Stellar Explosions

Nova: A white dwarf in a binary system that experiences a sudden increase in luminosity, followed by a slow decline back to its original brightness.
Mechanism: Hydrogen accretes onto the white dwarf from its companion, leading to a runaway nuclear fusion reaction, resulting in a bright outburst.
Location of Accretion: Material accretes onto an accretion disk, not directly onto the star.
Recurrent Novae: White dwarfs in binary systems can undergo repeated nova events.
Process:
- Material from the main-sequence companion falls onto the white dwarf.
- Fusion reignites suddenly when enough material has accreted, burning off the new material.
- Material keeps transferring, and the process repeats.
RS Ophiuchi: An example of a recurrent nova that brightened significantly in August 2021.

Novae can occur multiple times, while supernovae are one-time events that destroy the progenitor star.
Supernovae have higher luminosity than novae.

White dwarfs are supported by electron degeneracy pressure.
Chandrasekhar Limit: The maximum mass a white dwarf can have while still being supported by electron degeneracy, which is approximately $1.4$ solar masses.
If a white dwarf exceeds this limit, electron degeneracy can no longer prevent core collapse.
This leads to carbon fusion throughout the star almost simultaneously, resulting in a Type I supernova (carbon explosion).

High-mass stars can fuse elements in their cores up to iron.
Fusion reactions proceed faster as heavier elements are fused.
Example: A $20$ -solar-mass star burns carbon for about $10,000$ years but its iron core lasts less than a day.
Iron is the crossing point; when the core has fused to iron, no more fusion can take place because the reaction is no longer energetically favorable.
The core collapses due to enormous inward pressure, and the temperature increases.
Iron is broken down into protons, electrons, and neutrons: $p + e^- \rightarrow n + \nu$
Neutron degeneracy can keep the core collapsing, but the star overshoots equilibrium.
This causes a violent rebound and shockwaves that move through the star at high speed, resulting in a Supernova Type II.
The exploding star blows its shells and all elements into space and also creates some naturally occurring elements heavier than iron, although not all or in the amounts we observe.

Supernovae leave remnants—expanding clouds of material from the explosion.
The Crab Nebula (M1) is a remnant from a supernova explosion observed by Chinese astronomers in 1054.

Supernovae are incredibly luminous, more than a million times brighter than novae.

There are $81$ stable and $10$ radioactive elements on Earth.
Elements are created through stellar processes and supernovae.
Elements up to iron are formed in the cores of high-mass stars and released during Supernova Type II.
Most white dwarfs contain carbon, which is released during Supernova Type I; this contributes to carbon's high abundance.
All elements up to iron are formed within the cores of stars.
Iron is the most stable nucleus and cannot be fused.
Some heavier elements (above iron) are made during the first few seconds of a supernova explosion.
The heaviest elements are formed during the merger of two neutron stars.

Novae involve a white dwarf in a binary system that brightens and fades; mass is transferred to the accretion disk.
Novae can happen multiple times, unlike Supernova Type I.
Two types of Supernovae:
- Type I: Carbon-detonation supernova.
- Type II: Core-collapse supernova.
Type I supernovae also occur in binary systems, are much brighter, and happen only once.
Type I occurs when mass transfers onto a white dwarf, and it explodes when it exceeds the Chandrasekhar limit ( $1.4$ solar masses).
Stars greater than eight solar masses can fuse elements up to iron.
The iron core collapses, rebounds, and explodes as a Supernova Type II.
All elements heavier than helium (up to iron) are formed in stars.
Some elements heavier than iron are created during supernova explosions.
These elements are blown into space and form 2nd generation stars.