Extreme Physics in the Cosmos
Astrophysics applies the principles of physics to understand astronomical objects and phenomena.
Stellar Nucleosynthesis
This is the process by which elements are created within stars by nuclear fusion.
Main Sequence Stars (like the Sun): Fuse hydrogen into helium (Proton-Proton chain or CNO cycle).
Massive Stars: As they evolve, they develop an 'onion-like' structure, fusing progressively heavier elements in their core and in shells around the core: Hydrogen → Helium → Carbon → Neon → Oxygen → Silicon → Iron.
Supernovae: Iron is the last element that can be produced by fusion with a net release of energy. All elements heavier than iron are created during the intense conditions of a supernova explosion through processes like neutron capture.
Supernovae: Cosmic Explosions
A supernova is a powerful and luminous stellar explosion. There are two main types:
Type II Supernova (Core-Collapse): The death of a single, massive star. When the core can no longer support itself via fusion, it collapses catastrophically, and the outer layers rebound in a massive explosion. It leaves behind a neutron star or a black hole.
Type Ia Supernova (Thermonuclear): Occurs in a binary star system. A white dwarf accretes matter from a companion star. When the white dwarf's mass exceeds a critical limit (Chandrasekhar limit, about 1.4 solar masses), it triggers runaway carbon fusion, completely obliterating the star.
Because they always explode at the same mass, Type Ia supernovae have a very consistent peak luminosity, making them excellent 'standard candles' for measuring cosmic distances.
Pulsars
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation.
This beam sweeps across space like a lighthouse beam. If the beam happens to sweep across the Earth, we observe a regular 'pulse' of radiation.
They spin incredibly fast (some hundreds of times per second) due to the conservation of angular momentum during the core collapse.
Escape Velocity
Escape velocity is the minimum speed needed for a free, non-propelled object to escape from the gravitational influence of a massive body.
Formula: v_esc = √(2GM/R)
The escape velocity from Earth's surface is about 11.2 km/s.
The event horizon of a black hole is defined as the radius at which the escape velocity equals the speed of light.