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So in a discussion today I wanted to show someone the relationship between a star's color and their surface temperature.

Expecting to find a chart such as this one, I first found this one instead. The first one is from the Wikipedia article on black-body radiation, the second one from the article on incandescence.

I found myself unable to explain how both charts could exist and be true at the same time, even though I was sure there must be clear connection between the two.

Now I know the first one is definitely correct and used in practice for objects such as stars, the second one is used for objects such as glowing metals, and both scales apply to objects regardless of the chemical elements they consist of, i.e. their are both only a function of temperature.

Now even after a bit of googling I have found myself unable to reconcile both charts with each other. I am sure there is a relationship between the two, since both incandescence and black-body radiation are (as far as I understand it) defined as "the light emitted by an object due to its temperature" (as opposed to chemical reactions like in fire, quantum effects like in LEDs).

So the questions is: in which way are incandescence and black-body radation, and therefore their temperatur-to-color-scales, related to each other (until today I thought they were one and the same)?

Roger V.
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Mephane
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  • Note that there two figure you've linked are in different units, which explains a portion of the difference (but certainly doesn't invalidate the question). – dmckee --- ex-moderator kitten Jun 17 '17 at 23:22
  • @dmckee yes I thought about that, but adding 270 to the °C values to (roughly) convert them to K doesn't make up at all for the difference. – Mephane Jun 17 '17 at 23:43
  • the blackbody scale starts at around 800 really. If you zoom in you can tell better. The other one starts at 550 C or around 800 K. Hence these scales are very similar. So I don't see the problem. – Natsfan Jun 18 '17 at 02:10
  • @john the black-body picture is white somewhere between 5000K and 7000K whereas the incandescence one is shown white hot at 1300C (1600K). This is certainly off somehow. If this is not a mistake, I would only guess that the incandescence image might give a "subjective color" because a brighter object at the same average frequency will seem "whiter" and so on. – Void Jun 18 '17 at 04:29
  • @Void What about pyrometers? Would those be subjective? Incandescence uses black-body radiation as well. As far as I'm aware, stars aren't metallic. I can't say I'd know when something like hydrogen burns white. – Mea quidem sententia Jun 18 '17 at 17:13
  • Related: https://physics.stackexchange.com/q/639995/247642 – Roger V. Dec 01 '22 at 12:23

2 Answers2

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There is no inconsistency between the charts. The second chart is just a zoom on a portion of the first one. As mentioned by dmckee, the unites are also different.

Miladiouss
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  • In the wiki link it says clearly that "Incandescence is a special case of thermal radiation. Incandescence usually refers specifically to visible light, while thermal radiation refers also to infrared or any other electromagnetic radiation." As for the colors, one should not forget that perception of color is biologically determined, we do not see the rainbow spectrum, particularly the spectrum does not have white. see the perception http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colper.html – anna v Jun 21 '17 at 09:51
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Incandescence usually refers specifically to visible light, while thermal radiation or black body radiation refers also to infrared or any other electromagnetic radiation. So, incandescence is a special case of blackbody radiation [source:wikipedia] [1]: https://en.wikipedia.org/wiki/Incandescence

Aditya
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