What color are the stars?
- some pixel rgb values

Mitchell Charity <mcharity@lcs.mit.edu>
  > What color are the stars?
  >   > Star color - details
  >   > Star color - data comparison
  >   >   > Star color - data plots
  >   > Star color - blackbody approximation
  >   > What color is the Sun?
  > What color is a blackbody?

The idea: It would be nice to have at least a vague idea of what color stars are.
Here is some help on converting from stellar type/class (and color indexes) to pixel rgb values.

More formally - I have derived chromaticity and rgb pixel color, from spectra for various stellar types/classes, providing physically-motivated colors for astronomy presentations. Though the colors are still a bit rough.

The colors here are for the big balls of hot gas in space, rather than the pin-pricks of light in the local sky. I.e., atmospheric, interstellar medium, and relative motion effects are discarded...

The OBAFGKM table...

 Sp5(V)  r   g   b     rgb
  O     155 176 255  #9bb0ff  
  B     170 191 255  #aabfff  
  A     202 215 255  #cad7ff  
  F     248 247 255  #f8f7ff  
  G     255 244 234  #fff4ea  
  K     255 210 161  #ffd2a1  
  M     255 204 111  #ffcc6f  

Some familiar stars...

Sun viewed outside of atmosphere (ref) #fff3ea
Achernar (B3V)
#a5b9ff
Acrux (B0IV)
#9cb2ff (as B0V)
Adhara (B2II)
#a5c0ff
Aldebaran (K5III)
#ffcc8a
Alnilam (B0I)
#a1bdff
Alpha Centari A (G2V)
#fff5f2
Alpha Centari B (K1V)
#ffe0bc
Altair (A7V)
#c8d5ff
Antares (M1I)
#ffca8a
Arcturus (K1III)
#ffdfb5
Bellatrix (B2III)
#9fb4ff
Beta Centari
Hadar
(B1III)
#9eb1ff
Beta Crucis (B0III)
#9eb1ff
Betelgeuse (M1I)
#ffca8a
Canopus (F0II)
#cbd9ff
Capella (G5III)
#ffecd3
Deneb (A2I)
#c7d6ff
Elnath (B7III)
#abbfff
Fomalhaut (A3V)
#bfcfff
Gacrux (M3III)
#ffc877
Miaplacidus (A2IV)
#bbcbff (as A2V)
Pollux (K0III)
#ffe3be
Procyon (F5IV)
#f1efff
Regulus (B7V)
#adbfff
Rigel (B8I)
#b6ceff
Shaula (B2IV)
#9fb3ff
Sirius (A1V)
#b5c7ff
Spica (B1III)
#9eb1ff
Sun (G2V)
#fff5f2
Vega (A0V)
#b9c9ff
[Type/class data from Bright Star Catalog bsc5p.]

Some nearby stars...

Sun (G2V)
#fff5f2
Alpha Centauri A (G2V)
#fff5f2
Alpha Centauri B (K0V)
#ffeedd
Proxima Centauri (M5.5V)
#ffcc6f (as M5V)
Barnard's Star (M4.0V)
#ffc97f
Wolf 359 (M6.0V)
#ffc370
Lalande 21185 (M2.0V)
#ffc483
Sirius (A1V)
#b5c7ff
Sirius B (DA2)
#a8bdff (as D?2)
UV Ceti A (M5.5V)
#ffcc6f (as M5V)
UV Ceti B (M6.0V)
#ffc370
Ross 154 (M3.5V)
#ffce81 (as M3V)
Ross 248 (M5.5V)
#ffcc6f (as M5V)
epsilon Eridani (K2V)
#ffe3c4
Lacaille 9352 (M1.5V)
#ffcc8e (as M1V)
Ross 128 (M4.0V)
#ffc97f
EZ Aquarii (M5.0V)
#ffcc6f
Procyon (F5IV-V)
#f1efff (as F5IV)
61 Cygni (K5.0V)
#ffd2a1
61 Cygni B (?) (K7.0V)
#ffc78e
Gl 725 A (M3.0V)
#ffce81
Gl 725 B (M3.5V)
#ffce81 (as M3V)
Gl 15 A (M1.5V)
#ffcc8e (as M1V)
Gl 15 B (M3.5V)
#ffce81 (as M3V)
epsilon Indi (K5Ve)
#ffd2a1 (as K5V)
DX Cancri (M6.5V)
#ffc370 (as M6V)
tau Ceti (G8Vp)
#ffedde (as G8V)
RECONS 1 (M5.5V)
#ffcc6f (as M5V)
YZ Ceti (M4.5V)
#ffc97f (as M4V)
Luyten's Star (M3.5V)
#ffce81 (as M3V)
Kapteyn's Star (M1.5V)
#ffcc8e (as M1V)
[Type/class data from THE 100 NEAREST STAR SYSTEMS. Here is a hack colorized version. ]
Yes, to a zeroth approximation, all stars are type M :).
Hmm... so is Alpha Cent B a K0V or a K1V??

Stellar types

  O5(V)     157 180 255   #9db4ff
  B1(V)     162 185 255   #a2b9ff
  B3(V)     167 188 255   #a7bcff
  B5(V)     170 191 255   #aabfff
  B8(V)     175 195 255   #afc3ff
  A1(V)     186 204 255   #baccff
  A3(V)     192 209 255   #c0d1ff
  A5(V)     202 216 255   #cad8ff
  F0(V)     228 232 255   #e4e8ff
  F2(V)     237 238 255   #edeeff
  F5(V)     251 248 255   #fbf8ff
  F8(V)     255 249 249   #fff9f9
  G2(V)     255 245 236   #fff5ec
  G5(V)     255 244 232   #fff4e8
  G8(V)     255 241 223   #fff1df
  K0(V)     255 235 209   #ffebd1
  K4(V)     255 215 174   #ffd7ae
  K7(V)     255 198 144   #ffc690
  M2(V)     255 190 127   #ffbe7f
  M4(V)     255 187 123   #ffbb7b
  M6(V)     255 187 123   #ffbb7b

See Star color - details for greater detail.
These are not "magic" pixel values. Different approaches yield different values, but ones with quite similar appearance.

Other

Planetary Nebula     0.1378 0.3817     000 255 236 #00ffec Kurucz

Galaxy (spiral)     0.3523 0.3556     255 225 199 #ffe1c7 Kurucz
Galaxy (elliptical)     0.3617 0.3649     255 222 186 #ffdeba Kurucz
 
Quasar     0.2188 0.2764     073 214 255 #49d6ff Kurucz

Universe                 0.3450 0.3450     255 225 209 #ffe1d1 Glazebrook & Baldry

Notes

So, what should be born in mind looking at these colors?
These colors attempt to show "true" color. That is: without intervening atmosphere or interstellar medium; with the light dim enough to avoid saturating your cones (which would make the light appear white); and yet with the light bright enough that you are seeing color rather than rod grayscale.

So... looking at the star from space while wearing gray sunglasses (unpolarized:). Hmm... or better, through a gray window shade in a D65 white wall.

In applying these colors, remember that color perception is quite messy, so what you see is often not what you've got.

This page is concerned only with chromaticity (hue and saturation), but not brightness. The luminous power per unit area of star classes varies greatly. If we were doing brightness, and the G class star color were as above, then the M class star color would simply be black. See Blackbody colors - intensity.

The choice of white point, used in converting from abstract chromaticity to device-dependent rgb, has a large effect. Eg, see Sun. These colors use D65. I plan to add a set for D50.

Are you sure these colors are right? They seem odd.
No, I am not sure. And yes, they do. I was expecting rather different colors. As are many other folk, apparently, from my reading. Nevertheless...

Links

Color in Astronomical Photos
What colour is the Sun? and Where's purple
Sun Color
Stellar Classification Table for spaceflight simulation.
Solar Spectrum, Variability, and Atmospheric Absorption Graph of bb 5770 K vs unfiltered solar spectrum.

Thanks
Sigh. I just wanted some not-completely-bogus pixel values to color in the disks of a comparison of star sizes (How Big Are Things?). Now, n hours later... My thanks to John Walker's Colour Rendering of Spectra (specrend) and Dan Bruton's Color Science, who's pages got me started. To the CVRL Color & Vision database. To Andrew Hamilton for What colour is the Sun?, which suggested I was not completely confused. And to efg for his Color Reference Library. Without them, this page would not have happened. My thanks also to the stellar spectra sites.


Comments encouraged - Mitchell Charity <mcharity@lcs.mit.edu>

Notes:

Doables:
  Add D50 colors.
  Talk about D65 and D50 whitepoints.
  Get more domain folks to validate.
  Do a nicer stellar type/class color intro table.  
  Include "astronomers' pale pastel palette" and "colorful exaggeration" quotes.
  Add Hubble starcolor pretty picture.
  Individual spectra for near stars.
  Name site "stellar chromaticity"?

History:
  2002-Dec-30  Added link to spacegear.  Thanks to a reader.
  2002-Dec-14  Switched to a better Bright Star dataset.  Capella was wrong.  Thanks to a reader.
  2002-Nov-11  Fixed links broken by CVRL move.
  2002-Mar-14  Added universe color.  They got burned by specrend too.  Sigh.
  2001-Jun-23  Added near stars, white dwarfs.
  2001-Jun-22  Switched to sRGB from Rec.709 (similar matrix, different gamma).
  2001-Jun-15  New dataset combination approach.  Page of data plots.
  2001-Jun-08  Added GunnStryker spectra.
  2001-Jun-07  Added Kurucz's galaxies, quasar; Pickles spectra.
  2001-Jun-04  on-line