<p>Here is an enjoyable explanation: <div class="youtube-embed"><div>
                <iframe width="640px" height="395px" src="https://www.youtube.com/embed/Yt-SBT7nNfU?start=0"></iframe>
            </div></div></p>
<blockquote>
<p>Ok so I got doubt with neutrinos here that, how are they initiating the supernova explosion. Don't they interact rarely with matter?</p>
</blockquote>
<p>There are two things to point out. It is true that neutrinos rarely interact with matter, but on the other hand the neutrino pulse is very intense. There are just so many of them that the rare interactions can now happen frequently, at least for the few seconds that the pulse lasts.</p>
<p>Second, the the chance of an interaction between an outgoing neutrino and the matter surrounding the core goes up with increasing density. Because the core bounces, an immense shock wave is created traveling outward from the core. All waves are denser than the material they are traveling through, so the escaping neutrinos will preferentially interact with the matter in the shock front, rather than the less dense matter ahead of or behind it. The shock front is dense enough to absorb not a tiny fraction of a percent of the neutrinos, but maybe as many as 10% of them. It can gain an incredible amount of energy from those neutrinos, more than enough to power through the infalling matter and explode the entire star. (Of course if the star is big enough then even that won’t be enough and the infalling matter stifles the explosion, and ultimately a black hole is formed.)</p>
<blockquote>
<p>The other is thermal neutrinos, whose production is due to ? heat of hot neutron core? I really couldn't understand how they formed.</p>
</blockquote>
<p>Calculating the number of neutrinos created is quite hard. Of course there will be one neutrino for every proton converted to a neutron in the core, which is a lot, but then the <a href="https://en.wikipedia.org/wiki/Urca_process" rel="noreferrer">Urca process</a> creates many times this number of neutrinos as the core cools. At its simplest, imagine that a proton absorbs an electron to create a neutron. This neutron is carried by convection away from the interior of the core to a region of lower electron degeneracy. There it undergoes beta decay becoming a proton and electron again. Since both of those reactions release a neutrino (or antineutrino), and because the highly turbulent convection within the core and gain region can turn matter over many times per second, the number of neutrinos released will be quite large indeed.</p>
<p>(I am no expert myself, so I welcome any corrections!)</p>

CC BY-SA 4.0
2024-06-06T01:30:53.697Z
2024-06-06T05:43:49.470Z
2024-06-06T05:43:49.470Z
db48x
<p>Supernovae are powered by gravitational energy. Specifically, the change in gravitational potential energy when an electron-degenerate core, the mass of the Sun and radius of the Earth, collapsed rapidly (<span class="math-container">$&lt;1$</span> s) to a radius of <span class="math-container">$\sim 10$</span> km.</p>
<p>Where does this energy go? Initially it is shared between escaping neutrinos and making the the dense core extremely hot (<span class="math-container">$10^{11}$</span> K). However, as the central core achieves neutron star densities, the collapse is abruptly halted by the repulsion between closely packed nucleons and the consequent &quot;bounce&quot; drives a shockwave outwards.</p>
<p>The shock wave is <em>not</em> powerful enough to power the supernova. Instead, the driving force at play is the increased opacity of the hot, dense inner regions to neutrinos.</p>
<p>A basic rule of thumb is it takes a light year of lead to stop a neutrino. But neutron star densities are about <span class="math-container">$10^{13}$</span> times that of lead, so the equivalent is around 10 km of neutron star material. It is the absorption of the energy and momentum of just a small fraction (a few percent) of the neutrinos produced in the hot, but rapidly cooling, core, that drives the supernova explosion and throws off the outer envelope of the star.</p>
<p>Now to your specific question:</p>
<p>The hot core produces neutrinos chiefly via the URCA process. These are cycles of beta decay and inverse beta decay.
<span class="math-container">$$ n \rightarrow p + e + \bar{\nu}$$</span>
<span class="math-container">$$ p + e \rightarrow n + \nu $$</span>
At very high temperatures, these processes can come into thermal equilibrium (ordinarily, the second process is energetically unfavorable). The resultant (anti) neutrinos have a range of energies of order <span class="math-container">$k_B T \sim 10$</span> MeV and would be called &quot;thermal neutrinos&quot;.</p>
<p>Electron and positrons produced by pair production at high energies can also annihilate to produce neutrino/anti neutrinos pairs. Again, the energies of the neutrinos reflect the thermal energies of the electrons and positrons. There is also a scattering interaction between nucleons that can produce neutrino/anti neutrino pairs called &quot;neutrino bremsstrahlung&quot;.</p>
<p>These thermal neutrinos are a coolant. They take away energy from the core, allowing it to cool by an order of magnitude in seconds by emitting <span class="math-container">$\sim 10^{57}$</span> such neutrinos (the thermal energy of the core divided by 10 MeV). However, in these first few seconds after core collapse, the region just outside where these neutrinos are produced is able to absorb some of them, which drives the explosion.</p>

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2024-06-06T15:35:09.850Z
wait... electron/positron annihilation can produce neutrino/anti-neutrino pairs?
Michael
CC BY-SA 4.0
2024-06-06T16:23:16.113Z
@Michael yes. This conserves charge and lepton number. It is rarer than photon production because of the weak interaction involved.
ProfRob
CC BY-SA 4.0
2024-06-06T23:22:26.957Z
@Michael The probability of neutrino+antineutrino production is quite low, ~$10^{-19}$, but at that temperature there are a *lot* of electron+positron pairs being produced. Also see [The photo-neutrino process in astrophysical systems](https://doi.org/10.48550/arXiv.astro-ph/0309564)
PM 2Ring
CC BY-SA 4.0
2024-06-07T08:45:03.970Z
Ok so thanks for the answer. However I have two doubts. First the neutrinos interact with neutron core or the shockwave material? Also do they absorb energy or impart it to the shockwave, making it stronger?
Kshitij Kumar
CC BY-SA 4.0
2024-06-07T08:46:03.557Z
Also one more doubt is that the thermal energy of hot neutron core is spent in creation of neutrinos via these nuclear processes. Is my understanding correct?
Kshitij Kumar
CC BY-SA 4.0
2024-06-07T10:24:52.053Z
@KshitijKumar    "These thermal neutrinos are a coolant. They take away energy from the core, allowing it to cool by an order of magnitude in seconds"; "However, in these first few seconds after core collapse, the region just outside where these neutrinos are produced is able to absorb some of them, which drives the explosion."
ProfRob
CC BY-SA 4.0
2024-06-07T10:30:36.703Z
@KshitijKumar I would be reluctant to get into any more details than that.
ProfRob
CC BY-SA 4.0
2024-06-07T14:43:21.737Z
@ProfRob Thanks
Kshitij Kumar
CC BY-SA 4.0
2024-06-06T06:18:35.597Z
2024-06-06T16:25:55.723Z
2024-06-06T16:25:55.723Z
ProfRob
<p>I was trying to understand the type II supernovae's <a href="https://en.wikipedia.org/wiki/Type_II_supernova#Core_collapse" rel="nofollow noreferrer">core collapse mechanism</a> from Wikipedia.</p>
<blockquote>
<p>As the core's density increases, it becomes energetically favorable for electrons and protons to merge via inverse beta decay, producing neutrons and elementary particles called neutrinos.</p>
</blockquote>
<p>So far, everything's fine.</p>
<blockquote>
<p>Because neutrinos rarely interact with normal matter, they can escape from the core, carrying away energy and further accelerating the collapse, which proceeds over a timescale of milliseconds. As the core detaches from the outer layers of the star, some of these neutrinos are absorbed by the star's outer layers, beginning the supernova explosion.</p>
</blockquote>
<p><em>How do neutrinos initiate the supernova explosion</em>? Don't they rarely interact with matter?</p>
<blockquote>
<p>The newly formed neutron core has an initial temperature of about 100 billion kelvins, 10<sup>4</sup> times the temperature of the Sun's core. Much of this thermal energy must be shed for a stable neutron star to form, otherwise the neutrons would &quot;boil away&quot;. This is accomplished by a further release of neutrinos.</p>
</blockquote>
<blockquote>
<p>These 'thermal' neutrinos form as neutrino-antineutrino pairs of all flavors, and total several times the number of electron-capture neutrinos. The two neutrino production mechanisms convert the gravitational potential energy of the collapse into a ten-second neutrino burst, releasing about 10<sup>46</sup> joules (100 foe).</p>
</blockquote>
<p>We have two mechanisms of neutrinos production here. One is due to inverse beta decay leading to production of neutrons and neutrinos. The other is thermal neutrinos. What is their production due to? Heat of hot neutron core?</p>
<p>What is their role? They sap away thermal energy from hot neutron core, and convert gravitational potential energy into neutrino bursts? What does that mean?</p>
<p>My main question is: <strong>How are these neutrinos able to <em>cause</em> shockwave supernovae explosions?</strong></p>

CC BY-SA 4.0
2024-06-05T21:15:41.877Z
This question just reminds me very strongly of https://what-if.xkcd.com/73 and I strongly recommend reading it, if only because Randall Munroe is a very fun writer, but it also starts to answer this question.
Darth Pseudonym
CC BY-SA 4.0
2024-06-06T05:34:51.563Z
I have some info about thermal neutrino-antineutrino pair production during a core collapse supernova, at the end of https://physics.stackexchange.com/a/563986/123208 But also see  https://physics.stackexchange.com/q/636750/123208
PM 2Ring
CC BY-SA 4.0
2024-06-06T11:44:13.187Z
There's a lot in supernovae, but no doubt.
Mithoron
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2024-06-06T15:27:39.050Z
@DarthPseudonym One thing I didn't take away from reading that was how exactly the neutrinos "blast" could be so massive on Earth, and yet have no apparent effect on the core itself where the neutrino density is many orders of magnitude higher.  Is the core just too massive or have too much momentum to be affected?
Michael
CC BY-SA 4.0
2024-06-06T19:20:11.123Z
In the core that's currently collapsing into neutronium? I imagine it does have some neutrino interactions, but I assume it would either be a neutron capturing a neutrino and splitting into a proton + electron (the inverse of the collapse, slightly slowing the process) or a proton interacting with a neutrino which would IIRC just deflect the neutrino and generate some heat, I guess? I don't think the core fails to interact, it just doesn't have anything particularly special that happens when it does. As opposed to the layers just above the core, which can trigger fusion.
Darth Pseudonym
CC BY-SA 4.0
2024-06-07T10:29:13.347Z
The neutrinos we receive beyond the supernovae are from the "neutrinosphere" - that region close to the centre of the collapsed star from which neutrinos are able to escape. The neutrinosphere will initially be somewhere behind the shock front and will shrink back towards the surface of the proto-neutron star as it cools.
ProfRob
CC BY-SA 4.0
2024-06-09T00:39:21.343Z
@Michael The neutrinos (& antineutrinos) emitted during core collapse have a huge effect on the collapsing core: they cool it down, as ProfRob mentions.
PM 2Ring
<supernova><neutrinos>
How are neutrinos able to cause a supernova explosion?
Kshitij Kumar
# How are neutrinos able to cause a supernova explosion?

(Asked by: username_0 on 2024-06-05)

I was trying to understand the type II supernovae's [core collapse mechanism](https://en.wikipedia.org/wiki/Type_II_supernova#Core_collapse) from Wikipedia.

> As the core's density increases, it becomes energetically favorable for electrons and protons to merge via inverse beta decay, producing neutrons and elementary particles called neutrinos.

So far, everything's fine.

> Because neutrinos rarely interact with normal matter, they can escape from the core, carrying away energy and further accelerating the collapse, which proceeds over a timescale of milliseconds. As the core detaches from the outer layers of the star, some of these neutrinos are absorbed by the star's outer layers, beginning the supernova explosion.

*How do neutrinos initiate the supernova explosion*? Don't they rarely interact with matter?

> The newly formed neutron core has an initial temperature of about 100 billion kelvins, 104 times the temperature of the Sun's core. Much of this thermal energy must be shed for a stable neutron star to form, otherwise the neutrons would "boil away". This is accomplished by a further release of neutrinos.

> These 'thermal' neutrinos form as neutrino-antineutrino pairs of all flavors, and total several times the number of electron-capture neutrinos. The two neutrino production mechanisms convert the gravitational potential energy of the collapse into a ten-second neutrino burst, releasing about 1046 joules (100 foe).

We have two mechanisms of neutrinos production here. One is due to inverse beta decay leading to production of neutrons and neutrinos. The other is thermal neutrinos. What is their production due to? Heat of hot neutron core?

What is their role? They sap away thermal energy from hot neutron core, and convert gravitational potential energy into neutrino bursts? What does that mean?

My main question is: **How are these neutrinos able to *cause* shockwave supernovae explosions?**

**Tags:** supernova, neutrinos

> **Comments:**
> **username_1**: This question just reminds me very strongly of https://what-if.xkcd.com/73 and I strongly recommend reading it, if only because Randall Munroe is a very fun writer, but it also starts to answer this question.  
> **username_2**: I have some info about thermal neutrino-antineutrino pair production during a core collapse supernova, at the end of https://physics.stackexchange.com/a/563986/123208 But also see  https://physics.stackexchange.com/q/636750/123208  
> **username_3**: There's a lot in supernovae, but no doubt.  
> **username_4**: @username_1 One thing I didn't take away from reading that was how exactly the neutrinos "blast" could be so massive on Earth, and yet have no apparent effect on the core itself where the neutrino density is many orders of magnitude higher.  Is the core just too massive or have too much momentum to be affected?  
> **username_1**: In the core that's currently collapsing into neutronium? I imagine it does have some neutrino interactions, but I assume it would either be a neutron capturing a neutrino and splitting into a proton + electron (the inverse of the collapse, slightly slowing the process) or a proton interacting with a neutrino which would IIRC just deflect the neutrino and generate some heat, I guess? I don't think the core fails to interact, it just doesn't have anything particularly special that happens when it does. As opposed to the layers just above the core, which can trigger fusion.  
> **username_5**: The neutrinos we receive beyond the supernovae are from the "neutrinosphere" - that region close to the centre of the collapsed star from which neutrinos are able to escape. The neutrinosphere will initially be somewhere behind the shock front and will shrink back towards the surface of the proto-neutron star as it cools.  
> **username_2**: @username_4 The neutrinos (& antineutrinos) emitted during core collapse have a huge effect on the collapsing core: they cool it down, as username_5 mentions.  

---

## 2 answers

### Answer by username_5 on 2024-06-06. Score: 14 | Accepted answer

Supernovae are powered by gravitational energy. Specifically, the change in gravitational potential energy when an electron-degenerate core, the mass of the Sun and radius of the Earth, collapsed rapidly ($<1$ s) to a radius of $\sim 10$ km.

Where does this energy go? Initially it is shared between escaping neutrinos and making the the dense core extremely hot ($10^{11}$ K). However, as the central core achieves neutron star densities, the collapse is abruptly halted by the repulsion between closely packed nucleons and the consequent "bounce" drives a shockwave outwards.

The shock wave is *not* powerful enough to power the supernova. Instead, the driving force at play is the increased opacity of the hot, dense inner regions to neutrinos.

A basic rule of thumb is it takes a light year of lead to stop a neutrino. But neutron star densities are about $10^{13}$ times that of lead, so the equivalent is around 10 km of neutron star material. It is the absorption of the energy and momentum of just a small fraction (a few percent) of the neutrinos produced in the hot, but rapidly cooling, core, that drives the supernova explosion and throws off the outer envelope of the star.

Now to your specific question:

The hot core produces neutrinos chiefly via the URCA process. These are cycles of beta decay and inverse beta decay.
$$ n \rightarrow p + e + \bar{\nu}$$
$$ p + e \rightarrow n + \nu $$
At very high temperatures, these processes can come into thermal equilibrium (ordinarily, the second process is energetically unfavorable). The resultant (anti) neutrinos have a range of energies of order $k\_B T \sim 10$ MeV and would be called "thermal neutrinos".

Electron and positrons produced by pair production at high energies can also annihilate to produce neutrino/anti neutrinos pairs. Again, the energies of the neutrinos reflect the thermal energies of the electrons and positrons. There is also a scattering interaction between nucleons that can produce neutrino/anti neutrino pairs called "neutrino bremsstrahlung".

These thermal neutrinos are a coolant. They take away energy from the core, allowing it to cool by an order of magnitude in seconds by emitting $\sim 10^{57}$ such neutrinos (the thermal energy of the core divided by 10 MeV). However, in these first few seconds after core collapse, the region just outside where these neutrinos are produced is able to absorb some of them, which drives the explosion.

> **Comments:**
> **username_4**: wait... electron/positron annihilation can produce neutrino/anti-neutrino pairs?  
> **username_5**: @username_4 yes. This conserves charge and lepton number. It is rarer than photon production because of the weak interaction involved.  
> **username_2**: @username_4 The probability of neutrino+antineutrino production is quite low, ~$10^{-19}$, but at that temperature there are a *lot* of electron+positron pairs being produced. Also see [The photo-neutrino process in astrophysical systems](https://doi.org/10.48550/arXiv.astro-ph/0309564)  
> **username_0**: Ok so thanks for the answer. However I have two doubts. First the neutrinos interact with neutron core or the shockwave material? Also do they absorb energy or impart it to the shockwave, making it stronger?  
> **username_0**: Also one more doubt is that the thermal energy of hot neutron core is spent in creation of neutrinos via these nuclear processes. Is my understanding correct?  
> **username_5**: @username_0    "These thermal neutrinos are a coolant. They take away energy from the core, allowing it to cool by an order of magnitude in seconds"; "However, in these first few seconds after core collapse, the region just outside where these neutrinos are produced is able to absorb some of them, which drives the explosion."  
> **username_5**: @username_0 I would be reluctant to get into any more details than that.  
> **username_0**: @username_5 Thanks  

---

### Answer by username_6 on 2024-06-06. Score: 15

Here is an enjoyable explanation:

> Ok so I got doubt with neutrinos here that, how are they initiating the supernova explosion. Don't they interact rarely with matter?

There are two things to point out. It is true that neutrinos rarely interact with matter, but on the other hand the neutrino pulse is very intense. There are just so many of them that the rare interactions can now happen frequently, at least for the few seconds that the pulse lasts.

Second, the the chance of an interaction between an outgoing neutrino and the matter surrounding the core goes up with increasing density. Because the core bounces, an immense shock wave is created traveling outward from the core. All waves are denser than the material they are traveling through, so the escaping neutrinos will preferentially interact with the matter in the shock front, rather than the less dense matter ahead of or behind it. The shock front is dense enough to absorb not a tiny fraction of a percent of the neutrinos, but maybe as many as 10% of them. It can gain an incredible amount of energy from those neutrinos, more than enough to power through the infalling matter and explode the entire star. (Of course if the star is big enough then even that won’t be enough and the infalling matter stifles the explosion, and ultimately a black hole is formed.)

> The other is thermal neutrinos, whose production is due to ? heat of hot neutron core? I really couldn't understand how they formed.

Calculating the number of neutrinos created is quite hard. Of course there will be one neutrino for every proton converted to a neutron in the core, which is a lot, but then the [Urca process](https://en.wikipedia.org/wiki/Urca_process) creates many times this number of neutrinos as the core cools. At its simplest, imagine that a proton absorbs an electron to create a neutron. This neutron is carried by convection away from the interior of the core to a region of lower electron degeneracy. There it undergoes beta decay becoming a proton and electron again. Since both of those reactions release a neutrino (or antineutrino), and because the highly turbulent convection within the core and gain region can turn matter over many times per second, the number of neutrinos released will be quite large indeed.

(I am no expert myself, so I welcome any corrections!)


---

