Starlight energy falling on Earth

[Guest peepshow]

Doing some Googling I find that the total energy falling on Earth from the Sun is 174 Petawatts.

( 1 PW = 10 to the 15th watts).

 

I cannot find out what the total energy falling on Earth is from  all the stars.

 

Is it measured in mini watts, watts or in Kilowatts?  

Certainly, it's not in Petawatts.

Edited April 22, 2014 by peepshow

[Perkil8r]

The sun imparts way more than that, it's somewhere in the order of 1.3Kw per M^2

 

My mistake, I thought you were saying a petawatt was 1/15 of a watt

Edited April 22, 2014 by Perkil8r

[dawson]

A google search reveals this:

http://boards.straightdope.com/sdmb/showthread.php?t=616528

Jd

[Guest Tweedledum]

Had a dig and also put through wolfram alpha which came up with diddly squat. Did find introduction to stellar astrophysics but unable to cut & paste as was a protected page, url was :--- http://books.google.co.uk/books?id=JWrtilsCycQC&pg=PA49&lpg=PA49&dq=stellar+energy+reaching+earth&source=bl&ots=a5bO2CDGyO&sig=3cy47zgwVyDNoL8bzuWweDW71pA&hl=en&sa=X&ei=DKxWU56nEIra0QXTn4GYCA&ved=0CDYQ6AEwATge#v=onepage&q=stellar%20energy%20reaching%20earth&f=false

[Tweedledee]

I did a bit of googling and found a figure of 100 watts per square foot of energy falling on the earth from the sun, which multiplied by the earths surface area will be in the region of what Richard said, about 174 petawatts.

We could try to get a rough idea how much starlight hits the earth based on the difference in brightness between the sun and all the visible stars. The sun is about magnitude -26. Sirius, the brightest star is roughly magnitude -1.5, so we can be sure that the combined magnitude of all the starlight falling on earth is at least -1.5. The addition of the magnitudes of all the fainter stars will not increase the overall brightness to much above that figure.

 

So lets assume (very arbitrarily) that the total of all the stars amounts to the equivalent of just one magnitude -3 star.

 

Each magnitude differs from the next by the fifth root of 100 which is about 2.511.  The difference in magnitude between the starlight and the sun is 23 magnitudes. So the difference in brightness is 2.511^23 = about 1.6 billion.

 

So dividing 174 Petawatts by 1.6 billion gives 108,750,000 watts for the total visual energy of starlight hitting the earth. This equates to about 0.00000006 watts per square foot from starlight, a tiny and probably unmeasurable amount.

 

Very rough figures, but this may provide a rough ballpark figure for what you are looking for.
 

[Guest peepshow]

Thanks for that, Pete.  As is usual from you, a very scholarly approach. 

 

But 108.75 Megawatts hitting the Earth from starlight seems to be extremely high to me.

 

I would thought something in the region of a few hundred watts or so, but that's not based upon any

calculations.  Just a gut felling.

 

But I shall not be erecting any 'starlight' panels on my roof to generate electricity, that's for sure.

[Tweedledee]

Cheers Richard. Well, I gave it a shot

 

I don't know if that is anything like the correct method to work out the answer, but it looked logical to me. I attempted to do the type of calculation Brian Cox does so well whilst sitting on the sand in the middle of some exotic wilderness on his "Wonders of..." documentaries . I cheated though and used a calculator .

Personally, I don't thnk that the 108 megawatts is particularly high when you consider the collecting area on the whole surface of the earth. On that scale, 108 megawatts is a mere drop in the ocean. Breaking that down to the miniscule 0.00000006 watts per square foot, sounds like a reasonable figure to me.

I bet, if some of EMS's extremely talented, super intelligent, dark side members put their minds to it, they could extrapolate some far more accurate numbers for this problem by looking at the energy of photons falling on an imaging chip from some "average" star field

 

Maybe someone else can shed some light on this

 

I hope you find an answer to your query Richard, and let us know. I'm intrigued by this one. You pose some interesting questions
 

[Guest Tweedledum]

Pete,

 

There's plenty of info for the calculation of energy from individual stars, the benchmak is the sun!, once you move away the  you loose the correllation between radius which is used to calculate the radiation output. So if you take an average I bet you are not far out. Funnily enough there was a piece on the radio the other eve and there was a discussion on the energy from starlight and the rate of expansion also has a role to play!

[Tweedledee]

Here is an interesting fact I just found on the internet.

"The energy input from the sun in a single day could supply the needs for all of the Earth's inhabitants for a period of about 3 decades!"

"Obviously, there is no means conceivable (nor is it necessary) to harness all of the energy that is available; equally obvious is that capturing even a small fraction of the available energy in a useable form would be of enormous value."

[Tweedledee]

Cheers Damian.

 

The sun is a benchmark for a lot of stuff up there

 

If you have a link to that radio program, I would be interested to listen again.

[Guest Tweedledum]

From memory, mass loss from the sun is 4million tonnes / sec. Thats an awful lot of energy!

[Guest Tweedledum]

Hi Pete,

 

It was a Prof from Jodrell Bank,  after midnight on 4 I think!

[Guest peepshow]

I admit that I am out of my depth here, but if a star is twice as bright as another one,

does that mean that the energy reaching us is twice the amount too?

 

Is brightness perceived,  directly related to energy received?

[Tweedledee]

I admit that I am out of my depth here, but if a star is twice as bright as another one,

does that mean that the energy reaching us is twice the amount too?

 

Is brightness perceived,  directly related to energy received?

I think that is the case.

The question was about starlight (visible wavelengths). So comparing visual brightness and the energy received from that light should be proportional. It should be noted that a 2nd magnitude star is about 2.5 times fainter than a 1st magnitude one, and a 5 magnitude difference means a 100 times difference in brightness. This is all in accordance with the definition of the magnitude scale which is logarithmic.

There are many other wavelengths of the electromagnetic spectrum being received which contribute their own energies, but it seems we are just considering the visible light and its energy.

[Guest peepshow]

All this raises another interesting question then, the answer to which, I know not........

 

How much energy falls upon half the Earth from the moon, which is a reflector of the Sun's light ?

Edited April 25, 2014 by peepshow