Supernovae are broadly divided into two types: Type I, which shows hydrogen in the spectrum, and Type II, which does not. Type III, IV, and V supernovae have been proposed but not confirmed as distinct types of events.
Supernovae come in two types: core-collapse supernovae and Type Ia supernovae.
Core-Collapse Supernovae (Type II, Type Ib and Type Ic)
When we usually think of a supernova, we think of core-collapse supernovae, the dramatic end of short-lived massive stars. Core-collapse supernovae primarily occur in stars with original masses exceeding roughly eight times that of our Sun. Such behemoths burn through their nuclear fuel at a ferocious rate, sequentially fusing heavier and heavier elements in their cores, progressing from hydrogen to helium, carbon, neon, oxygen, and finally silicon. Each fusion stage lasts a shorter duration than its predecessor, with silicon fusion lasting merely days.
The culmination of this fusion cascade is the formation of an iron core. Iron atoms cannot be fused to release more energy than it takes to combine its protons, electrons, and neutrons into heavier atoms. Thus, the furnace that has powered the star and supported it against gravity’s pull for millions of years suddenly extinguishes, unable to produce more energy than it uses to sustain itself.
With the cessation of fusion, the iron core is rendered inert, and an unsettling silence descends. Deprived of its energy source and overwhelmed by its own gravitation, the core implodes rapidly. This contraction causes protons and electrons to merge, forming neutrons and releasing a vast number of neutrinos.
Almost paradoxically, the core’s implosion rebounds. The outer layers of the star, still hurtling inward, encounter a now-rigid neutron core and a surge of neutrinos. This results in a shock wave that races outward, tearing the star apart in a titanic explosion.
Post-explosion, the core’s fate depends on its mass. A core between 1.4 and 3 times the Sun’s mass becomes a neutron star, an incredibly dense object where a sugar-cube-sized amount of material would weigh as much as a mountain. If the core’s mass exceeds 3 solar masses, its gravitational pull becomes unstoppable, collapsing it into a black hole, a region of space where gravity is so strong that nothing, not even light, can escape.
Most core-collapse supernovae are classified as Type II, though Type Ib and Ic supernovae are also core-collapse supernovae. Type Ib and Ic supernovae are caused by stars that have shed more of their outer layers; Type Ib supernovae lack hydrogen and silicon, while Type Ib lacks helium as well.
Type Ia Supernovae
Unlike core-collapse supernovae, Type Ia Supernovae don’t originate from the death of a single massive star. Instead, a Type Ia supernova is the result of a catastrophic event in a binary system with a white dwarf, like a nova. If the white dwarf accumulates enough matter—either from a companion star or by merging with another white dwarf—it can surpass the Chandrasekhar limit, triggering a thermonuclear explosion that completely obliterates the star.
Type Ia supernovae are extremely important. Due to their consistent intrinsic brightness, they serve as “standard candles” for astronomers, helping measure cosmic distances and the expansion rate of the universe. Type Ia supernovae were pivotal in the 1998 revelation that the universe’s expansion is accelerating, a discovery that led to the Nobel Prize in Physics in 2011.
