
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 Type  Temperature  lambda_max 
B8  10,000
K  3000 A 
G2  5800  5000 
M2  3500  8300 
(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.
