apple green

I've often joked about the colourful (pun intended) descriptions of double stars. Yellow and dull brown. Pale topaz and lilac tint. Rosey. Ashy white. Light grey. Straw yellow and silvery azure. What?! Typical terms in double stars for small telescopes. Smyth describes 95 Herculies as "light apple green; cherry red." What the hey?! I've often joked, "I wonder what they were smoking." Or ingesting. Or shooting. Or however they took drugs in the 1800s.

Green stars. Pshaw.

When one knows about the Hertzsprung–Russell diagram and the composition of stars and their life cycles, these kinds of colours are not possible.

I've read some of the articles on star colours at Southern Astronomical Delights web site. A long time ago I read Paul Baize's article on colour (written perhaps in 1956). There are many factors like difference in optics, magnitude, contrast, proximity, perception, etc.

But I had never considered the effect of the Doppler shift! For example, an intrinsically yellow star moving rapidly toward the observer would get blue shifted and the blue would additively combine with the yellow. The result? A green tint!

OK. I could be wrong about the whole colour thing...