visible light - Why are stars white?

That is may be an easy question, but I am not a professional. The sun is a star, and when I look at the sun it is usually yellow. Why are stars in the night white?

I suppose it is for the distance. What is the explanation?

Answer

It has more to do with physiology of the eye rather than the spectrum of light produced by stars.

Stars emit light over the full range of visible wavelengths. Hot stars emit more blue/violet light, cool stars emit more red light. The Sun is relatively neutral in that regard, so does not have a strong colouration, but many other stars in the sky have temperatures as low as 3,000K (should appear reddish) to 20,000K or more (should be blueish).

However, light received by the eye has to be reasonably bright to trigger the colour sensitive cells (the cones). So whereas the Sun appears to be yellow(ish) (or to be more exact your eyes perceive the mix of wavelengths as yellowish), there are relatively few stars that are bright enough to look distinctly coloured. Betelgeuse is an example that most people see as reddish. Arcturus is distinctly orange (to me).

But fainter stars all appear white because they are mainly being seen by the rod cells in your eye, which are not very colour sensitive (and have no sensitivity to light at the extremes of red and blue). This is known as scotopic vision.

APPENDIX: I have found a few interesting links that appear to confirm this line of argument. Firstly, one can predict what stellar spectra would look like to the eye under conditions where the cone cells are operational. Here we can see that many stars would appear either pink (K-stars) through orange (M-stars). G-stars like the Sun are just off-white, whilst hot stars are distinctly blue (see also here).

These calculations do not include the effects of the Earth's atmosphere which preferentially absorb and scatter blue light. However, the effect of this on the colours cannot be that serious (it will have no effect at all on rod cells since they are not colour sensitive). Cone cells are sensitive over the range 450-650 nm. The typical atmospheric transmission at zenith for these wavelengths ranges from 85% to 93% (see Fig.1 of Burke et al. 2010), but would reduce to perhaps 70% to 85% for an object only 30 degrees above the horizon. Even at this low altitude, the differential effect is quite small. The colours of stars would not be changed by the atmosphere until they got quite close to the horizon.

Stars would have different colours, ranging from pale blue through to an orangey-red, if they were bright enough to excite the eye's cone cells.