Supernovae

For a few brief weeks, a single exploding star can outshine every other star in its galaxy combined. A supernova is the most violent event in stellar evolution — and, remarkably, the name covers two completely different physical processes that happen to look similar from the outside.

What a supernova looks like

A supernova is a sudden, extremely energetic explosive event marking the violent end (or transformation) of certain stars' lives. Observationally, a supernova is defined by a characteristic light curve: an extremely rapid, dramatic rise in absolute magnitude (brightness) over a period of days, followed by a much slower decline over weeks to months as the ejected material expands and cools. This is broadly similar in spirit to the Type Ia light curve introduced in the Standard Candles lesson, though as this lesson makes clear, the underlying cause is not always the same.

What makes supernovae worth treating as a single lesson, even though (as covered below) there are two quite different physical mechanisms behind them, is this observational feature they share: an object that was a normal, comparatively faint star suddenly and briefly becomes one of the most luminous single objects in its entire galaxy.

Two very different routes to a supernova

This topic has already introduced the physics behind both of the main supernova mechanisms in earlier lessons — this section brings them together and makes the distinction between them explicit, since both are simply called "supernovae" despite being physically very different events.

Core-collapse supernovae

As covered in the Neutron Stars and Black Holes lessons, a sufficiently massive star eventually exhausts the nuclear fuel available to its core, at every stage from hydrogen through to progressively heavier elements. Once the core can no longer generate enough energy through fusion to support itself against gravity, it collapses catastrophically, on a timescale of a fraction of a second. This sudden collapse releases a colossal amount of gravitational potential energy, triggering a violent explosion that blows the star's outer layers off into space at very high speed, while the core itself collapses into either a neutron star or a black hole, depending on its final mass. This is a core-collapse supernova.

Type Ia (thermonuclear) supernovae

As covered in the White Dwarfs and Standard Candles lessons, a white dwarf in a binary system can gradually accrete matter pulled from a companion star. If enough matter accretes that the white dwarf's mass approaches the Chandrasekhar limit (roughly 1.4 solar masses), the resulting increase in core temperature and pressure can trigger a runaway thermonuclear reaction that detonates the entire white dwarf in one explosive event. This is a Type Ia supernova — and because it always occurs at essentially the same white dwarf mass, it produces explosions of very consistent peak luminosity, which is exactly why Type Ia supernovae are used as standard candles for measuring cosmological distances.

Just how much energy is released?

The scale of energy involved in a supernova is difficult to overstate. A single supernova can, for a period of days to weeks, radiate roughly as much energy as the Sun will radiate over its entire main-sequence lifetime.

Bringing this topic together

Across this topic, the full range of ways a star's life can unfold has now been covered. Every star begins with the gravitational collapse of a nebula (Formation of Stars) and, once hydrogen fusion ignites, spends most of its life in the stable main-sequence phase (Main Sequence Stars). Once core hydrogen is exhausted, the star swells into a red giant, powered by shell burning and, if the core gets hot enough, helium fusion (Red Giants). From there, its fate depends on mass: a low-to-intermediate mass star sheds its outer layers and settles as a white dwarf, supported by electron degeneracy pressure (White Dwarfs) — unless that white dwarf later accretes enough matter to reach the Chandrasekhar limit and detonates as a Type Ia supernova, as covered in this lesson. A sufficiently massive star instead ends with core collapse: a supernova explosion leaving behind either a neutron star, supported by neutron degeneracy pressure (Neutron Stars), or, for the most massive cores, a black hole from which nothing can escape (Black Holes).