Color index

Sample calibration colors^{[1] [failed verification]}

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

In

To measure the index, one observes the

. 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 as black bodies, using Ballesteros' formula^[7] (also implemented in the PyAstronomy package for Python):^[8]

${\displaystyle T=4600\,\mathrm {K} \left({\frac {1}{0.92\;(B{\text{-}}\!V)+1.7}}+{\frac {1}{0.92\;(B{\text{-}}\!V)+0.62}}\right).}$

Color indices of distant objects are usually affected by

, defined as the difference between the observed color index and the normal color index (or intrinsic 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:

${\displaystyle E_{{\text{B-}}\!{\text{V}}}={B{\text{-}}\!V}_{\text{observed}}-{B{\text{-}}\!V}_{\text{intrinsic}}.}$

The

is sometimes called the Johnson–Cousins filter system, named after the originators of the system (see references). These filters were specified as particular combinations of glass filters and photomultiplier tubes. M. S. Bessell specified a set of filter transmissions for a flat response detector, thus quantifying the calculation of the color indices.^[9] 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:

Color indices of Solar System bodies^[10][11]

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

Quantitative color index terms

Quantitative color index terms^[11]

Color	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	Aldebaran, Betelgeuse, Antares
Orange	0.80-1.40	K	G4-K3	G1-K2	Pollux, Arcturus
Yellow	0.60-0.80	G	G0-G3	F8-G0	Sun, Capella
"Green"[a]	0.30-0.60	F	F	F4-7	Polaris
White	0.00-0.30	A	A	A0-F3	Sirius, Canopus
Blue	-0.33-0.00	OB	OB	OB	Rigel, Bellatrix

See also

• Asteroid color indices
• Color–color diagram
• Distant object color indices
• UBV photometric system
• Zero point

Notes

References

Further reading

• Query for Johnson, H. L. and Morgan, ApJ 117, 313 (1953)
• Query for Cousins, A. W. J., MNRAS 166, 711 (1974)
• Query for Cousins, A. W. J., MNASSA 33, 149 (1974)
• Query for Bessell, M. S., PASP 102, 1181 (1990)