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The Question

(Submitted March 13, 1997)

A while ago, I saw a curve which showed that 550 nm was the best wavelength for visible light for our Sun. On this curve, there was also information given showing that other stars had a wavelength for optimal viewing but in a different region of the electromagnetic spectrum. I remember seeing stars like Betelgeuse and Rigel on this curve, but I'm unable to remember which parts of the electromagnetic spectrum they matched up with and I can't locate this curve anywhere. What I'd like is to know which stars had it's light most visible in the ultraviolet and infrared regions. I'd appreciate it if you could let me know today, but I definitely understand if it's too much to ask.

The Answer

I don't know of a reference to a graph like the one you describe, but it is straightforward to predict the wavelength at which the spectrum of a star of a given type will reach a maximum. The Wien displacement law for black bodies has:

lambda_max = 2.898 x 10^7/T

where lambda_max is in angstroms, and T is in K. For the Sun, if it were a black body with T= T_eff = 5770, this implies that lambda_max=5000 angstroms. The observed max of the solar spectrum is a little bit short of this, I think, at about 4500 angstroms. For other stars, the predictions are as follows:

Spectral TypeTemperaturelambda_max
B810,000 K 3000 A
G258005000
M235008300

(B8 is Rigel's spectral type, and M2 is Betelgeuse)

Actual stellar distributions are more complex, and computer simulations to predict. Looking at Silva & Cornell 1992, ApJS, 81, 865, it looks like the actual peaks are at 4000 (B8), 4500 (G2), and 9000 (M2).

Hope that this helps.

Tim Kallman and Steve Drake
for the Ask an Astrophysicist team.

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