<p>There are several processes at work:</p>

<ul>
<li>Fission (of unstable elements) increases the number of atoms.</li>
<li>Fusion (occurs in stars and supernovae) decreases the number of atoms.</li>
<li>you could claim the formation of neutron stars decreases the number of atoms: it reduces atoms to just neutrons</li>
<li>you could claim the formation of black holes decreases the number of atoms: it moves atoms out of reach of the 'normal' universe. </li>
</ul>

<p>The next question is which of these processes produce/annihilate the largest number of atoms. Haven't found data for that yet. As Peter Erwin pointed out, any atom undergoing fission must have been built up from hydrogen and helium to start with. </p>

CC BY-SA 4.0
2019-11-18T21:27:25.903Z
Every atom undergoing fission was originally formed by multiple rounds of fusion. (E.g., an atom of U-235 started as a large number of hydrogen and helium atoms.) So that tells you that the reduction in number of atoms due to fusion has to be greater than any increase in the number due to fission.
Peter Erwin
CC BY-SA 4.0
2019-11-19T11:13:03.500Z
Fission is *very* rare. Even fissile isotopes like U-235 mostly undergo alpha decay, unless there's a critical mass so that the neutrons released by fission can induce further fission. OTOH, I suppose you can consider alpha decay as a kind of fission, since an alpha particle is a helium nucleus. But in terms of sheer numbers, fusion totally dominates over alpha decay & various forms of fission.
PM 2Ring
CC BY-SA 4.0
2019-11-19T19:48:13.730Z
You could add that every proton currently in existence started out as a Hydrogen ion,  depending on your point of view.
Carl Witthoft
CC BY-SA 4.0
2019-11-18T14:44:20.263Z
2019-11-19T09:51:18.453Z
2019-11-19T09:51:18.453Z
Hobbes
<p>One other option for destroying atoms is called photodisintegration. When massive stars near the end of their life, just before they blow themselves up in a supernovae, the energy of photons in the core is high enough to smash apart a heavy atom (like iron) into lighter atoms. </p>

CC BY-SA 4.0
2019-11-29T07:32:05.080Z
2019-11-29T07:32:05.080Z
Rob
<p>As per the title, are there only a set number of atoms in the universe? Or does the universe make more atoms, and if so, how does it work?</p>

<p>I'm a high school student that just had a question during science class, I didn't ask my teacher, since he has more knowledge in biology than anything.</p>

CC BY-SA 4.0
2019-11-18T09:29:05.520Z
I've suggested an edit for your question.  It's best to put the question in the actual body of the question, and to keep the title short.  (I also fixed some obvious spelling errors.)
Martin Bonner supports Monica
CC BY-SA 4.0
2019-11-18T10:06:08.547Z
It is very hard to guess what the teacher had in mind by asking this. Current models do not consider creation of atoms. The question is more sensible in such terms https://physics.stackexchange.com/questions/47941/dumbed-down-explanation-how-scientists-know-the-number-of-atoms-in-the-universe   look for all answers and comments.
Alchimista
CC BY-SA 4.0
2019-11-18T13:47:15.330Z
Are you asking about the whole universe, or just the observable universe (the part we can see with our eyes & telescopes)? The whole universe is quite likely to be infinite, and if so, it contains an infinite number of atoms.
PM 2Ring
CC BY-SA 4.0
2019-11-19T08:59:08.463Z
I see now that is your own question. Again, whilst Hobbes and related comments are relevant, once having them in mind is better you refer to the link I gave. Rather considering *matter* instead of atoms, as this is indeed cosmology. Also consider that universe, visible, observable, or as a whole, it's basically hydrogen and helium.
Alchimista
<big-bang-theory>
Are there only a set number of atoms in the whole universe?
HHM
# Are there only a set number of atoms in the whole universe?

(Asked by: username_0 on 2019-11-18)

As per the title, are there only a set number of atoms in the universe? Or does the universe make more atoms, and if so, how does it work?

I'm a high school student that just had a question during science class, I didn't ask my teacher, since he has more knowledge in biology than anything.

**Tags:** big-bang-theory
> **Comments:**
> **username_1**: I've suggested an edit for your question.  It's best to put the question in the actual body of the question, and to keep the title short.  (I also fixed some obvious spelling errors.)  
> **username_2**: It is very hard to guess what the teacher had in mind by asking this. Current models do not consider creation of atoms. The question is more sensible in such terms https://physics.stackexchange.com/questions/47941/dumbed-down-explanation-how-scientists-know-the-number-of-atoms-in-the-universe   look for all answers and comments.  
> **username_3**: Are you asking about the whole universe, or just the observable universe (the part we can see with our eyes & telescopes)? The whole universe is quite likely to be infinite, and if so, it contains an infinite number of atoms.  
> **username_2**: I see now that is your own question. Again, whilst Hobbes and related comments are relevant, once having them in mind is better you refer to the link I gave. Rather considering *matter* instead of atoms, as this is indeed cosmology. Also consider that universe, visible, observable, or as a whole, it's basically hydrogen and helium.  

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## 2 answers

### Answer by username_4 on 2019-11-18. Score: 9

There are several processes at work:

* Fission (of unstable elements) increases the number of atoms.
* Fusion (occurs in stars and supernovae) decreases the number of atoms.
* you could claim the formation of neutron stars decreases the number of atoms: it reduces atoms to just neutrons
* you could claim the formation of black holes decreases the number of atoms: it moves atoms out of reach of the 'normal' universe.

The next question is which of these processes produce/annihilate the largest number of atoms. Haven't found data for that yet. As Peter Erwin pointed out, any atom undergoing fission must have been built up from hydrogen and helium to start with.

> **Comments:**
> **username_5**: Every atom undergoing fission was originally formed by multiple rounds of fusion. (E.g., an atom of U-235 started as a large number of hydrogen and helium atoms.) So that tells you that the reduction in number of atoms due to fusion has to be greater than any increase in the number due to fission.  
> **username_3**: Fission is *very* rare. Even fissile isotopes like U-235 mostly undergo alpha decay, unless there's a critical mass so that the neutrons released by fission can induce further fission. OTOH, I suppose you can consider alpha decay as a kind of fission, since an alpha particle is a helium nucleus. But in terms of sheer numbers, fusion totally dominates over alpha decay & various forms of fission.  
> **username_6**: You could add that every proton currently in existence started out as a Hydrogen ion,  depending on your point of view.  

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### Answer by username_7 on 2019-11-29. Score: 2

One other option for destroying atoms is called photodisintegration. When massive stars near the end of their life, just before they blow themselves up in a supernovae, the energy of photons in the core is high enough to smash apart a heavy atom (like iron) into lighter atoms.


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