Which scientist explained why binary stars change color?

Answer 1

Carl Vogel and Edward Pickering discovered the first spectroscopic binaries, and used Doppler's effect to explain their results. However, this question is rather confused. Binary stars do not change colour (except as part of their evolution, which happens to any star eventually). I think you may be misunderstanding what we call a spectroscopic binary. Some binary stars are close enough to us to be separable in telescopes, but many are too far away for that. One of the ways we can tell a star is binary is by spectroscopy. This does have to do with the wavelengths of light, which correspond to colours, but it is a misunderstanding to think that this involves the star changing colour. Spectroscopy involves dispersing light into a spectrum, that is spreading out the wavelengths into a 'rainbow'. When a light source is moving towards us, wavelengths are shifted towards the blue end of the spectrum and when it moves away, they are red shifted. This is called the Doppler effect. With the continuous spectrum of a star, this does not show up as a change of colour, because everything that is shifted is replaced with wavelengths from further along (the spectrum continues beyond the visible at both ends). The reason we can tell shifting happens is that there are dark lines across a stellar spectrum caused by elements in the star's atmosphere which absorb some of the emitted light. The frequencies at which these lines appear are always the same for the same element. If these lines are found at different frequencies than expected, then the source is moving. Binary stars orbit each other, so part of the time they are moving towards us and part of the time they are moving away. This results in the frequency of the absorption lines' shifting back and forward with time.