Stars are massive, luminous celestial objects that generate light and heat through nuclear fusion in their cores, which are composed primarily of hydrogen and helium. They undergo various stages of evolution, from formation in nebulae to becoming main-sequence stars and eventually ending as white dwarfs, neutron stars, or black holes. The study of stars provides insights into the fundamental processes of the universe and the lifecycle of celestial objects.

1. 1. Formation

• Nebula: Stars form from clouds of gas and dust called nebulae. Under the influence of gravity, these clouds collapse and condense, leading to the formation of a protostar.
• Nuclear Fusion: When the core temperature of the protostar becomes high enough, nuclear fusion reactions begin, converting hydrogen into helium and releasing energy.

1. 2. Structure

• Core: The central region where nuclear fusion occurs. It’s extremely hot and dense.
• Radiative Zone: The layer surrounding the core where energy is transported outward through radiation.
• Convective Zone: The outer layer where energy is transported by convection currents.
• Photosphere: The visible surface of the star that emits light and heat.
• Chromosphere: The layer above the photosphere, often visible during solar eclipses as a reddish rim.
• Corona: The outermost layer of a star’s atmosphere, which can extend millions of kilometres into space and is visible as a halo during a solar eclipse.

1. 3. Types of Stars

• Main Sequence Stars: Stars in the stable phase of their lifecycle, where they fuse hydrogen into helium. Examples include the Sun and most of the stars in the universe.
• Red Giants: Large, luminous stars with expanded outer layers that are cooler and appear red.
• White Dwarfs: Dense, hot remnants of stars that have exhausted their nuclear fuel. They are the final evolutionary state of low to medium-mass stars.
• Neutron Stars: Extremely dense remnants of supernova explosions, composed mostly of neutrons.
• Black Holes: The remnants of very massive stars that have collapsed under their own gravity to a point where not even light can escape.

1. 4. Lifecycle

• Main Sequence: The longest phase of a star’s life where it fuses hydrogen into helium.
• Red Giant/Supergiant: The star expands and cools after exhausting hydrogen in the core.
• End States: Depending on the star’s mass, it can end its life as a white dwarf, neutron star, or black hole.

1. 5. Classification

• Hertzsprung-Russell Diagram: A chart used to classify stars based on their luminosity and temperature. It includes categories such as main sequence, giants, and dwarfs.
• Spectral Types: Stars are classified by their spectra into types O, B, A, F, G, K, and M, from hottest to coolest.

1. 6. Examples

• • • The Sun: A G-type main-sequence star (G dwarf) and the closest star to Earth.
    • Sirius: The brightest star in the night sky, classified as an A-type main-sequence star.
    • Betelgeuse: A red supergiant star in the constellation Orion.