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?

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    $\begingroup$ I'm not doubting the timeframe for peak magnitude, but can you edit your question to provide a link to your source? $\endgroup$ Oct 8, 2018 at 1:02
  • $\begingroup$ @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. $\endgroup$ Oct 8, 2018 at 2:23

3 Answers 3


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.

  • $\begingroup$ 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! $\endgroup$ Oct 10, 2018 at 1:14
  • $\begingroup$ Re your last paragraph in brackets, Co-56 decay actually becomes the dominant source of energy from peak luminosity onwards. $\endgroup$ Oct 10, 2018 at 11:10
  • $\begingroup$ @Chappo -- You have a good point (though the timing is coincidental); I'll update the answer. $\endgroup$ Oct 12, 2018 at 14:04

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, 1982.

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

  • $\begingroup$ 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. $\endgroup$ Oct 8, 2018 at 18:04
  • $\begingroup$ 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? $\endgroup$ Oct 9, 2018 at 3:34
  • $\begingroup$ 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. $\endgroup$ Oct 9, 2018 at 11:56
  • $\begingroup$ @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! $\endgroup$ Oct 10, 2018 at 1:08
  • $\begingroup$ 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? $\endgroup$ Oct 10, 2018 at 11:05

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.

  • $\begingroup$ 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? $\endgroup$ Oct 8, 2018 at 2:27
  • $\begingroup$ Radioactive decay; 56Ni → 56Co → 56Fe. $\endgroup$
    – Kyle
    Oct 8, 2018 at 3:01
  • $\begingroup$ 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. $\endgroup$
    – Kyle
    Oct 8, 2018 at 3:09
  • $\begingroup$ Kyle, the prevailing view is that the Chandrasekhar limit is not involved in Type Ia SNE. $\endgroup$ Oct 8, 2018 at 20:54
  • $\begingroup$ 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. $\endgroup$
    – Kyle
    Oct 8, 2018 at 21:18

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