Chris Peterson wrote:
Cousin Ricky wrote:I haven’t seen very many detailed paintings of blue stars, but on those that I have seen, the stellar prominences (and chromospheres, for that matter) are depicted in blue. Don’t prominences get their color from H-alpha emissions? Wouldn’t they be red regardless of the star’s temperature?
The color of a prominence depends on the atoms that are ionized. Prominences aren't really red... reddish would be a better description, since Ha is only one of the emission lines. Still, a several solar mass blue star has a hydrogen envelope, so it's prominences should be substantially similar in color to what we see on the Sun.
Note, however, that we only see the color of the prominences of the corona of the Sun during solar eclipses. That is because the solar disk is so much brighter than the prominences. The blue star in today's APOD isn't eclipsed, and its disk is much, much brighter than the disk of the Sun. Admittedly its prominences would likely also be brighter than the prominences of the Sun, but still, a picture like this shouldn't realistically show prominences of the corona. We do seem to see the prominences, and if we do see them they should be reddish in color, as Chris said. But in my opinion we shouldn't see them at all.
Do blue giants show limb-darkening?
I can't think of any reason they wouldn't. But keep in mind that limb darkening is essentially a continuum phenomenon. In narrow spectral bands, it may not be present, or you can even get limb brightening. And this artistic representation pretty clearly shows the blue star as it would appear in some narrow band, not white light.
I don't find the color of this blue star very unrealistic (except for the blue prominences), so in my opinion, this could be a "white-light picture". The problem is that the star is not nearly bright enough. Hot blue stars have fantastically bright photospheres. The illustration makes the star look almost dark in places, which is beyond impossible. A closeup of a blue star like this one would be so overwhelmingly bright that we wouldn't be able to see the accretion disk of the black hole at all, except its innermost, very bright parts.