Why is the Sun red at sunset?

And why is the sky blue? Both occur because sunlight is scattered by atmospheric molecules. Short wavelength (such as blue) are scattered more, and short ones (that is red) scatter less.

The scattering changes the direction in which light moves. Some of it is redirected upwards, which is why Earth viewed from the Moon appears like a big blue marble. And some is scattered down, reaching the eye from some direction other than that of the Sun, since it was redirected--from somewhere in the sky. Hence the sky is blue.

At sunset light reaching the eye has traveled at a shallow angle and traversed a great thickness of air. It has encountered a lot of scattering, and much of its blue light had been redirected. Remove the blue from balanced sunlight and what is left is the long-wave stuff, with a lot of red.

Postscript: Dust in space between stars also scatters light, and once again, long wavelengths are scattered less. For many years the center of our galaxy has been a mystery, because the dust surrounding it scatters all visible light, blanketing it from the view of telescopes.
Not any more: a large European telescope in Chile has probed it with infra-red light (even longer wavelength than red) and established that it contains a humongous black hole, 3-4 million times as massive as the Sun. For more about it, see: http://www.phy6.org/stargaze/Sblkhole.htm

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