Supernovae are among the most energetic and explosive events in the universe. We can use them to map out the universe, understand how stars die, and even model neutrinos! Like almost everything else in astronomy, in order to grasp the physics behind these events, we need to understand how light propagates through the system. In that spirit, let’s try to understand what powers supernova light curves by answering the following questions: What are all the different sources of energy that can power supernova light curves? And then what happens to the light as it passes through the layers of ejecta before it gets observed by our telescopes?
The Exploding Star
First, let’s remind ourselves what actually happens when a supernova happens. There are broadly two kinds of supernovae: Thermonuclear Explosions (which come from an exploding White Dwarf progenitor) and Core-collapse Explosions (which come from a massive star progenitor). To understand the types of supernovae in more detail, check out our bite on classifying them.
In thermonuclear explosions: a white dwarf accretes enough matter to start explosively burning. A white dwarf is a degenerate ball of Carbon and Oxygen (and sometimes Neon; read here and here), meaning that all of the electrons making up the white dwarf are as tightly packed as is physically allowed. As the explosion front travels through the star, a shockwave rips through the white dwarf and destroys the entire structure.
In core-collapse explosions: the core of a massive star stops producing enough radiation to hold itself up against gravitational collapse. For most of the star’s life, it has been fusing Hydrogen atoms into Helium. As it ages, the Helium will begin to fuse, producing heavier elements, which will in turn fuse. This pattern will continue until the star produces significant amounts of Iron-56, which is the heaviest atom that can be produced via nuclear fusion while releasing energy. At this stage, the star has an onion-like structure, as shown in Figure 1. Once enough Iron-56 is produced, any further fusion requires a net input of energy, meaning that radiation is no longer going to flow out of the core. Once this happens, the stellar core collapses in on its own gravity, forming what will eventually become either a neutron star or black hole.
Soon after, the rest of the star will also fall onto the already degenerate neutron star, creating a violent collision between the outer envelope of the star and the degenerate core of the star. This collision drives a shockwave that will rip through the star, imparting a lot of outward momentum to the envelope of the star.
In both thermonuclear and core-collapse supernovae, a shockwave is created, and once the shock wave travels through the entire structure of the progenitor (either a white dwarf or a massive dying star), the supernova explosion will be visible to any outside observers.

Figure 1: A late-stage high-mass star that has fused hydrogen into heavier elements, ending up with Iron in its core. Credit: coursehero study guide, high-mass stellar evolution
Sources of Energy
Depending on the composition, mass, and environment of the exploding star, what ends up powering a supernova can vary dramatically. Here, let’s enumerate some of the most common sources of energy in supernovae. In all cases, the source of energy creates photons that are released into the ejected material of the supernova (the ejecta). At least for the first several months, the ejecta of most supernovae is very thick and almost completely opaque to these photons. Therefore any injected photons will be absorbed and re-emitted by the atoms of the ejecta countless times before they are free to travel to any external observer. Therefore, any observations of light from supernovae (i.e. light curves or spectra) will be influenced by the physics of both the energy source and the ejecta. Figure 2 demonstrates the path photons will take, undergoing a random walk while they “diffuse” through the layers of the ejecta.

Figure 2: Visual representation of photons from the central source diffusing through ejecta before being released to the surrounding environment
Radioactive Decay
Both thermonuclear and core collapse supernovae will produce and liberate a host of unstable radioactive isotopes of heavy elements. The elements will radioactively decay, meaning they will spontaneously change into different elements. During this process, the isotopes emit photons into the ejecta of the supernova, thereby powering the supernova light curve. If a supernova light curve is primarily driven by radioactive decay, the most relevant radioactive decay pathway is likely that of Nickel-56. Nickel-56 radioactively decays into Cobalt-56 with a half life of about 6 days. Cobalt-56 is also radioactive and will decay into Iron-56 with a half life of about 80 days. In the absence of any other sources of energy (i.e. assuming radioactive decay of Nickel-56 is the only power source for a supernova), one could in theory infer the total mass of Nickel-56 by looking at the peak brightness of the light curve.
Hydrogen Recombination
If a star has continuously lost mass to its surroundings for many centuries before exploding, it should be surrounded by a very thick and extended shell of (mostly) Hydrogen from the star’s envelope. Once this star explodes, the resulting ejecta will travel through this extended envelope of Hydrogen, exciting and ionizing much of the Hydrogen. Over the subsequent months, the ionized Hydrogen will slowly recombine (i.e. freed electrons once again become bound to protons), once again becoming neutral. A neutral Hydrogen actually has less energy than a free proton and electron, so when a proton and electron recombine into a neutral hydrogen atom, energy is released into the surroundings. This energy can power the supernova light curve. For many supernovae (particularly for Type-II supernovae, see here), the progenitor star will have released so much mass into its surroundings that Hydrogen recombination will be the dominant contributor to the light curve.
Shock Breakout and Shock Cooling
Imagine that a massive star is puffed up before exploding instead of the star losing mass to its surroundings for many centuries, it is simply a little puffed up. In this situation, the shock from the initial collapse of the core of the star will take time to fully travel through the star. Once the shock breaks out of the star, a burst of radiation will become visible to external observers. This usually happens so immediately (usually about ~hours) after the explosion that it is almost never detected (here’s an example of it being detected for a closeby supernova). However, after shock breakout, the envelope that was excited by the shock will then cool off, emitting light through shock cooling. After both shock breakout and shock cooling, another source of energy will take over powering the light curve (like radioactive decay).
Interactions with Circumstellar Material
In the years to decades before exploding as a core-collapse supernova, a star can have a bout of mass loss into its surroundings, where it “burps” off a shell of mass into its surroundings. The subsequent supernova ejecta will then interact with this material surrounding the star (circumstellar material; CSM). When the quickly expanding cloud of ejecta collides with this surrounding CSM, this will excite the atoms in the CSM, just like the Hydrogen atoms we discussed earlier. These excited atoms will then “relax” and “cool” as their electrons de-excite to lower energy levels, emitting photons in the process. Usually, most atoms in the ejecta are moving much faster than the speed of sound through the CSM, so the collision will create a shock wave. This shockwave will in turn rapidly excite the atoms in its wake. For most supernovae, this process will happen so quickly that we cannot detect this shock wave passing through the CSM. Instead, we only can detect the later cooling period, where the excited atoms in the CSM are slowly cooling off. Usually such shock cooling periods will be short (~days) compared to the entire time of the light curve, and the light curve can otherwise be powered primarily by radioactive decay. Because shells of CSM could have been ejected by the star at almost any point during the star’s evolution, CSM interaction can drive bumps in the observed supernova light curve at any phase during its evolution. However, it is most common to see light curve bumps in the CSM early-on, around when the supernova is first detected.
A connection to Observations
Almost all supernovae (including both thermonuclear and core-collapse) produce some mass of Nickel-56. Therefore, light curves of almost all supernovae have at least some contribution from radioactive decay. Supernovae that have, for example, environments that are dusty or are filled with mass from stellar expulsions, can also have other sources of light, including CSM interaction and Hydrogen recombination. Stellar explosions from massive stars that have not been stripped (e.g. by a binary companion) of their outer envelopes will likely retain a lot of the Hydrogen, allowing for a lot more Hydrogen recombination after the supernova (resulting in a Type II supernova). Stripped envelope supernovae can lose their hydrogen and sometimes even helium envelopes, and may be primarily powered by radioactive decay (and are Type I supernovae). Stars that have lost mass before exploding may have CSM interactions, powering bumps in their supernova light curves. All stellar explosions are unique, and have the potential to tell a detailed story about how the star lived. More complex stellar explosions can have multiple energy sources powering the light curve, with different sources turning on at different times during the light curve. We can learn so much about the physics of how stars live their lives and how they die just by watching supernovae!
Edited by Megan Masterson
Featured image adopted from Hubble/ESA and LadyOfHats in the public domain