What is the total energy in joules received on Earth per year from all the Galaxies in the Universe?

Answer 1

There are a couple ways to figure this out. Perhaps the most straightforward is using the stellar magnitude of objects.

To do this, I will compare everything to the only stellar source that I can figure to received energy for, that's the Sun. The Sun gives us about 1300 watts/m^2 which comes out to 5x10^24 joule/year.

The Sun has a magnitude of -26.0. Magnitude is scaled so that an object 5 magnitudes lower is 100 times brighter. The fraction of energy received from a body of magnitude m compared to the Sun is: 100^((-26-m)/5).

Using the formula for magnitude, you can figure the amoutn of energy received from any galaxy by scaling it to the Sun. The brigtest galaxy (assuming of course you are excluding the milky way and companions in all this) is the Andromeda galaxy at magnitude 3.5. The energy from Andromeda in one year is:

5x10^24 joule/year * 100^((-26-3.5)/5) = 8 x 10^12 joules per year.

There are about 100 other galaxies around magnitude 9 or so for another:

5x10^24 joule/year * (100^((-26-9)/5))*100 = 5 x 10^12 joules per year.

And probably another 400 around magnitude 12:

5x10^24 joule/year * (100^((-26-12)/5))*400 = 1 x 10^12 joules per year.

There are more and more as you go farther out but they contribute less and less total energy. Adding these together, a reasonable estimate is maybe 2 x 10^13 joules per year. This might sound like a lot but this is about the same amount of energy we recieve from the planet Venus or Jupiter and less than 1% of the energy we get from the Moon.

Answer 2

There's one particular star in the Milky Way galaxy that provides the earth with 4 * 10^18 Joules per year... are you ruling that one out?

Obviously, the energy input of all other stars & galaxies is vanishingly small in comparison, so the total energy is right around 4 * 10^18 Joules per year.

Answer 3

I know that starlight flux is about 6E-6 cd m^-2. So the total energy from starlight, for the whole Earth per year is about 1E15 J (plus or minus a factor of 5 or so).

Now, how much of the starlight flux is from galaxies? I would have to guess here -- about 0.1%? 0.001%? Lets say 0.01% for now.

Then the total energy would be about 1E11 J per year, or about 3000 Watts. Or maybe it only 300 Watts. Of course, that's not counting the cosmic background radiation ...