| Class | B−V | U−B | V−R | R−I | Teff (K) |
|---|---|---|---|---|---|
| O5V | −0.33 | −1.19 | −0.15 | −0.32 | 42,000 |
| B0V | −0.30 | −1.08 | −0.13 | −0.29 | 30,000 |
| A0V | −0.02 | −0.02 | 0.02 | −0.02 | 9,790 |
| F0V | 0.30 | 0.03 | 0.30 | 0.17 | 7,300 |
| G0V | 0.58 | 0.06 | 0.50 | 0.31 | 5,940 |
| K0V | 0.81 | 0.45 | 0.64 | 0.42 | 5,150 |
| M0V | 1.40 | 1.22 | 1.28 | 0.91 | 3,840 |
Inastronomy, thecolor index is a simplenumericalexpression that determines thecolor of an object, which in the case of astar gives itstemperature. The lower the color index, the moreblue (or hotter) the object is. Conversely, the larger the color index, the morered (or cooler) the object is. This is a consequence of the inverselogarithmic magnitude scale, in which brighter objects have smaller (more negative) magnitudes than dimmer ones. For comparison, the whitishSun has a B−V index of0.656 ± 0.005,[2] whereas the bluishRigel has a B−V of −0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B−V = −0.03).[3] Traditionally, the color index usesVega as azero point. Theblue supergiantTheta Muscae has one of the lowest B−V indices at −0.41,[4] while thered giant andcarbon starR Leporis has one of the largest, at +5.74.[5]
To measure the index, one observes themagnitude of an object successively through two differentfilters, such as U and B, or B and V, where U is sensitive toultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also:UBV system). The set of passbands or filters is called aphotometric system. The difference in magnitudes found with these filters is called the U−B or B−V color index respectively.
In principle, the temperature of a star can be calculated directly from the B−V index, and there are several formulae to make this connection.[6] A good approximation can be obtained by considering stars asblack bodies, using Ballesteros' formula[7] (also implemented in the PyAstronomy package for Python):[8]
Color indices of distant objects are usually affected byinterstellar extinction, that is, they areredder than those of closer stars. The amount of reddening is characterized bycolor excess, defined as the difference between theobserved color index and thenormal color index (orintrinsic color index), the hypothetical true color index of the star, unaffected by extinction.For example, in the UBV photometric system we can write it for the B−V color:
Thepassbands most opticalastronomers use are theUBVRI filters, where the U, B, and V filters are as mentioned above, the R filter passes red light, and the I filter passesinfrared light. Thissystem of filters is sometimes called the Johnson–Kron–Cousins filter system, named after the originators of the system (see references).[9] These filters were specified as particular combinations of glass filters andphotomultiplier tubes.M. S. Bessell specified a set of filter transmissions for a flat response detector, thus quantifying the calculation of the color indices.[10] For precision, appropriate pairs of filters are chosen depending on the object's color temperature: B−V are for mid-range objects, U−V for hotter objects, and R−I for cool ones.
Color indices can also be determined for other celestial bodies, such as planets and moons:
| Celestial body | B-V color index | U-B color index |
|---|---|---|
| Mercury | 0.97 | 0.40 |
| Venus | 0.81 | 0.50 |
| Earth | 0.20 | 0.0 |
| Moon | 0.92 | 0.46 |
| Mars | 1.43 | 0.63 |
| Jupiter | 0.87 | 0.48 |
| Saturn | 1.09 | 0.58 |
| Uranus | 0.56 | 0.28 |
| Neptune | 0.41 | 0.21 |
| Color (Vega reference) | Color index (B-V) | Spectral class (main sequence) | Spectral class (giant stars) | Spectral class (supergiant stars) | Examples |
|---|---|---|---|---|---|
| Red | ≥1.40 | M | K4-M9 | K3-M9 | Betelgeuse,Antares |
| Orange | 0.80-1.40 | K | G4-K3 | G1-K2 | Arcturus,Pollux |
| Yellow | 0.60-0.80 | G | G0-G3 | F8-G0 | Sun,Rigil Kent |
| Green | 0.30-0.60 | F | F | F4-7 | Procyon |
| White | 0.00-0.30 | A | A | A0-F3 | Sirius,Vega |
| Blue | -0.33-0.00 | OB | OB | OB | Spica,Rigel |
The common color labels (e.g. red supergiant) are subjective and taken using the star Vega as the reference. However, these labels, which have a quantifiable basis, do not reflect how the human eye would perceive the colors of these stars. For instance, Vega has a bluish white color, while the Sun, from outer space, would look like a neutral white somewhat warmer than theilluminant D65 (which may be considered a slightly cool white). "Green" stars would be perceived as white by the human eye.[12]
