Low Mass Star Lifecycle

Initial stages (all masses)

• The life cycle of a star follows predictable stages. The exact route a star's development takes depends on its initial mass.
• The first four stages in the life cycle of stars are the same for stars of all masses: nebula, protostar, main sequence star, and then the lifecycle branches depending on the star's mass.
• Low mass: stars with a mass of less than about 8 times the mass of the Sun ($< 8M_\odot$). The Sun is assumed to be a low mass star and follows this evolution.
• High mass: stars with a mass of more than about 8 times the mass of the Sun ($> 8M_\odot$).

1. Nebula

• All stars form from a giant cloud of hydrogen gas and dust called a nebula. Gravitational attraction between individual atoms forms denser clumps of matter (gravitational collapse).

2. Protostar

• The gravitational collapse causes the gas to heat up and glow, forming a protostar. The temperature eventually reaches millions of degrees kelvin and fusion of hydrogen nuclei to helium nuclei begins.

3. Main sequence star

• The star reaches a stable state when the inward and outward forces are in equilibrium. The gas pressure and radiation pressure act outwards to balance the gravitational force acting inwards.
• A star will spend most of its life on the main sequence. About 90% of stars are on the main sequence at any given time.
• Main sequence stars can vary in mass from about 10% of the mass of the Sun to 200 times the mass of the Sun.
• The Sun has been on the main sequence for 4.6 billion years and will remain there for an estimated 6.5 billion years.

4. Red giant

• Hydrogen fuelling the star begins to run out. Most of the hydrogen nuclei in the core have been fused into helium. Nuclear fusion slows and the energy released by fusion reactions decreases.
• The star initially shrinks and compresses the core until fusion can continue in the shell around the core.
• Once fusion reactions start again, the outer layers expand and cool as a red giantA large, low-temperature, luminous star in which helium nuclei are fused into more massive nuclei such as beryllium, carbon and oxygen. It forms when hydrogen in the core is exhausted. forms.

5. Planetary nebula

• The outer layers of the star are released. Core helium burning releases massive amounts of energy in fusion reactions, ejecting the outer layers into space.
• The carbon-oxygen core is not hot enough for further fusion, so the core collapses.

6. White dwarf

• The solid core collapses under its own mass, leaving the remnant of the core called a white dwarf.
• A white dwarf is an extremely dense, hot star powered by the gravitational potential energy released as it contracts, rather than by nuclear fusion.
• The key part is the complete sequence: nebula → protostar → main sequence → red giant → planetary nebula → white dwarf (and eventually a black dwarf as it cools).