What colour
is the Sun? What colour
is the Sun?
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Where's purple?
Blackbody
The above is supposed to show, as accurately as possible on a computer screen, how the colour of a radiating blackbody varies with its temperature. The two vertical bars of colour at each end of the colour chart are the colours at zero and infinite temperature. Evidently the limiting colours are not much different from the colours at 1,000 K and 50,000 K. I confess a little disappointment at the high temperature result: I was hoping that the colour there would be more violet.
The blackbody colours in the colour chart at top and in the diagram at
right were computed by integrating the monochromatic tristimuli X,
Y, Z from the CIE
2¢X tables over a Planck distribution. The resulting blackbody
tristimuli were transformed to R, G, B, then
gamma-corrected to R?/font>, G?/font>, B?/font>, as described
on the Blackbody colours at temperatures around 5000-7000 K are nearly
white, and the choice of white point is important if one wants to render
these colours accurately. On these pages I have adopted CIE D65
as white, but of course this may not accurately represent the white point
R = G = B of your monitor.
Introduced by CIE in 1963, the D series of standard illuminants are
intended to represent `daylight' at various correlated colour
temperatures, with D65 to be used whenever possible. The
precise (slightly complicated) definition of these illuminants can be
found in R. W. G. Hunt (1987) ``Measuring Colour''. The
`correlated colour temperature' is the temperature of the blackbody that
is, in a certain well-defined sense, nearest in colour. The subscript 65
on D65 signifies a correlated colour temperature that was
originally 6500 K, but thanks to a revision of the combination
hc/k of fundamental constants in the Planck formula is now defined
to be (1.4388/1.4380) 6500 K ?/font> 6504 K (the latest NIST value is hc/k = 1/0.6950387 cm K = 1.438769 cm K,
but apparently the definition of correlated colour temperature has not
been revised again).
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Sun The Sun
has an effective surface temperature of 5780 K. However, the Sun's
spectrum is not a precise blackbody. The graph at right shows the spectrum
of the Sun as seen from above the Earth's atmosphere, together with the
spectrum of a blackbody at 5780 K with the same total flux as the
Sun, and the spectrum of CIE D65 `daylight' normalized to the
solar flux at 560 nm. The Sun's spectrum was taken from R. L. Kurucz,
I. Furenlid, J. Brault & L. Testerman (1984) Solar Flux Atlas from 296
to 1300 nm. The spectrum in the Solar Flux Atlas has a resolution
of l/Dl ?/font> 800,000. The spectrum plotted at right is
heavily binned, into 5 nm bins. A table of the spectral energy
distribution of CIE D65 is available at the UCSD Color and Vision database.
The colour of the background of this page and accompanying pages on this site is the colour of the Sun relative to D65 white. As shown in the table, the Sun's colour is in fact rather similar to that of a 5780 K blackbody. It looks peach pinkish, not yellow, doesn't it? Strange. But I think it's a pretty colour. It has to be emphasized that white is a relative thing, at least to a certain extent. It is relative to the D65 white that the Sun is peachy pink. Item 16 of the Color FAQ states that for most people D65 has a little hint of blue. So maybe the Sun is really white? |
Star colours
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Back to Where's purple?
Updated 19 June 2001
Forward to Primary Colours
The
chromaticity diagram at right shows the blackbody trajectory as a black
line labelled with temperature in Kelvin. This chromaticity diagram,
constructed using a corrected version of John Walker's 
Where's purple?