### Re: How many photons does a Sun sized star emit in a second?

Date: Wed Apr 23 11:39:45 2003
Posted By: Pauline Barmby, Astrophysicist
Area of science: Astronomy
ID: 1042328987.As
Message:

Dear Terry,
Thanks for an interesting question, and sorry it took us a while to answer it. I thought that there might be some measured value for the "photon luminosity" for the Sun, but I couldn't find one in a quick flip through my reference books. So here is a reasonable, order-of-magnitude type estimate.

The Sun's luminosity is 3.83e26 W (where "e" here always means "times 10 to the following power"). This luminosity comes out at a lot of different wavelengths, but we need to pick one to convert energy to nmuber of photons. So let's say that 40% of the Sun's energy is emitted at a wavelength of 550 nm (which is pretty reasonable). The energy of a 550 nm photon is E=hc/lambda=3.6e-19 J.

So the number of photons emitted at 550 nm is roughly: 3.82e26 J/s * 0.4 / (3.6e-19 J/photon)= 4.2e44 photons/s. Wow - what a huge number! I wouldn't have guessed it would be nearly so big. The total number of photons is larger than this, but to know the exact number you'd need to do a more complicated calculation summing over the Sun's entire spectrum.

While researching this answer I ran across the following interesting webpage: Extrasolar Planet Thought Experiment, where the author computes how long you would have to wait to see a photon from a planet orbiting another star. It's not as long as I would have guessed, which goes to show that intuition is not always the best guide when dealing with these big numbers.

Pauline

[This number is the number emitted from the surface of the Sun. The number of photons we receive from the Sun is smaller, and the number of photons that we receive from a Sun-like star at a typical interstellar distance is even vastly smaller. In fact, the number of photons that we receive every second from a distant Sun-like star is sufficiently small that it might be possible for an extraterrestrial civilization to "out-shine" their star for a brief period of time (much less than a second) using a powerful laser. (Assuming that there are any ET civilizations!) This possibility motivates many optical searches for extraterrestrial intelligence (OSETI) programs. Moderator]

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