photons per stellar magnitude

[John Cincotta]

*** REPLACE THIS LINE WITH YOUR MESSAGE ***
I have a question for the experts

How many photons per second per square meter are recieved from a star of
say 10th magnitude on the surface of the earth??

Name: John Cincotta
Mail: Saber Technology, 2381 Bering Drive, San Jose, California 95131
AT&T: 945-9600
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[William L. Sebok]

In article <18...@saber.UUCP> j...@saber.UUCP (John Cincotta) writes:
>How many photons per second per square meter are received from a star of

>say 10th magnitude on the surface of the earth??

From Allen's "Astrophysical Quantities" (Chapter 10):

log f (V) = -0.4m - 8.43
lambda V

-2 -1 -1
where f (V) is visual flux in erg cm Angstrom sec outside the Earth's
lambda

atmosphere near 5500 Angstroms. "This relation is almost unchanged from B to
M stars."

Therefore (plugging in numbers), for m = 10
V
-13 -2 -1 -1
f = 3.72 x 10 erg cm sec Angstrom
lambda

Now a photon at a wavelength of 5500 Angstrom has an energy of

E = h(c/lambda)

-27 18
= (6.626 x 10 erg-sec) (2.998 x 10 Angstrom/sec) / (5500 Angstrom)

-12
= 3.612 x 10 ergs

this yields
-2 -1
Flux(5500 Angstrom) = 0.11 photons cm Angstrom

-2 -1
= 110 photons meter Angstrom

For a 1000 Angstrom bandpass (roughly since I am too lazy now to do the
integral and the whole business is approximate anyway) this is about
110,000 photons / square meter for that 10th magnitude star.
--
Bill Sebok Princeton University, Astrophysics
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