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My question refers to an overview of the biggest stars we know: http://farm5.static.flickr.com/4138/4820647230_faba1c9f3b_o.jpg

Why are some of those blue?

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The picture you've linked to is of stars not planets (and it's a huge file). –  dmckee Feb 26 '11 at 16:53
    
Well, my planet is blue and is big too! Its called Earth. –  Pratik Deoghare Feb 26 '11 at 20:32
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up vote 7 down vote accepted

dmckee's right: the picture you link to shows stars, not planets.

The color of a star is almost entirely determined by its temperature. The light coming from a star is, to a good approximation, blackbody radiation (except for absorption lines in its spectrum, which are very important tools for learning about the star but have little effect on its color). The spectrum of a blackbody depends on its temperature, in such a way that it shifts from longer to shorter wavelengths as the temperature increases. So hot stars look blue, and cool stars look red. Even cooler objects, such as you, don't glow significantly at all in the visual part of the spectrum but do in the infrared.

At the moment, we know little or nothing about the colors of planets other than those in our solar system. Extrasolar planets are detected indirectly, via their effect on the star they orbit. They are not yet seen directly themselves.

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Note: We actually have directly seen a few exoplanets, just not too many and not necessarily in many wavebands. That said, people are working very hard to even get spectra of planets' atmospheres in cases where they transit in front of their stars, which would very much get you a color if the atmosphere is anything but clear. –  Chris White Jun 16 '13 at 16:01
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I guess that some or many planets of this kind are similar to Uranus but I can't guarantee that the chemical composition below is universal for blue planets.

Uranus' atmosphere is composed of hydrogen, helium, and methane. Methane in the upper atmosphere is what matters: it absorbs red light from the Sun but reflects the blue light.

The Earth is blue mostly because of the oceans of liquid water that have a similar optical property as methane on Uranus.

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