Stellar characteristics and stellar evolution

Stellar Characteristics

Stars are celestial bodies made mostly of hydrogen and helium that produce light and heat from nuclear reactions (nuclear fusion).
Stars are categorised by spectral class (O, B, A, F, G, K, or M), with each class corresponding to a range of surface temperatures and colours.
Stars fall into three broad sequences: dwarfs, giants and supergiants, depending on their luminosity and temperature.
Temperature and colour are related; hot stars are bluish and cooler stars are redder.
An individual star’s specific classification is represented by a letter-number combination. For example, our sun is a G2 star.

Stellar Evolution

Stars are born within the interstellar clouds of dust and gas called nebulae.
Stars go through a lifecycle over billions of years, evolving through stages: Protostar – Main Sequence Star – Red Giant /Red Super Giant – Planetary Nebula/Supernova – White Dwarf/Neutron Star/Black Hole.
A star spends the most significant part of its life in the main-sequence phase, where it’s in equilibrium as it fuses hydrogen into helium.
After the main-sequence phase, depending on its mass, a star can become a red giant or a supergiant, where helium fusion begins.
When fusion ends, the star will eject its outer layers as a planetary nebula or explode in a supernova, leaving only the core behind.
Depending upon the mass of the original star, it could then become a white dwarf, neutron star, or black hole.

Hertzsprung-Russell (HR) Diagram

The HR diagram classifies stars by their luminosity, spectral class, temperature, and intrinsic colour.
On the HR diagram, the vertical axis is the Luminosity or Absolute magnitude, and the horizontal axis is the Temperature or spectral type.
The main sequence, giants, supergiants, and white dwarfs all have different positions on the HR diagram.
Stars move within the plot on the HR diagram as they evolve and change their characteristics.

Stellar Nuclear Fusion

Nuclear fusion is the process by which energy is produced in the core of a star.
Stars initially fuse hydrogen atoms to form helium in a process called the proton-proton chain.
Heavier stars will continue fusion processes to create elements up to iron.
Fusion requires a high temperature and pressure, and produces a huge amount of energy, responsible for a star’s glow.

Stellar Nucleosynthesis

Stellar nucleosynthesis is the process by which elements are created within stars due to nuclear fusion.
All elements heavier than helium are produced in the stars.
The process of nucleosynthesis includes several paths like the proton-proton chain, carbon-nitrogen-oxygen (CNO) cycle, and triple-alpha process.
Heaviest elements (heavier than iron) are formed during supernova explosions.