<p>Any gas that has a lower atomic mass than iron. The only condition on the gas is that nuclear fusion with it should be exothermic (the energy required to fuse the nuclei should be less than the energy obtained). </p>

<p>However, usually a majority of hydrogen is the best. If the majority gas is of a higher element, we'll need a sufficient gravitational force to initiate the fusion, and that's not always easy.</p>

<p>While it is possible for <a href="http://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction" rel="noreferrer">just hydrogen to start fusion in a star</a>, some carbon or nitrogen is a good thing to have to <a href="http://en.wikipedia.org/wiki/CNO_cycle" rel="noreferrer">start fusion more efficiently</a>.</p>

<p>Different initial conditions lead to different kinds of stars.</p>

CC BY-SA 3.0
2013-09-27T12:41:21.893Z
2013-09-27T12:41:21.893Z
Manishearth
<p>It depends on the kind of star you want to make.</p>

<p>Basically, what you really need is hydrodgen; as soon as enough hydrodgen is gathered somewhere and collapse, you end up with a star.</p>

<p><strong>But</strong>, if you just have hydrodgen, you will get a very massive star, and with a history different than the stars observed in the local Universe. Why so?</p>

<ul>
<li><a href="http://www.cfa.harvard.edu/swas/science1.html">hydrodgen is a poor coolant</a>, therefore the <a href="http://en.wikipedia.org/wiki/Jeans_instability#Jeans_mass">Jeans' mass</a> (the minimum mass for a density structure to gravitationally collapse), which strongly depends on temperature, will be much higher if the temperature is higher. In practice, it means that the formed stars will be much more massive.</li>
<li>you need at least a small fraction of carbon to launch the CNO cycle in stellar cores (CNO cycle is one of the two ways to burn hydrodgen and to turn it into helium); stellar cores are then hotter and denser, and stars become hotter and more luminous.</li>
</ul>

<p>So if you want to make a star as we typically observe in our galaxy, you will need also some molecules (hopefully <a href="https://en.wikipedia.org/wiki/List_of_interstellar_and_circumstellar_molecules">there are plenty of them</a>) and also some carbon. Else, you'll get big, massive, hot and luminous stars. Like the first generation of stars (called Population III stars, you can see <a href="http://www.google.fr/url?sa=t&amp;rct=j&amp;q=&amp;esrc=s&amp;source=web&amp;cd=1&amp;ved=0CDAQFjAA&amp;url=http%3A%2F%2Fwww.astro.psu.edu%2Fusers%2Frbc%2Fa534%2Fredman.pdf&amp;ei=S0ZFUq3BGamH0AWAhYCQBA&amp;usg=AFQjCNHgmWxmdBhD8QOM4VJYuLrWUhr66A&amp;sig2=tsibya0AEYsbbx4iKJClDw&amp;bvm=bv.53217764,d.bGE">here</a> if you want more dirty details) in the early Universe.</p>

CC BY-SA 3.0
2017-07-30T18:40:34.197Z
[Ethan Siegel](https://medium.com/starts-with-a-bang/this-one-imperfection-in-nuclear-physics-allowed-earth-to-exist-195ecc0da9ca) points out that the hydrogen must include a small amount of deuterium.
Keith McClary
CC BY-SA 3.0
2013-09-27T12:18:38.793Z
2013-09-27T12:26:31.847Z
2013-09-27T12:26:31.847Z
MBR
<p>What gases do you need to start the creation of a star, and why do you need these gases? What are their functions?</p>

CC BY-SA 3.0
2013-10-23T15:23:22.513Z
You don't really need 'gases' per say - you just need matter, specifically atoms with low nuclear weights. If somehow a huge amount of frozen water (i.e. hydrogen and oxygen) got together, you'd also get a star.
john3103
<star>
What gases are needed to make a star?
Timtech
# What gases are needed to make a star?

(Asked by: username_0 on 2013-09-26)

What gases do you need to start the creation of a star, and why do you need these gases? What are their functions?

**Tags:** star

> **Comments:**
> **username_1**: You don't really need 'gases' per say - you just need matter, specifically atoms with low nuclear weights. If somehow a huge amount of frozen water (i.e. hydrogen and oxygen) got together, you'd also get a star.  

---

## 2 answers

### Answer by username_3 on 2013-09-27. Score: 10 | Accepted answer

It depends on the kind of star you want to make.

Basically, what you really need is hydrodgen; as soon as enough hydrodgen is gathered somewhere and collapse, you end up with a star.

**But**, if you just have hydrodgen, you will get a very massive star, and with a history different than the stars observed in the local Universe. Why so?

* [hydrodgen is a poor coolant](http://www.cfa.harvard.edu/swas/science1.html), therefore the [Jeans' mass](http://en.wikipedia.org/wiki/Jeans_instability#Jeans_mass) (the minimum mass for a density structure to gravitationally collapse), which strongly depends on temperature, will be much higher if the temperature is higher. In practice, it means that the formed stars will be much more massive.
* you need at least a small fraction of carbon to launch the CNO cycle in stellar cores (CNO cycle is one of the two ways to burn hydrodgen and to turn it into helium); stellar cores are then hotter and denser, and stars become hotter and more luminous.

So if you want to make a star as we typically observe in our galaxy, you will need also some molecules (hopefully [there are plenty of them](https://en.wikipedia.org/wiki/List_of_interstellar_and_circumstellar_molecules)) and also some carbon. Else, you'll get big, massive, hot and luminous stars. Like the first generation of stars (called Population III stars, you can see [here](http://www.google.fr/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDAQFjAA&url=http%3A%2F%2Fwww.astro.psu.edu%2Fusers%2Frbc%2Fa534%2Fredman.pdf&ei=S0ZFUq3BGamH0AWAhYCQBA&usg=AFQjCNHgmWxmdBhD8QOM4VJYuLrWUhr66A&sig2=tsibya0AEYsbbx4iKJClDw&bvm=bv.53217764,d.bGE) if you want more dirty details) in the early Universe.

> **Comments:**
> **username_4**: [Ethan Siegel](https://medium.com/starts-with-a-bang/this-one-imperfection-in-nuclear-physics-allowed-earth-to-exist-195ecc0da9ca) points out that the hydrogen must include a small amount of deuterium.  

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### Answer by username_2 on 2013-09-27. Score: 6

Any gas that has a lower atomic mass than iron. The only condition on the gas is that nuclear fusion with it should be exothermic (the energy required to fuse the nuclei should be less than the energy obtained).

However, usually a majority of hydrogen is the best. If the majority gas is of a higher element, we'll need a sufficient gravitational force to initiate the fusion, and that's not always easy.

While it is possible for [just hydrogen to start fusion in a star](http://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction), some carbon or nitrogen is a good thing to have to [start fusion more efficiently](http://en.wikipedia.org/wiki/CNO_cycle).

Different initial conditions lead to different kinds of stars.


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