
Stars tend to twinkle for two main reasons: first, stars are very far away (the closest star is about 4 light-year from the Sun) and are therefore seen as point sources.

Second, Earth has an atmosphere. Earth's atmosphere is turbulent, and therefore all images view through it tends to "swim". Therefore, sometimes a single point in "object space" is mapped to several points in "image space", and sometimes it is not mapped at all. Since stars are seen as single points, they sometimes seems brighter, sometimes even seems to disapear. If you look at it in another planet of our solar system, it will depends on the planet's own atmosphere. If you look at stars on Mars, the atmosphere being very thin, the stars won't twinkle that much. Same for Mercury. On Venus, the atmosphere is so thick than you won't see anything apart from the atmosphere itself (if you are not crunched by the atmosphere pressure, by the way. ).

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