# Does the associated colour of a spectral class (red and class M, for example) come from the peak black body radiation of stars in that class?

Each spectral class O through to M has an associated colour. Class O stars are described as blue stars, for example. Is this colour determined by the peak wavelength of their black body radiation? Do all class O stars have a peak visible light wavelength that is blue?

In that case, why are F class stars considered to be white? The peak wavelength of a star cannot be white, as white is a combination of light of all other visible wavelengths.

Furthermore, the Sun's peak visible wavelength is in the green part of the spectrum, and yet it is classed as a G star, which has a corresponding colour of yellow.

Either my initial assumption that the associated colour of each spectral class comes from the peak visible wavelength of the stars in that class is wrong, or I'm misunderstanding the above two examples. I would appreciate an explanation, as I am very confused!

• And I would add that sun is already pretty white. Worth to look again at the classification in details – Alchimista Jan 23 '18 at 20:09
• Anyway a point to consider is that a given T correspond to a precise wl in Wien displacement law. In such a sense the wl of white is that peaking in a bb spectrum that we perceive as the most white.... – Alchimista Jan 23 '18 at 20:20

The spectral classes are defined by the presence and depth of certain absorption features in their atmospheres. The pattern was discovered to correlate to the star's surface temperature after the scheme was fixed (though they're still discovering spectral classes on the low end in the form of brown dwarf stars).

In that case, why are F class stars considered to be white? The peak wavelength of a star cannot be white, as white is a combination of light of all other visible wavelengths.

Furthermore, the Sun's peak visible wavelength is in the green part of the spectrum, and yet it is classed as a G star, which has a corresponding colour of yellow.

Really, the sun is white, too. The perception that it's yellow comes from the distortion of its spectrum when we view it low in the sky, because that's when it's least painful to catch glimpses of it. When the sun's light passes through that much atmosphere, the blue light is more blocked than the red and green. Mix red and green light, and you get yellow. Where did the blue go? Well, that's why the sky is blue, after all.

See, the peaks of black-body spectra (and stars are only approximately black-bodies) are really quite wide. So when the peak is anywhere near the visible spectrum (in log-scale of wavelengths) the star is going to look basically white because the spectrum is pretty flat. So, the color of hot objects really goes from red, through white, to blue/ultraviolet. The spectrum is never narrow enough to appear green or yellow, even though it can appear orange-ish in the transition to red, because the high frequency cutoff is sharper than the low frequency one.

• Thank you for your answer. I understand what you have written, but I am still confused as to why each class has an associated colour. Why are G stars considered to be yellow, for example? – Pancake_Senpai Jan 23 '18 at 20:19
• $\mathrm{G}$ stars are considered "yellow" because our sun is a $\mathrm{G}$ star and is considered "yellow" for the reason I described. Other than it, it's a convenient (if slightly misleading) way to remember the sequence of temperatures. 7 spectral classes (for main sequence stars, anyway), "7" colors of the rainbow. Also, keep in mind that when an astronomer says "color" they don't mean "what you see", they mean a ratio of light observed by two different filters (frequently, $B$ and $V$). – Sean E. Lake Jan 23 '18 at 20:35
• This Wikepedia page (en.wikipedia.org/wiki/Stellar_classification) says the colour attached to each spectral class is its chromaticity, so that would mean that these colours do in fact refer to the colours of stars, and not a ratio of light. If these colours are really nothing more than an attempt to make the spectral classes more memorable, then that would mean they are completely meaningless, which I would struggle to believe. – Pancake_Senpai Jan 23 '18 at 21:34
• @Pancake_Senpai "chromaticity" is a fancy way of saying "ratio of fluxes relative to some standard". Notice how there are two different "chromaticity" columns in their table that have different values for the sun? How can the stars have two different apparent colors? As I said in the post, there's a continuum from red through orange and white to blue in the star's observed, physical, color. – Sean E. Lake Jan 23 '18 at 21:54
• I think I've just been too hung up on the exact colour descriptions, and haven't taken into account how general they are. The peak wavelength of the Sun is green, which is a whole lot closer to yellow than it is to blue. The colour description of G class stars therefore applies to the Sun, as the colours for spectral classes are very general. As for why the colour of F class stars is white, I guess it's just that most of those stars generally tend to be white, as opposed to a red or blue giant (for example). Would you say that this is correct? – Pancake_Senpai Jan 23 '18 at 21:59