Analyzing the Universe - Course Wiki: The Sky is Falling!

Core Collapse Supernovae

Last week we learned about how neutron stars become the compact stellar objects that are left behind when massive stars ($M_{star}>10_{\odot}$) end their lives violently in a core-collapse supernova. This week we will explore the physical processes that explain these tremendous explosions.

As we discussed in the stellar evolution wiki article, after the hydrogen is depleted in the core of a massive star, successive stages of fusion ensue in the core and around it. The figure below depicts the elemental makeup of these regions with the heavier elements appearing closer to the core. This is generally referred to as an onion-skin make-up, but this is a grossly simplified view, as there would sometimes be mixing between layers as the star evolves. Moreover, there are other elements (such as sulfur) in some of the layers but the general idea applies. In particular, we must pay close attention to the iron core enshrouded by a shell of silicon and sulfur burning material. This shell generates more iron which grows the core’s mass. Unlike all other stages of the star’s evolution, the final stage sees the fires of elemental fusion extinguish in the core.

This fate is ensured by the fact that, unlike all of the previous stages of nuclear fusion that generated energy, iron requires energy to fuse into heavier elements. In a sense the core becomes a massive energy sink and as its mass nears the Chandrasekhar mass limit ($\sim1.4\,M_{\odot}$), the atoms become relativistic (in addition to having the electrons degenerate) and the core begins to collapse, unable to exert the needed outward pressure to resist the pull of gravity towards the star’s center.

The core of the star, about the size of earth, collapses until neutron degeneracy pressure can balance that of gravity. By this point the core is about the size of Manhattan ($\sim10\;km$). Let us take a short time-out on the action to explain the collapse in detail.

As the core collapses all of the protons are transformed into neutrons through numerous physical processes. The simplest process is inverse-$\beta$ decay, wherein a proton combines with an electron to become a neutron (neutral by the fact that electrons and protons have exactly the same magnitude charge, yet opposite in sign). In addition to forming a neutron, a neutrino is also emitted. Think of a neutrino as the near-massless, chargeless, soul of the electron that has ceased to be; in this case it would be an electron neutrino, typically symbolizes as $\nu_{e^-}$. After supernova 1987A exploded in the Large Magellanic Cloud, a massive stream of neutrinos was detected by way of anomalously high detection rate at neutrino detection experiments that were, and still are, housed far underground to shield against spurious detections not associated with cosmic events. We can visually represent this nuclear decay in a simple equation:

$$p+e^-\longrightarrow n+\nu_{e^-}.\:\:\:\left(1\right)$$

(Note that for each neutron thus formed, a neutrino is generated as well). Now, we pick up where the action left off.... The core reaches its minimum size of about 10 km, but then it begins to rebound. As it does so, shock waves are driven into the stellar material that is also trying to fall in to the center. After this rebound the core will again collapse, but this time it is more or locked into place as a fully formed neutron star. In certain respects, this resembles a ringing bell.

Meanwhile, the shock wave drives through the infalling stellar material and is strengthened by the tremendous flux of neutrinos that results from the neutron-star formation. This shock wave rips apart the star in an event we call a supernova. The diagram below shows a great cartoon and caption from the wikipedia page on Type II Supernovae, and depicts the various stages of the core-collapse.

"Within a massive, evolved star (a) the onion-layered shells of elements undergo fusion, forming a nickel-iron core (b) that reaches Chandrasekhar-mass and starts to collapse. The inner part of the core is compressed into neutrons (c), causing infalling material to bounce (d) and form an outward-propagating shock front (red). The shock starts to stall (e), but it is re-invigorated by neutrino interaction. The surrounding material is blasted away (f), leaving only a degenerate remnant".- From https://en.wikipedia.org/wiki/Core_collapse_supernova.