<p>It is conceivable that pycnonuclear fusion could result in conversion of a cold white dwarf into iron (or at least, iron-peak elements) if the density in the white dwarf is very high. That is the lowest energy density state for relatively (compared with a neutron star) low density material.</p>
<p>The Chandrasekhar mass for iron is about 1.1 solar masses. If the white dwarf mass exceeds this then electron capture onto the iron nuclei becomes energetically favourable. This leads to instabity, collapse, neutronisation, a hardening of the equation of state, core bounce and <em>maybe</em> some weird kind of supernova.</p>
<p>No nuclear fusion is involved, <em>unless</em> the outer parts of the collapsing white dwarf have managed to remain as carbon and oxygen. In which case, these might be detonated by the core collapse leading to thermonuclear kind of type Ia supernova, driven by fusion of the remaining carbon and oxygen.</p>
<p>However, this all sounds fantastically speculative. Because, to make iron by pycnonuclear fusion you have to somehow avoid detonating the star at an earlier stage  since pycnonuclear fusion rates for carbon would be much faster than the fusion that produces iron.</p>

CC BY-SA 4.0
2024-09-23T18:27:12.223Z
2024-09-23T18:27:12.223Z
ProfRob
<p>According to wikipedia:</p>
<blockquote>
<p>pycnonuclear (density-based) fusion processes much of the star to iron, which would lower the Chandrasekhar limit for some black dwarfs below their actual mass. If this point is reached, it would then collapse and initiate runaway nuclear fusion.</p>
</blockquote>
<p>but of what element? It most likely can't be iron, since fusion of iron is endothermic, but then what element do black dwarf supernovae hypothetically burn? Is it the residual carbon in the core, or something else created by the pyronuclear reactions?</p>

CC BY-SA 4.0
2024-09-23T15:00:18.820Z
It seems to be electron capture on the iron nuclei. See [Black dwarf supernova in the far future](https://academic.oup.com/mnras/article/497/4/4357/5884975) by M E Caplan. Also see https://johncarlosbaez.wordpress.com/2021/04/14/black-dwarf-supernovae/
PM 2Ring
CC BY-SA 4.0
2024-09-23T18:01:52.120Z
",According to wikipedia..." there's your problem. Please cite the original source.
ProfRob
<supernova><fusion><black-dwarf>
What element do black dwarf supernovae burn?
Astrovis
# What element do black dwarf supernovae burn?

(Asked by: username_0 on 2024-09-23)

According to wikipedia:

> pycnonuclear (density-based) fusion processes much of the star to iron, which would lower the Chandrasekhar limit for some black dwarfs below their actual mass. If this point is reached, it would then collapse and initiate runaway nuclear fusion.

but of what element? It most likely can't be iron, since fusion of iron is endothermic, but then what element do black dwarf supernovae hypothetically burn? Is it the residual carbon in the core, or something else created by the pyronuclear reactions?

**Tags:** supernova, fusion, black-dwarf
> **Comments:**
> **username_1**: It seems to be electron capture on the iron nuclei. See [Black dwarf supernova in the far future](https://academic.oup.com/mnras/article/497/4/4357/5884975) by M E Caplan. Also see https://johncarlosbaez.wordpress.com/2021/04/14/black-dwarf-supernovae/  
> **username_2**: ",According to wikipedia..." there's your problem. Please cite the original source.  

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## 1 answers

### Answer by username_2 on 2024-09-23. Score: 7 | Accepted answer

It is conceivable that pycnonuclear fusion could result in conversion of a cold white dwarf into iron (or at least, iron-peak elements) if the density in the white dwarf is very high. That is the lowest energy density state for relatively (compared with a neutron star) low density material.

The Chandrasekhar mass for iron is about 1.1 solar masses. If the white dwarf mass exceeds this then electron capture onto the iron nuclei becomes energetically favourable. This leads to instabity, collapse, neutronisation, a hardening of the equation of state, core bounce and *maybe* some weird kind of supernova.

No nuclear fusion is involved, *unless* the outer parts of the collapsing white dwarf have managed to remain as carbon and oxygen. In which case, these might be detonated by the core collapse leading to thermonuclear kind of type Ia supernova, driven by fusion of the remaining carbon and oxygen.

However, this all sounds fantastically speculative. Because, to make iron by pycnonuclear fusion you have to somehow avoid detonating the star at an earlier stage since pycnonuclear fusion rates for carbon would be much faster than the fusion that produces iron.


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