We apologize for the absence, as we have been extremely and unexpectedly busy.
It’s Christmas time folks! And what do we have at the top of the tree (if you celebrate)? A star, right? The perfect reminder to bring us back to our lovely discussion on stars! So, we have discussed the life of a star. Now things get interesting. We have reached the end of a star’s lonely existence. Yet another difference between a star’s end and a human’s end is that in some cases a star can have one of the most awesome events in the universe: a supernova. Summarized, it is an explosion of a star. But in detail there are multiple types, all fascinating.
Each type has a progenitor, which makes spectra important as they show the amount of elements present and rate of fusion. For example, during an SN enough energy is released such that higher than average fusion can take place. So, if you ever wondered where iron or even heavier than iron elements came from, thank SNe. Other ways to determine the type follow similar reasoning, including light curves, energy, and progenitor events. The main summary of the types of SNe is the popular Minkowski-Zwicky scheme or classification. We will now begin splitting up these wonderful things, despite the fact that SNe involve fusion. There are two types: I and II. A major way to characterize them is by spectra. Type I SNe lack Balmer hydrogen emission lines, while type II SNe show Balmer hydrogen emission lines in their spectra.
The Type Ia (thermonuclear) SN is the most different. It consists of a binary pair of two stars, one being a white dwarf. As stated previously in the WD, PN, Type Ia post , they go through an uncontrolled thermonuclear explosion as the white dwarf becomes unstable upon reaching the Chandrasekhar limit. The mechanism of explosion is important to note as the other three main types explode from gravitational collapse.
The other three main types are known as core-collapse SNe, which we explained at the end of our post on high-mass stellar evolution. As the name indicates they collapse from the sheer amount of matter in one space, creating the appropriately named implosion-explosion event. Type Ib and Ic are thought to mostly come from WR stars, so they appeared to be very similar. But they were differentiated by spectra, where the Type Ib has shed its hydrogen shell before collapsing, while the Type Ic has shed its helium shell too. (Despite being caused by totally different mechanisms, both Type Ia and Type Ic lack hydrogen and helium lines; they are differentiated by the strong silicon lines in Type Ia.) As we said last time, the shedding of layers before explosion makes them sometimes named stripped-core collapse SNe. Type II SNe are massive stars, of generally about 8 to 12 solar masses that have enough mass to collapse and then either continue collapsing or explode.
They also differ by location. Type Ia are located in most areas as they only require a white dwarf and a binary system, and this is a more likely outcome for most stars. Type Ib, Ic, and II all are generally in the galactic spiral arms in areas of relatively recent star formation since they require more massive, population I stars, which have to be massive enough to explode.
Another difference involves energy and light curves. Type Ia SNe have the highest and most consistent peak; the rest depend on mass. Also, out of the SNe, the only one with the possibility for a plateau or non-linear falling luminosity is the type II SN. Visually the light curves are like so (click to enlarge):
Next is the Type II SN and its core-collapse. As the core collapses, protons and electrons are pushed together, forming neutrons and neutrinos. This accounts for the high neutrino signatures that come from these types. In fact, the relatively recent issue of faster than light neutrinos involved this very type of SN! The neutrinos have such high densities that they develop radiation pressure, but the star essentially collapses in on itself, rebounds, and creates a shock wave. These effects are named neutrino outburst and rebound shock. Type II SNe can also be split up into types P, L, n, and b. Type II-P and II-L stand for plateau and linear, which are the shapes of the light curve. The plateau type forms from hydrogen ionizing after the explosion, increasing luminosity, while the linear type fully expels its hydrogen layer. Type IIn SNe are detected by narrow hydrogen emission lines in spectra, sometimes thought to be caused by LBVs. Type IIb SNe show weak hydrogen lines initially, but the lines later become untraceable and it starts to resemble the spectra of a Type Ib SN. The famous example of this is Cassiopeia A. Another very high mass explosion is the hypernova, or a pair instability SN. The pair instability occurs when atoms collide with gamma rays to form electrons and positrons, which creates a pressure to slightly reduce the core’s overall pressure. This leads to a large-scale collapse.
For all the supernovae remnants (SNRs) can form. For type Ia SNe they will always be large nebulae, but the other types can leave massive objects too. These include a neutron star or a black hole. The cause for the neutron star is that neutronization or neutrino outburst from neutron degeneracy pressure, analogous to the electron degeneracy pressure holding up a white dwarf, stops full collapse. Black holes, on the other hand, collapse to a point. Both objects have very odd and interesting properties.
To finish off we aren’t fooling that there are also supernova impostors, originally thought to be a type V SN. These are also possibly LBV eruptions, but they don’t seem to be full supernovae. As stated LBVs are unstable, so they can throw off large amounts of mass and heat that up. They are recent but important events that are still being researched.
The easiest summary of supernovae is that they are characterized by many ways. But they have great importance. They explain how we could have any jewelry (in case you wanted to get some for a holiday present). Even greater, they allow for a better understanding of the universe. This is by giving ways to calibrate distance measures and understand properties of stars. Just to the point, they’re awesome. It’s not Thanksgiving, but we should certainly thank SNe anyway.
References and further reading:
SNe in general:
Type Ib+Ic SNe:
Type II SNe: