 |
|
|
|
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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'
|
|
| |
| |
|
|
From: Christian Froeschlin
Subject: Re: Stellar colors: someone is wrong
Date: 10 Nov 2016 19:27:35
Message: <58251077@news.povray.org>
|
|
|
| |
| |
|
|
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.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
"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.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
"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
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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
|
|
| |
| |
|
|
|
|
| |
| |
|
|
"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.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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
Alpha 1 Cru -0.24 24000 20918 -3082 14633 -9367
Alpha Vir -0.23 22400 19845 -2555 14354 -8046
Alpha Eri -0.16 15000 14320 -680 12692 -2308
Alpha CMa 0.00 9940 8908 -1032 10125 185
Alpha PsA 0.09 8590 8421 -169 9125 535
Alpha Car 0.15 6998 8111 1113 8571 1573
Alpha 1 Cen 0.71 5790 5716 -74 5568 -222
Alpha 2 Cen 0.88 5260 5139 -121 5051 -209
Alpha Tau 1.54 3910 3402 -508 3734 -176
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.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
