by VictorBorun » Sat Dec 10, 2022 11:56 am
Chris Peterson wrote: ↑Fri Dec 09, 2022 1:57 pm
VictorBorun wrote: ↑Fri Dec 09, 2022 1:59 am
I can see 2 quotes here stating that coloured light (of red dwarfs or blue giants) gives way to white (of white dwarfs):
(1)
A 100,000 K star will photograph as white with RGB photography. Because it is white... nearly equal intensity across all visible wavelengths.
Indeed. Nearly flat across the
visible spectrum. Which is
precisely what we get from the Rayleigh–Jeans law. And a spectrum which varies in intensity by only a few percent from blue to red is essentially white to the eye and to an RGB camera.
the Rayleigh–Jeans law, B ~ ν²/T, gives a spectrum shape that is dependent not on T, but rather on ν = c/λ. The frequency does rise 2 times from red to violet making Brightness vary 4 times. Well, in fact 3 times, when we crop extreme margins, red and violet, that are poorly seen, but the result is readily seen as bluish. I mean "blue" is the colour that S-cones report as bright as white pixels in the scene, the L-cones report as dark as black pixels in the scene, and M-cones report as middle-bright.
And I mean "bluish" is the colour that the observer interpret as mixing tiny particles of white with tiny particles of blue. A white dwarf must look bluish for a comfortably set observer, because L-cones will register some yellow and red photons and report the thing as half-bright, just like they report a mixture of white and blue powders in equal portions.
the Rayleigh–Jeans law gives as bluish a colour as they come.
To get that colour from a thermal source you must heat it to 100,000°K or more.
If your source is mere 12,000°K, the spectrum will be flatter; you only get a pale bluish, like a mixture of much white powder and little blue powder.
Oh, now I get it! The reason why the myth of the Flat Thermal Spectrum of white dwarfs was born.
The physicists discussed the broad band of a 100,000°K source and plotted Brightness that varied much from light to X-rays.
To fit the plot to window they had to compress the Brightness scale a lot.
Then the plot did show the 1000 times variations, but sadly pictured 3 times variations at red-to-violet band as flat zero.
[quote="Chris Peterson" post_id=327689 time=1670594252 user_id=117706]
[quote=VictorBorun post_id=327677 time=1670551153 user_id=145500]
I can see 2 quotes here stating that coloured light (of red dwarfs or blue giants) gives way to white (of white dwarfs):
(1)
A 100,000 K star will photograph as white with RGB photography. Because it is white... nearly equal intensity across all visible wavelengths.
[/quote]
Indeed. Nearly flat across the [i]visible [/i]spectrum. Which is [i]precisely [/i]what we get from the Rayleigh–Jeans law. And a spectrum which varies in intensity by only a few percent from blue to red is essentially white to the eye and to an RGB camera.
[/quote]
the Rayleigh–Jeans law, B ~ ν²/T, gives a spectrum shape that is dependent not on T, but rather on ν = c/λ. The frequency does rise 2 times from red to violet making Brightness vary 4 times. Well, in fact 3 times, when we crop extreme margins, red and violet, that are poorly seen, but the result is readily seen as bluish. I mean "blue" is the colour that S-cones report as bright as white pixels in the scene, the L-cones report as dark as black pixels in the scene, and M-cones report as middle-bright.
And I mean "bluish" is the colour that the observer interpret as mixing tiny particles of white with tiny particles of blue. A white dwarf must look bluish for a comfortably set observer, because L-cones will register some yellow and red photons and report the thing as half-bright, just like they report a mixture of white and blue powders in equal portions.
the Rayleigh–Jeans law gives as bluish a colour as they come.
To get that colour from a thermal source you must heat it to 100,000°K or more.
If your source is mere 12,000°K, the spectrum will be flatter; you only get a pale bluish, like a mixture of much white powder and little blue powder.
Oh, now I get it! The reason why the myth of the Flat Thermal Spectrum of white dwarfs was born.
The physicists discussed the broad band of a 100,000°K source and plotted Brightness that varied much from light to X-rays.
To fit the plot to window they had to compress the Brightness scale a lot.
Then the plot did show the 1000 times variations, but sadly pictured 3 times variations at red-to-violet band as flat zero.