Life Cycle of a Low Mass Star (AQA A Level Physics)

Evolution of a Low Mass Star

• The life cycle of a star follows predictable stages

• The exact route a star's development takes depends on its initial mass

Initial stages for all masses

• The first four stages in the life cycle of stars are the same for stars of all masses

• After these stages, the life-cycle branches depending on whether the star is:

  • Low mass: stars with a mass of less than about 8 times the mass of the Sun (< 8M_Sun)

    • 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_Sun)

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

  • This inward movement of matter is called gravitational collapse

2. Protostar

• The gravitational collapse causes the gas to heat up and glow, forming a protostar

  • Work done on the particles of gas and dust by collisions between the particles causes an increase in their kinetic energy, resulting in an increase in temperature

  • Protostars can be detected by telescopes that can observe infrared radiation

• Eventually, the temperature will reach millions of degrees Kelvin and the fusion of hydrogen nuclei to helium nuclei begins

  • The protostar’s gravitational field continues to attract more gas and dust, increasing the temperature and pressure of the core

  • With more frequent collisions, the kinetic energy of the particles increases, increasing the probability that fusion will occur

3. Main Sequence Star

• The star reaches a stable state when the inward and outward forces are in equilibrium

  • As the temperature of the star increases and its volume decreases due to gravitational collapse, the gas pressure increases

• The star joins the main sequence when fusion reactions begin in the star's core

• A main sequence star is one in which radiation pressure is produced by the thermonuclear fusion of hydrogen nuclei into helium nuclei

Forces acting on a main sequence star

The balanced inward and outward forces will remain that way for millions, or even billions of years

• A star will spend most of its life on the main sequence

  • 90% of stars are on the main sequence

  • Main sequence stars can vary in mass from ~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

Next Stages for Low Mass Stars

• The fate of a star beyond the main sequence depends on its mass

  • The cut-off point for a low-mass star is less than about 8 times the mass of the Sun

  • A low-mass star will become a red giant before turning into a white dwarf

Evolution of a Low-Mass Star

The lifecycle of a low mass star

4. Red Giant

• Hydrogen fuelling the star begins to run out

  • Most of the hydrogen nuclei in the core of the star have been fused into helium

  • Nuclear fusion slows

  • 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 giant forms

• A red giant is a large, low-temperature, luminous star in which helium nuclei are fused into more massive nuclei such as beryllium, carbon and oxygen

 5. Planetary Nebula

• The outer layers of the star are released

• Core helium burning releases massive amounts of energy in fusion reactions

 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