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The colors of stars are recorded as scalar magnitude differences, called
color indexes. Theoretically, the color of a star can be reconstructed
from a color index by correlating the value to a temperature, then
calculating the black body color.
The second step is easy, as it's already been done in a popular POV-Ray
suite. :-) But the first step requires astronomical research. A few
years ago, I found a curve fit by Cameron Reed of Alma College.
Yesterday, while trying to distract myself from the realization that
almost half my fellow citizens hate my guts, I chanced upon another
formula, by F. J. Ballesteros of Universitat de València.
I created a color map derived from each of the formulas. (The curves
show the sRGB compositions of the derived colors.) The resulting images
are quite different. It looks like I'll have to look up a few actual
stars and see which one of these curves matches the stellar data more
closely.
Post a reply to this message
Attachments:
Download 'blackbody_b-v1.png' (37 KB)
Download 'blackbody_b-v2.png' (35 KB)
Preview of image 'blackbody_b-v1.png'
Preview of image 'blackbody_b-v2.png'
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From: Christian Froeschlin
Subject: Re: Stellar colors: someone is wrong
Date: 10 Nov 2016 19:27:35
Message: <58251077@news.povray.org>
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On 10.11.2016 22:14, Cousin Ricky wrote:
> The colors of stars are recorded as scalar magnitude differences, called
> color indexes. Theoretically, the color of a star can be reconstructed
> from a color index by correlating the value to a temperature, then
> calculating the black body color.
There is probably no one true answer since color index is based
on broad-band filters with calibration determined empirically. Stars
are not quite perfect blackbodies and observed B-V is not intrinsic B-V
due to extinction effects, so if you calibrate the observed B-V against
temperature / class these effects are likely included in the mapping.
For precise temperature determination a spectrum is better.
But B-V can be determined easily for thousands of stars at once, even
faint ones a thus yields large datasets for statistical analysis.
> Yesterday, while trying to distract myself from the realization that
> almost half my fellow citizens hate my guts
Sorry about that. So crazy.
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Christian Froeschlin <chr### [at] chrfr de> wrote:
> For precise temperature determination a spectrum is better.
> But B-V can be determined easily for thousands of stars at once, even
> faint ones a thus yields large datasets for statistical analysis.
If I had a spectrum, I could go directly to color; I wouldn't need temperature
as a proxy.
However, aside from full spectral data being less available, I'm not sure it
would get me better results for ray tracing purposes, as it would disregard the
interstellar extinction effects.
A direct spectrum would be best if I were doing a portrait of a single star. It
would also get me accurate colors for carbon stars, which, you might remember
from an earlier post, turned out hot pink when I used a B-V-to-temperature
correlation.
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"Cousin Ricky" <rickysttATyahooDOTcom> wrote:
> However, aside from full spectral data being less available, I'm not sure it
> would get me better results for ray tracing purposes, as it would disregard the
> interstellar extinction effects.
On second thought, no it wouldn't.
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"Cousin Ricky" <rickysttATyahooDOTcom> wrote:
> Christian Froeschlin <chr### [at] chrfrde> wrote:
> > For precise temperature determination a spectrum is better.
> > But B-V can be determined easily for thousands of stars at once, even
> > faint ones a thus yields large datasets for statistical analysis.
>
> If I had a spectrum, I could go directly to color; I wouldn't need temperature
> as a proxy.
>
> However, aside from full spectral data being less available, I'm not sure it
> would get me better results for ray tracing purposes, as it would disregard the
> interstellar extinction effects.
>
> A direct spectrum would be best if I were doing a portrait of a single star. It
> would also get me accurate colors for carbon stars, which, you might remember
> from an earlier post, turned out hot pink when I used a B-V-to-temperature
> correlation.
Star spectrum...Do you mean the black-body radiation?
https://en.wikipedia.org/wiki/Planck%27s_law
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Am 10.11.2016 um 22:14 schrieb Cousin Ricky:
> I created a color map derived from each of the formulas. (The curves
> show the sRGB compositions of the derived colors.) The resulting images
> are quite different. It looks like I'll have to look up a few actual
> stars and see which one of these curves matches the stellar data more
> closely.
The kink near -0.02 in the Reed curves looks pretty suspicious; are you
sure you got the formula right?
Post a reply to this message
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"And" <49341109@ntnu.edu.tw> wrote:
> Star spectrum...Do you mean the black-body radiation?
> https://en.wikipedia.org/wiki/Planck%27s_law
Star spectra are similar to black body radiation, but they are not the same.
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clipka <ano### [at] anonymousorg> wrote:
> Am 10.11.2016 um 22:14 schrieb Cousin Ricky:
>
> > I created a color map derived from each of the formulas. (The curves
> > show the sRGB compositions of the derived colors.) The resulting images
> > are quite different. It looks like I'll have to look up a few actual
> > stars and see which one of these curves matches the stellar data more
> > closely.
>
> The kink near -0.02 in the Reed curves looks pretty suspicious; are you
> sure you got the formula right?
It's actually a two-part formula, and the kink is where the formula cuts over.
The Reed formula is an empirical curve fit. I don't know about the Ballesteros
formula.
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Cousin Ricky <ric### [at] yahoocom> wrote:
> It looks like I'll have to look up a few actual
> stars and see which one of these curves matches the stellar data more
> closely.
These are my results for the brightest stars, plus a few dimmer, but colorful
stars:
Surface Reed Ballesteros
Star B-V Temperature Pred. Diff. Pred. Diff.
---- --- ----------- ----- ----- ----- -----
Mu Col -0.28 33000 26417 -6583 15882 -17118
Upsilon Ori -0.26 33400 ± 200 23387 -10013 15229 -18171
Alpha 1 Cru -0.24 24000 20918 -3082 14633 -9367
Beta Cru -0.23 27000 ± 1000 19845 -7155 14354 -12646
Alpha Vir -0.23 22400 19845 -2555 14354 -8046
Beta Cen -0.23 25000 ± 2000 19845 -5155 14354 -10646
Alpha Eri -0.16 15000 14320 -680 12692 -2308
Alpha Leo -0.11 12460 ± 200 11712 -748 11744 -716
Beta Ori -0.03 12100 ± 150 9077 -3023 10516 -1584
Alpha CMa 0.00 9940 8908 -1032 10125 185
Alpha Lyr 0.00 9602 ± 180 8908 -694 10125 523
Alpha PsA 0.09 8590 8421 -169 9125 535
Alpha Cyg 0.09 8525 ± 75 8421 -104 9125 600
Alpha Car 0.15 6998 8111 1113 8571 1573
Alpha Aql 0.22 7700 ± 800 7764 64 8011 311
Alpha CMi 0.42 6530 ± 50 6851 321 6776 246
Alpha 1 Cen 0.71 5790 5716 -74 5568 -222
Alpha Aur 0.80 4970 ± 50 5403 433 5281 311
Alpha 2 Cen 0.88 5260 5139 -121 5051 -209
Beta Gem 1.00 4666 ± 95 4768 102 4743 77
Alpha Boo 1.23 4286 ± 30 4130 -156 4251 -35
Alpha Tau 1.54 3910 3402 -508 3734 -176
Alpha Sco 1.83 3400 ± 200 2838 -562 3356 -44
Alpha Ori 1.85 3590 2803 -787 3333 -257
Mu Cep 2.35 3750 2051 -1699 2845 -905
Reed's formula is a better match at bluer than -0.1, although it still isn't
very good. Both formulas are good up to about 1.0, and Ballesteros' is better
for the reddest stars.
The B-V values were copied directly from the Yale Bright Star Catalogue, 5th
edition; and the temperature data are from Wikipedia, which in turn got its data
from a variety of sources.
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Cousin Ricky <ric### [at] yahoocom> wrote:
> Yesterday, while trying to distract myself from the realization that
> almost half my fellow citizens hate my guts
You have piqued my curiosity! What country do you live in? Why do people who
don't know you hate you? What are you basing this statement on? I'm very
curious! When a population turns against the educated, intelligent members of
it's own society there's a big problem! Whoever you are, whatever country you're
in, I consider you my friend (for what that's worth).
Regards,
Dave Blandston
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On 11/19/2016 8:25 AM, Dave Blandston wrote:
> Cousin Ricky <ric### [at] yahoocom> wrote:
>> Yesterday, while trying to distract myself from the realization that
>> almost half my fellow citizens hate my guts
>
> You have piqued my curiosity! What country do you live in? Why do people who
> don't know you hate you? What are you basing this statement on? I'm very
> curious! When a population turns against the educated, intelligent members of
> it's own society there's a big problem! Whoever you are, whatever country you're
> in, I consider you my friend (for what that's worth).
>
> Regards,
> Dave Blandston
>
could be wrong but i'm guessing the US ... just after the election
waiting for the elevator at my local VA (veterans administration)
hospital this odd-ish fellow with an "i voted" sticker on his jacket
announced that he had voted for trump. i politely said "good for you"
... he persisted (followed me to my appointment) and when i wouldn't
engage him further he shouted ... "you voted for the her didn't you"
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On 11/19/2016 10:05 AM, Jim Holsenback wrote:
>> ... "you voted for the her didn't you"
oops ... scratch that "the"
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Jim Holsenback <spa### [at] nothanksnet> wrote:
> could be wrong but i'm guessing the US
Hey, what do you know? A veteran AND a POV-Ray aficionado! If you live near
Riverside, California let's get together for a beer or two! (I'm not a veteran,
but chances are that we'll get along very well. I work for the same corrupt
puppet masters.)
I was guessing the same thing based on the timing of the original post. The
break-down in civility and rule of law in America is very concerning.
Regards,
Dave Blandston
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On 11/19/2016 4:06 PM, Dave Blandston wrote:
> Jim Holsenback <spa### [at] nothanksnet> wrote:
>> could be wrong but i'm guessing the US
>
> Hey, what do you know? A veteran AND a POV-Ray aficionado! If you live near
> Riverside, California let's get together for a beer or two! (I'm not a veteran,
> but chances are that we'll get along very well. I work for the same corrupt
> puppet masters.)
>
Oh! No. Not another spook.
The place is hoaching with them. :)
> I was guessing the same thing based on the timing of the original post. The
> break-down in civility and rule of law in America is very concerning.
>
From the people I've met, he lives in a very nice part of the world. :)
--
Regards
Stephen
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