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**Physics, Astronomy & Cosmology / Re: Distant astronomy objects**

« **on:**04/08/2014 20:58:24 »

Do you know inverse square law?

f.e.

Energy/Area = Initial Energy / 4*PI*r^2

or

Power/Area = Initial Power / 4*PI*r^2

In the case of Sun, each 1 m^2 of Earth (including what is absorbed by atmosphere) is receiving 1360 W/m^2

Reverse equation:

1360 W/m^2*4*PI*150 mln km^2 = 3.8453*10^26 Watts (power of Sun)

(knowing it, you can apply inverse square law again with any other distance, and calculate f.e. energy that Jupiter, Saturn or anything further is receiving)

If we assume average photon has wavelength = 532 nm (green in middle of visible spectrum). Such photon has h*c/wavelength = 3.734*10^-19 J.

1360 W/m^2 is equivalent to 3.64*10^21 photons with energy 3.734*10^-19 J each per m^2 per second.

If you will do similar calcs for 1000 light years distant object that is Sun-alike (same power), you will get something like 915,000 photons per m^2. It's far far from 1 photon per m^2.

Is that 915,000 photons m

^{–2}s

^{–1}?