If I could somehow reliably count all the carbon, nitrogen, hydrogen, oxygen, and phosphorus atoms in the observable universe, what number would I come up with?

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    $\begingroup$ A big one, that's for sure. Phosphate is not an atom. Other than that, see en.wikipedia.org/wiki/… $\endgroup$ Jan 14, 2023 at 18:40
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    $\begingroup$ Nobody knows. Every number estimate will be a conjecture about the unknown worlds. Where does the universe start and where does it end? And who will define the entire universe? $\endgroup$
    – AChem
    Jan 14, 2023 at 20:36
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    $\begingroup$ @achem Astronomers. Whose job is partly observing how much stuff there is and what it is made of. $\endgroup$
    – matt_black
    Jan 14, 2023 at 20:38
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    $\begingroup$ @AChem It isn't about knowing everything, it is about using actual observations to make a reasonable estimate. Yhey don't, for example, need to know everything about the sun to estimate its mass and composition very reliably. $\endgroup$
    – matt_black
    Jan 14, 2023 at 22:55
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    $\begingroup$ I changed this pesky "entire" as it indeed pretty much disqualified the question. Answer tells about observable universe and that's what question should ask (also no "phosphate atoms" in body...) If a user wants to answer a question, that is having serious issues, it should be fixed quickly. $\endgroup$
    – Mithoron
    Jan 15, 2023 at 16:31

1 Answer 1


Two key numbers can be estimated from known observable things in astronomy (and cosmology where current big bang models explain the processes that created the "light" elements and their abundance and observations broadly agree with theory). Spectroscopy allow astronomers to estimate the abundance of many elements in stars and galaxies.

There are also some estimates of how many nuclei there are (based on estimates of the mass of the observable universe.

We can combine the estimates about the number of nuclei with their relative abundance to give some approximate counts for elements.

The number of nuclei is estimated to be between $\pu{10^78}$ and $\pu{10^82}$ (which are both crazy big numbers though the upper estimate is 10,000 times the lower estimate so don't expect precision).

But we also understand the relative abundance, though most estimates count mass proportion (counting nucleons not atoms). Using the numbers in Wikipedia and adjusting the estimates for the mass of each elemental nucleus gives the following proportions:

H 92%
He 7.5%
O 0.08%
C 0.05%
N 0.01%

If we take the mid estimate of total nuclei as $\pu{10^80}$ then there are approximately $\pu{8E76}$ oxygen atoms, $\pu{5E76}$ carbon atoms and $\pu{1E76}$ nitrogen atoms in the observable universe (if I've done my maths right).

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    $\begingroup$ I think the number just changed, so you have to update it. Like the joke of dinosaurs going extinct 65 million and 5 years ago. $\endgroup$
    – Karsten
    Jan 14, 2023 at 23:19
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    $\begingroup$ I gave the error bars of a factor of 10^4. Did it change by more than that? ;-) $\endgroup$
    – matt_black
    Jan 15, 2023 at 0:33
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    $\begingroup$ Mid? Pretty sure the halfway point between those two estimates is still something like $5 \times 10^{81}$ :) $\endgroup$
    – chepner
    Jan 15, 2023 at 17:27
  • $\begingroup$ @chepner I good point. I'm thinking logarithmically, as an astronomer probably would. $\endgroup$
    – matt_black
    Jan 15, 2023 at 17:54
  • $\begingroup$ And yet, apparently the average density is only one proton every couple cubic meters. $\endgroup$ Jan 16, 2023 at 1:11

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