<p>The early light curve of a Type Ia SN (i.e., the fact that the brightness increases to a peak, then declines) is the result of the combination of two things, both of which are decreasing over time: energy from the decay of Ni-56 to Co-56 (and Co-56 to Fe-56), and the opacity of the explosion as it expands.</p>

<p>Just after the actual explosion, the expanding fireball is dense and opaque. Only light from the very outermost layer escapes to the outside.[*] Since the Ni-56 created in the thermonuclear burning that caused the explosion is in the interior regions, the initially visible part of the expanding fireball has little or no Ni-56, and thus no energy from its decay.</p>

<p>As the fireball expands, it gets less dense, and thus less opaque, to the point where light from the layers heated by radioactive decay can actually escape. Over time, more and more of the inner part of the fireball becomes visible, and so we see more and more of the regions where Ni-56 (and its daughter product Co-56) is decaying. Roughly speaking, the peak of the light curve is when the opacity is low enough that all the light from the Ni-56 + Co-56 decay can escape.[**]</p>

<p>But since there are fewer and fewer radioactive nuclei as time goes by, the amount of energy produced by the decay decreases with time. This is why the light curve turns over: now we're seeing all the light produced by Ni-56 and Co-56 decay, but <em>that</em> just keeps getting fainter over time.</p>

<p>[*] At this point, the SN is too faint to be seen. Even the initial thermonuclear explosion is too faint to be seen under most circumstances.</p>

<p>[**] Ni-56 and Co-56 actually produce gamma rays (and energetic positrons) when they decay, so what "the light from Ni-56 + Co-56 decay" means is thermal radiation from the gas heated by the gamma rays. Also, as I understand it, what we're seeing is really the combination of energy from current decay <em>and</em> residual heat from previous radioactive decay.</p>

CC BY-SA 4.0
2018-10-10T01:14:51.270Z
The key factor is a combination of the opacity variation with time and the energy from the Ni-56 decay. Thank you for your detailed answer!
cosmozaka46
CC BY-SA 4.0
2018-10-10T11:10:48.360Z
Re your last paragraph in brackets, Co-56 decay actually becomes the dominant source of energy from peak luminosity onwards.
Chappo Hasn't Forgotten
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2018-10-12T14:04:26.093Z
@Chappo -- You have a good point (though the timing is coincidental); I'll update the answer.
Peter Erwin
CC BY-SA 4.0
2018-10-09T08:05:22.100Z
2018-10-12T14:10:59.873Z
2018-10-12T14:10:59.873Z
Peter Erwin
<p>SN Ia is a thermonuclear explosion. At very early time, Ni-56 is the main source of energy. Since the hard photons from the Ni-56 decay have to travel from the inside out, they interact with ejecta. The diffusion timescale due mainly to the mass of ejecta determines the peak timescale of a light curve. Check Arnett <a href="http://adsabs.harvard.edu/abs/1980ApJ...237..541A" rel="nofollow noreferrer">1980</a>, <a href="http://adsabs.harvard.edu/abs/1982ApJ...253..785A" rel="nofollow noreferrer">1982</a>.</p>

<p>Fe-56 is not radioactive. Co-56 supplies energy at later times after the peak.</p>

CC BY-SA 4.0
2018-10-08T18:04:22.253Z
Can you edit  so that your reference "Arnett" is a link to something, or else put in the full biblio entry, i.e. title, publisher, etc.
Carl Witthoft
CC BY-SA 4.0
2018-10-09T03:34:01.687Z
One question you haven't addressed is: how much of the pre-peak light curve is due to *nucleosynthesis* rather than Ni-56 decay? A second question is the proportion of Ni-56 produced compared to other fusion products; Arnett's 1982 paper notes that several results "rule out models which convert essentially all the star to 56Ni. However, this is not awkward if the explosion burns to completion only the inner part of the star." Is there a difference in products from the (inner, hydrostatic) carbon detonation compared to what's produced by the shock wave hitting the outer part of the star?
Chappo Hasn't Forgotten
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2018-10-09T11:56:00.460Z
I don't understand what you meant by "nucleosynthesis rather than Ni-56 decay." For the pre-peak light curve, a simple explanation is just the diffusion, which is covered in Arnett 1980, 1982. 

Second question is about nucleosynthesis, which has nothing to do with this originial question.

For the last one, not sure what you are looking for. But general answer is yes.
Kornpob Bhirombhakdi
CC BY-SA 4.0
2018-10-10T01:08:19.023Z
@KornpobBhirombhakdi I see, that means the time of peak luminosity is determined by the Ni-56 and decay and diffusion timescale. That makes sense, thank you!
cosmozaka46
CC BY-SA 4.0
2018-10-10T11:05:18.027Z
Ni-56 *synthesis* (releasing energy through the hydrostatic burning process) occurs during the few seconds of carbon detonation initiating the supernova, whereas Ni-56 *decay* necessarily occurs **after** the Ni-56 has been created. So is the light near pre-peak predominantly from initial synthesis (with the photons taking time to make their way to the surface), or from much later decay?
Chappo Hasn't Forgotten
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2018-10-10T11:17:59.457Z
Energy from Ni-56 dominates the explosion energy in Type Ia.
Kornpob Bhirombhakdi
CC BY-SA 4.0
2018-10-08T12:05:33.030Z
2018-10-08T18:53:42.383Z
2018-10-08T18:53:42.383Z
Kornpob Bhirombhakdi
<p>It has to do with the Chandrasekhar limit, and how all type 1a supernova are created from the destruction of a white dwarf. It takes the same amount of time to hit absolute magnitude because they convert the same amount of mass.</p>

CC BY-SA 4.0
2018-10-08T02:27:02.077Z
Thank you for your reply. Actually, what I really want to know is not the time universality but the mechanism why they reach after around 20 days since their explosion. Do you have any ideas?
cosmozaka46
CC BY-SA 4.0
2018-10-08T03:01:59.813Z
Radioactive decay; 56Ni → 56Co → 56Fe.
Kyle
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2018-10-08T03:09:35.910Z
I just realized you might be looking for the process which initiates the decay, so I'd also like to clarify that it's due to an energy release from carbon detonation.
Kyle
CC BY-SA 4.0
2018-10-08T20:54:32.257Z
Kyle, the prevailing view is that the Chandrasekhar limit is *not* involved in Type Ia SNE.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2018-10-08T21:18:26.017Z
It's not that the white dwarf exceeds the limit, so technically I guess it's not involved; however, my understanding is that the event that leads to the type 1a supernova is a long period of convection which is initiated by the approaching (~99%) of that white dwarf mass limit. Sometime during that convection, a deflagation flame front triggers carbon fusion, which then leads to a runaway effect that causes the star to explode. If the mass if the same before (99% of the limit) that explains the reproducible light curve and magnitude maximum due to E=mc^2.
Kyle
CC BY-SA 4.0
2018-10-08T01:35:15.240Z
2018-10-08T01:35:15.240Z
Kyle
<p>I'm not sure why type Ia supernova reaches its peak magnitude 15-20 days after the explosion. Is this luminosity generated only by the radioactive decay of nickel-56 to cobalt-56?(considering a half-life, I don't think the decay to iron-56 can be involved with it) Or is there any combination that produces the peak luminosity?</p>

CC BY-SA 4.0
2018-10-08T01:02:30.667Z
I'm not doubting the timeframe for peak magnitude, but can you [edit] your question to provide a link to your source?
Chappo Hasn't Forgotten
CC BY-SA 4.0
2018-10-08T02:23:23.747Z
@Chappo Ah sorry, actually the source is a Japanese slide found on the web. I also see the brief summary of Ia supernova on Wikipedia.
cosmozaka46
<cosmology><supernova>
Why type Ia supernova reaches its peak absolute magnitude(~ -19.3mag) at around 15-20 days after the explosion?
cosmozaka46
# Why type Ia supernova reaches its peak absolute magnitude(~ -19.3mag) at around 15-20 days after the explosion?

(Asked by: username_0 on 2018-10-08)

I'm not sure why type Ia supernova reaches its peak magnitude 15-20 days after the explosion. Is this luminosity generated only by the radioactive decay of nickel-56 to cobalt-56?(considering a half-life, I don't think the decay to iron-56 can be involved with it) Or is there any combination that produces the peak luminosity?

**Tags:** cosmology, supernova
> **Comments:**
> **username_1**: I'm not doubting the timeframe for peak magnitude, but can you [edit] your question to provide a link to your source?  
> **username_0**: @Chappo Ah sorry, actually the source is a Japanese slide found on the web. I also see the brief summary of Ia supernova on Wikipedia.  

---

## 3 answers

### Answer by username_2 on 2018-10-09. Score: 3 | Accepted answer

The early light curve of a Type Ia SN (i.e., the fact that the brightness increases to a peak, then declines) is the result of the combination of two things, both of which are decreasing over time: energy from the decay of Ni-56 to Co-56 (and Co-56 to Fe-56), and the opacity of the explosion as it expands.

Just after the actual explosion, the expanding fireball is dense and opaque. Only light from the very outermost layer escapes to the outside.[\*] Since the Ni-56 created in the thermonuclear burning that caused the explosion is in the interior regions, the initially visible part of the expanding fireball has little or no Ni-56, and thus no energy from its decay.

As the fireball expands, it gets less dense, and thus less opaque, to the point where light from the layers heated by radioactive decay can actually escape. Over time, more and more of the inner part of the fireball becomes visible, and so we see more and more of the regions where Ni-56 (and its daughter product Co-56) is decaying. Roughly speaking, the peak of the light curve is when the opacity is low enough that all the light from the Ni-56 + Co-56 decay can escape.[\*\*]

But since there are fewer and fewer radioactive nuclei as time goes by, the amount of energy produced by the decay decreases with time. This is why the light curve turns over: now we're seeing all the light produced by Ni-56 and Co-56 decay, but *that* just keeps getting fainter over time.

[\*] At this point, the SN is too faint to be seen. Even the initial thermonuclear explosion is too faint to be seen under most circumstances.

[\*\*] Ni-56 and Co-56 actually produce gamma rays (and energetic positrons) when they decay, so what "the light from Ni-56 + Co-56 decay" means is thermal radiation from the gas heated by the gamma rays. Also, as I understand it, what we're seeing is really the combination of energy from current decay *and* residual heat from previous radioactive decay.

> **Comments:**
> **username_0**: The key factor is a combination of the opacity variation with time and the energy from the Ni-56 decay. Thank you for your detailed answer!  
> **username_1**: Re your last paragraph in brackets, Co-56 decay actually becomes the dominant source of energy from peak luminosity onwards.  
> **username_2**: @Chappo -- You have a good point (though the timing is coincidental); I'll update the answer.  

---

### Answer by username_3 on 2018-10-08. Score: 1

SN Ia is a thermonuclear explosion. At very early time, Ni-56 is the main source of energy. Since the hard photons from the Ni-56 decay have to travel from the inside out, they interact with ejecta. The diffusion timescale due mainly to the mass of ejecta determines the peak timescale of a light curve. Check Arnett [1980](http://adsabs.harvard.edu/abs/1980ApJ...237..541A), [1982](http://adsabs.harvard.edu/abs/1982ApJ...253..785A).

Fe-56 is not radioactive. Co-56 supplies energy at later times after the peak.

> **Comments:**
> **username_4**: Can you edit  so that your reference "Arnett" is a link to something, or else put in the full biblio entry, i.e. title, publisher, etc.  
> **username_1**: One question you haven't addressed is: how much of the pre-peak light curve is due to *nucleosynthesis* rather than Ni-56 decay? A second question is the proportion of Ni-56 produced compared to other fusion products; Arnett's 1982 paper notes that several results "rule out models which convert essentially all the star to 56Ni. However, this is not awkward if the explosion burns to completion only the inner part of the star." Is there a difference in products from the (inner, hydrostatic) carbon detonation compared to what's produced by the shock wave hitting the outer part of the star?  
> **username_3**: I don't understand what you meant by "nucleosynthesis rather than Ni-56 decay." For the pre-peak light curve, a simple explanation is just the diffusion, which is covered in Arnett 1980, 1982.   Second question is about nucleosynthesis, which has nothing to do with this originial question.  For the last one, not sure what you are looking for. But general answer is yes.  
> **username_0**: @KornpobBhirombhakdi I see, that means the time of peak luminosity is determined by the Ni-56 and decay and diffusion timescale. That makes sense, thank you!  
> **username_1**: Ni-56 *synthesis* (releasing energy through the hydrostatic burning process) occurs during the few seconds of carbon detonation initiating the supernova, whereas Ni-56 *decay* necessarily occurs **after** the Ni-56 has been created. So is the light near pre-peak predominantly from initial synthesis (with the photons taking time to make their way to the surface), or from much later decay?  
> **username_3**: Energy from Ni-56 dominates the explosion energy in Type Ia.  

---

### Answer by username_5 on 2018-10-08. Score: -3

It has to do with the Chandrasekhar limit, and how all type 1a supernova are created from the destruction of a white dwarf. It takes the same amount of time to hit absolute magnitude because they convert the same amount of mass.

> **Comments:**
> **username_0**: Thank you for your reply. Actually, what I really want to know is not the time universality but the mechanism why they reach after around 20 days since their explosion. Do you have any ideas?  
> **username_5**: Radioactive decay; 56Ni → 56Co → 56Fe.  
> **username_5**: I just realized you might be looking for the process which initiates the decay, so I'd also like to clarify that it's due to an energy release from carbon detonation.  
> **username_1**: Kyle, the prevailing view is that the Chandrasekhar limit is *not* involved in Type Ia SNE.  
> **username_5**: It's not that the white dwarf exceeds the limit, so technically I guess it's not involved; however, my understanding is that the event that leads to the type 1a supernova is a long period of convection which is initiated by the approaching (~99%) of that white dwarf mass limit. Sometime during that convection, a deflagation flame front triggers carbon fusion, which then leads to a runaway effect that causes the star to explode. If the mass if the same before (99% of the limit) that explains the reproducible light curve and magnitude maximum due to E=mc^2.  

---

