# Why stars are white? According to Rayleigh Scattering

According to Rayleigh Scattering, the red waves are capable of travelling a long distance, so that only we are seeing the Sun as reddish during Sunset and Sunrise. If this was true then all other stars must also appear red in colour, as the distance through the atmosphere travelled by those lights is further during sunset or sunrise.

Why do stars still appear white?

• Rayleigh scattering happens on small particles. What would it happen on in space? – Jimmy360 Apr 16 '15 at 11:44
• It really doesn't have much to do with Lord Rayleigh and plus stars are not all white. – Xiaolei Zhu Apr 16 '15 at 15:09
• I believe this question is about the colour as perceived on Earth, where starlight certainly is affected by Rayleigh scattering as much as light from our own Sun. – gerrit Apr 16 '15 at 19:16
• I checked out this link, the more the light travels in atmosphere gets scattered. – Kanni1303 Apr 17 '15 at 8:37

The colour of stars as observed by an observer on Earth varies just like the colour of our own Sun, depending on where in the sky the source is relative to the observer.

However, the light of stars is generally too faint to notice this as clearly with the naked eye, because we cannot perceive colour for weak light sources.

• As per my knowledge, mix of all colors leads to white so only we are viewing the white source. If we split that into spectrum we can see Violet to Red visible waves. I am not sure that weak light source will also be white? in case of stars looking white? – Kanni1303 Apr 17 '15 at 8:22

No, Rayleigh scattering models the probability (and angle) of scattering as a function of wavelength and of the particle sizes. All wavelengths travel a long way but the path followed (scatter or nonscatter) varies.

Since space is mostly "empty", there's little scattering. Beyond that, your understanding of stars is quite incomplete. THey do in fact have a variety of "colors," aka peak lambda outputs, depending on size, age, etc.

"White," as a color, is a perceived effect and can occur either when a bunch of wavelengths hit the retina simultaneously OR when the input intensity is extremely high. Then there's a third case: in very dim light (e.g. most stars during a very dark night), our color receptors don't respond and only the 'grayscale' cones in the retina respond.

• I don't think this answers what the OP meant to ask. Stellar light reaching an observer on Earth has exactly as much Rayleigh Scattering as light from our own Sun. – gerrit Apr 16 '15 at 19:19
• Space is not all that "empty" in terms of scattering - e.g. irsa.ipac.caltech.edu/applications/DUST – Kyle Oman Apr 16 '15 at 21:03
• If this was the case, then moon light during rise and set should also be red in color. The scattering is happening in space also. As daily tons and tons of dust are coming to earth from space due to celestial actions like bursting of star, planets etc. – Kanni1303 Apr 17 '15 at 8:29
• @gerrit perhaps so. I got the impression that he was comparing the total distance travelled from stars. – Carl Witthoft Apr 17 '15 at 11:26
• @Kanni1303 First of all, the moon does turn orange near the horizon, most easily seen when it's a full moon. Second, as I stated, for dim objects in a dark sky, your eyes' color receptors don't work very well. – Carl Witthoft Apr 17 '15 at 11:27
• Starlight, as emitted by a star, comes in a wide range of colours. For instance see the picture below. Now this is a picture, and pictures can often be tricky with their representation of colour, so you'll have to take my word for it that Betelgeuse does look significantly redder to the naked eye than say Vega until you get a chance to go look yourself on a dark clear night.
• Contrary to what some of the other answers are saying, scattering in space can be substantial. In the context of this question, Mie scattering is probably the most relevant process (Rayleigh scattering is a special case of Mie scattering). Mie scattering causes interstellar reddening - stars that have a lot of dust along the line of sight appear redder than they would in the absence of dust.
• Rayleigh scattering in the Earth's atmosphere affects starlight the same as sunlight (the light incident at the top of the atmosphere may be redder or bluer than sunlight, but it will be redder by the time it gets to the ground, just like sunlight).
• Finally, as others have mentioned, human eyes operating in low light are bad at seeing colours, but alright at seeing variations in intensity, so faint stars may appear whiter than they otherwise should to a human eye ("cones" need higher intensities to work than "rods").
• Rayleigh scattering goes with λ^-4 but Mie scattering does not necessarily do so — that's why the clear sky is blue, but clouds are white. Doesn't the observation that it does cause reddening require that the scattering particles are much smaller than the wavelength of the light — and that we are in the Rayleigh regime? – gerrit Apr 16 '15 at 21:15
• @gerrit As I understand it, the Mie scattering cross section will either be independent of wavelength, or a negative power of wavelength (depending on relative size of wavelength and grain size), or $\sim$zero. The scattering will therefore either redden or have no effect on colour. There is a good mix of dust grain sizes out there, so there's a good chance there will be some reddening. Sometimes there will be uniform extinction, sometimes no scattering, but there is no "bluening". – Kyle Oman Apr 16 '15 at 21:22