Part 5: Colors and temperatures of stars#

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B-V Color Index#

Using filters#

One way to measure the color of a star is by viewing the star through lens filters.

Here is an example of magnitudes through some lens filters:

Rigel fluxes:
U = -0.56
B = 0.10
V = 0.13
R = 0.13
I = 0.15
J = 0.22
H = 0.20
K = 0.18

U is ultraviolet; I is infrared; J, H, and K are near infrared wavelengths; B is blue; V is visual (green); and R is red. The lower the number the brighter the light through the filter. Therefore, the brightest light for Rigel is in the blue part of the spectrum.

The only visual light filters here are B, V, and R. We can simply look up B-V in a color index table to determine the color of the star.

\(B-V\)
\(0.10-0.13=-0.03\)

Notice that -0.03 is between the star classes A0V and B0V, but much closer to A0V. Rigel is a blue star.

This is called the Johnson magnitude system, but there are many others.