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.
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.
How hot is an accretion disc, and how fast does the temperature go up the nearer to the event horizon? The one depicted here goes from red hot to white hot, but if these things emit x-rays, I would think that they would appear blue towards the center.
The maximum temperature for an accretion disc around a small black hole is around 107
K. This occurs near the inside, but not completely so. That will appear bluish-white to the eye, although the peak output will be well into the gamma rays. The temperature is approximately proportional to r-3/4
over most of the radius, so the temperature rise from outside to inside will be gradual. But it will rapidly appear white. I'd expect the disk to have a very narrow outer band going from red to orange to yellow, then the rest essentially white or blue-white.
Would the red shift caused by the gravity well sufficiently counteract the wavelengths of the thermal emissions to keep them from appearing blue?
I don't think it would have any visible impact.
What color would the gas outside the photosphere be? Would it cool off as it approaches the accretion disc? Does it reflect the light from the star and the accretion disc? Would it be the same pearly white color as the Sun’s corona? (And why does the Sun’s corona glow white when it is x-ray hot?)
Good question. I'd guess it might be similar to the corona- very hot but very tenuous, so not very bright.