Supernovae
A supernova is an explosive event in which a star brightens dramatically over a very short time, producing a large change on the absolute-magnitude scale.
Two routes described in stellar evolution lead to supernovae:
| Type | Origin | Trigger |
|---|---|---|
| Type II | High-mass supergiant star | The stellar core collapses after its nuclear fuel is exhausted |
| Type Ia | White dwarf in a binary system | The white dwarf gains matter from its companion until a critical mass is reached |
In the high-mass route, the core collapses rapidly when fusion can no longer support the star. The outer layers fall inwards and are driven back out in a powerful shock wave, ejecting stellar material into space.
The extreme conditions associated with the shock wave allow nuclei heavier than iron to be produced and expelled into the surrounding space.
Gamma-ray bursts
A gamma-ray burst, GRB, is a brief and extremely energetic burst of gamma radiation associated with the collapse of a supergiant star. The collapse leaves a compact remnant, forming a neutron star or, for a sufficiently massive core, a black hole.
- GRBs can release about to .
- In a very short time, the energy released can be comparable with the Sun's total output over its roughly -billion-year lifetime.
- The gamma radiation can be strongly collimated into narrow beams.
- These beams are emitted from the polar regions of the collapsing star.
Exam Tip: If asked to compare a gamma-ray burst with the Sun, state both the enormous energy released and the very short timescale over which the burst occurs.