What color is a blackbody?

A blackbody, in this context, is just an object glowing with its own heat in a way unaffected by its composition. Things like hot iron or the interior of a glass furnace.

As the temperature rises, a blackbody starts to get brighter very rapidly. But its color also changes. We are familiar with red going to yellow going to white as an object gets hotter, but is that all that happens? What does it look like? (1/n)

In principle this isn't too hard to calculate: our color vision is based to the responses of three photopsins (light-sensing pigments) to the incoming light; if you simply write down these amounts as X Y and Z you get a description of the color as received by the human eye. The response of each photopsin to light of various frequencies is tabulated.

Since we know the spectrum of blackbody light, we can easily calculate the X Y Z color we'd see. Convert this to RGB and you have it. (2/n)

There are perectly reasonable colors that can't be represented in RGB (they are out of the "gamut"). In particular, for very cool blackbodies the color you'd see is a deeper red than exists in RGB. But we're used to cramming the world's vibrant colors into the few that our screens can reproduce.

I have done these calculations and produced a color gradient (attached) that captures the heating of a blackbody from deep red through yellow and white up to a sort of baby blue. (3/n)

One odd thing you'll notice is that as the blackbody keeps getting hotter, the color settles down to a particular baby blue.

Although blackbodies this hot are actually putting out loads of ultraviolet and X-rays, the part our eyes can actually see is far below the peak of the blackbody spectrum. And in this (Rayleigh-Jeans) regime, the spectrum just looks like a straight line of a particular slope regardless of temperature. This means it looks the same color. (4/5)

In short: all blackbodies that are hot enough - including accretion disks and neutron stars - look the same powder blue.

Also: if you want to use a blackbody color map for your black hole image, this is what it actually ought to look like. (5/5)

@anne This is a very cool thread. It’s counterintuitive that the color settles (visually) on this light blue, but your explanation makes it clear why. It’s also a very pretty color, so there is that. 😊

@tsturm
For some reason nobody likes it when I print their cataclysmic accretion disks in a pretty baby blue

@anne No, the color is fine, but the extra-sparkly background stars are just too much. 😂

@tsturm
You joke but I figured all this out and implemented it to make this video:
youtu.be/DwiaBPdshNs

Which does indeed have extra-twinkly stars.

And also a view into another universe, though you only get to see it once you're doomed to be torn apart by the singularity.

@anne Finally had a chance to look at the video - very interesting. So this would be a giant black hole hanging out in empty space without any kind of accretion disc, correct? I assume the disc would be so bright to obscure a view of the black hole up ahead...

@tsturm
Not only does this one not have an accretion disc (which would indeed drown out your view of the distant stars), if you want to see the other universe, there can never have been any matter falling into the black hole. It had to have been formed as is through quantum black magic of some kind.

@anne Ah. Never considered what it would mean to look at the insides of a black hole. It's probably like the super-hot equivalent of a full vacuum cleaner bag.

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