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# Density of light from stars

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This is something I just can't get my head around.

For example, after night has fallen I can see Vega. It is 25 ly away, that is 2.365 e+17 meters away. Now, if I move my eyes a centimetre (or whatever small distance ) I can still see Vega. I am guessing too that there are a lot of photons from it falling into my eyes sine I can see it. I also assume that would be the case anywhere my distance from Vega. A quick sphere calculation  with Vega in the center and my distance from it as a radius gives a surface area of 7 e+35 square meters. Now that's a lot of photons in one instance. And then integrate over the the time Vega has been into existence. A quick Google of the number of atoms in the observable universe is in the ballpark of 10 + e 80. Vega is is one (bright) star, there are a few more around.... It bewilders me.

Edited by Viktiste

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Indeed - number of photons is much larger than that!

Vega is 0 mag star and this means that flux at the top of our atmosphere (to exclude any atmospheric extinction) is about 880000 photons per second per cm squared. This is only in visible light (300-700nm). Vega shines in much broader spectrum.

If you take all of this and calculate number of photons for past N years of Vega's existence - you'll get massive number.

Nothing strange with that. Number of photons is not well defined value and you should not think of them as particles with mass - that can be counted regularly. Best to think of them as packets of energy. When you do another calculation - count number of photons at particular wavelength, convert into energy, integrate over wavelengths and then see how much matter you need to convert via E=mc2 - you'll get only a fraction of mass of a star.

Another interesting calculation can be following:

How much do our Sun and Vega repel each other due to their shine? Photons carry momentum and when photons from one star hit another - they create pressure - trying to push it away (solar sail principle). Can you calculate magnitude of this pressure / force exerted by two stars at a distance?

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The Sun converts approx 10^17kg of matter into energy every year. A small fraction of its mass but a lot of individual nuclear reactions.

Another fact to ponder is there are about 10^10 photons for each baryon (protons, neutrons mainly) in the observable universe!

Regards Andrew

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1 hour ago, andrew s said:

Another fact to ponder is there are about 10^10 photons for each baryon (protons, neutrons mainly) in the observable universe!

Regards Andrew

That is impressive, but I can already hear the wise crack from the back of the class "can you name them ?"

Jim

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20 minutes ago, saac said:

That is impressive, but I can already hear the wise crack from the back of the class "can you name them ?"

Jim

Yes most are called CMB

It's also why the matter excess (over anti matter) is only 1 part in 10^10 the balance annihilating into the photons of the CMB.

Regards Andrew

PS I was that child.

Edited by andrew s
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2 minutes ago, andrew s said:

Yes most are called CMB

It's also why the matter excess (over anti matter) is only 1 part in 10^10 the balance annihilating into the photons of the CMB.

Regards Andrew

PS I was that child.

Oh good answer Andrew I'm making a note of that one for future use.      Lucie Green's book  "15 Million Degrees A Journey To The Centre Of The Sun"  is full of fascinating facts and explanations of the amazing processes that go on in the Sun.    In one section she describes the amount of energy liberated in one proton-proton chain reaction as 40000 billionths of a joule.  The implication she is making being clear, that it is the sheer number  of these reactions that take place every second that power the sun - pointing to the unimaginable conversion of mass per second.  She goes on  to describe that "if you could scoop up a coffee cup amount of the material at the core - they would release just 4 thousands of a joule  - equivalent to eating 100 grams of corn chips over 16 years ! ".  Gram for gram we create more energy from our food than the sun does from its nuclear reactions !   It is a very readable book and would highly recommend it to anybody interested in stellar physics without requiring a phd to get past the foreword

Jim

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I really need to start reading again, I used to love learning (and sharing) these kinds of unexpected facts.

As a boy I enjoyed reading Asimov's Guide to Science and Children of The Universe (H. von Ditfurth). They introduced me to just how wonderful science is and how it can be described in layman's terms.

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57 minutes ago, Paul M said:

I really need to start reading again, I used to love learning (and sharing) these kinds of unexpected facts.

As a boy I enjoyed reading Asimov's Guide to Science and Children of The Universe (H. von Ditfurth). They introduced me to just how wonderful science is and how it can be described in layman's terms.

You're not alone Paul - I don't read enough either - life just seems to get in the way.  It's one of the things that I'm looking forward to in retirement; I have a stack of books to get through.

Jim

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7 hours ago, andrew s said:

Another fact to ponder is there are about 10^10 photons for each baryon (protons, neutrons mainly) in the observable universe!

So how did these photons come into existence?  10^10 electron jumps for each atom around? I guess my physics learning from the 90's need some refreshing/updating

5 hours ago, saac said:

Lucie Green's book  "15 Million Degrees A Journey To The Centre Of The Sun"

Looks like interesting reading. On my Christmas list now, thanks.

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5 hours ago, Viktiste said:

So how did these photons come into existence?  10^10 electron jumps for each atom around? I guess my physics learning from the 90's need some refreshing/updating

They come from matter antimatter annihilation producing gamma rays that are now the CMB due to expansion of the Universe

Regards  Andrew

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