<p><em>Extending Zephyrs comment as community wiki</em></p>

<p>See <a href="https://physics.stackexchange.com/questions/196366/number-density-of-cmb-photons">https://physics.stackexchange.com/questions/196366/number-density-of-cmb-photons</a> </p>

<p>In this response, the number of photons per cubic meter is estimated as $n_\lambda = 10^8$. Since the observable universe has a volume of the order $10^{80}\ m^3$, there are about $10^{88}$ photons in the observable universe.</p>

CC BY-SA 3.0
2017-01-07T23:23:10.463Z
2017-01-07T23:23:10.463Z
2017-04-13T12:40:15.377Z
James K
<p>I'll take number per average star, multiple by appropriate numbers , star/Galaxy, galaxies/universe, 14 billion years, and try to figure their location.</p>

<p>I was looking for a rough number and a distribution function.</p>

CC BY-SA 3.0
2017-01-04T19:27:47.793Z
By universe you mean observable universe, right?
called2voyage
CC BY-SA 3.0
2017-01-04T19:52:05.207Z
See [this related question](http://physics.stackexchange.com/questions/196366/number-density-of-cmb-photons). Note that a large fraction of the photons in our universe don't come from stars, but rather are a part of the Cosmic Microwave Background (CMB) radiation. That will make your calculation woefully short of the real number of photons in the universe.
zephyr
CC BY-SA 3.0
2017-01-05T11:13:39.017Z
Not only that, but single objects (e.g. active galactic nuclei) sometimes produce photon outputs larger than entire galaxies of stars!
probably_someone
CC BY-SA 3.0
2017-01-05T13:46:46.783Z
There really isn't any such number.  You might as well try to calculate the total mass or energy equivalent of the entire universe and pretend it's one giant black body, and calculate the spectral radiation.
Carl Witthoft
CC BY-SA 3.0
2017-01-06T22:29:43.283Z
Also, how will you take into account that the number of stars in a galaxy changes with time as stars are born from infalling gas?  And, what about dust grains and planets that reprocess optical photons into a different number of infrared photons?  And, what about various radio sources like neutron stars and black holes?  Radio waves can also be photons.  And supernovae and novae?  And energetic particles radiating in magnetic fields everywhere?   The universe is perhaps more interesting than you think!
eshaya
<galaxy><cosmology>
Number of photons in universe? I'll take number from a star in unit time and extrapolate
Tony
# Number of photons in universe? I'll take number from a star in unit time and extrapolate

(Asked by: username_0 on 2017-01-04)

I'll take number per average star, multiple by appropriate numbers , star/Galaxy, galaxies/universe, 14 billion years, and try to figure their location.

I was looking for a rough number and a distribution function.

**Tags:** galaxy, cosmology
> **Comments:**
> **username_1**: By universe you mean observable universe, right?  
> **username_2**: See [this related question](http://physics.stackexchange.com/questions/196366/number-density-of-cmb-photons). Note that a large fraction of the photons in our universe don't come from stars, but rather are a part of the Cosmic Microwave Background (CMB) radiation. That will make your calculation woefully short of the real number of photons in the universe.  
> **username_3**: Not only that, but single objects (e.g. active galactic nuclei) sometimes produce photon outputs larger than entire galaxies of stars!  
> **username_4**: There really isn't any such number.  You might as well try to calculate the total mass or energy equivalent of the entire universe and pretend it's one giant black body, and calculate the spectral radiation.  
> **username_5**: Also, how will you take into account that the number of stars in a galaxy changes with time as stars are born from infalling gas?  And, what about dust grains and planets that reprocess optical photons into a different number of infrared photons?  And, what about various radio sources like neutron stars and black holes?  Radio waves can also be photons.  And supernovae and novae?  And energetic particles radiating in magnetic fields everywhere?   The universe is perhaps more interesting than you think!  

---

## 1 answers

### Answer by username_6 on 2017-01-07. Score: 1

*Extending Zephyrs comment as community wiki*

See <https://physics.stackexchange.com/questions/196366/number-density-of-cmb-photons>

In this response, the number of photons per cubic meter is estimated as $n\_\lambda = 10^8$. Since the observable universe has a volume of the order $10^{80}\ m^3$, there are about $10^{88}$ photons in the observable universe.


---

