Edexcel A Level (IAL) Physics-5.44 The Life Cycle of Stars- Study Notes- New Syllabus

Relating the Hertzsprung–Russell Diagram to the Life Cycle of Stars

The Hertzsprung–Russell (H–R) diagram not only classifies stars, but also shows how stars change position as they evolve during their life cycle.

Birth of a Star (Protostar Stage)

Stars form from large clouds of gas and dust called nebulae.

• Gravitational collapse increases temperature.
• Nuclear fusion has not yet begun.
• The object is called a protostar.

H–R diagram position:

• Low temperature
• Low luminosity
• Bottom-right region

Main Sequence Stage

When nuclear fusion of hydrogen begins in the core, the star enters the main sequence.

• Hydrogen fuses into helium.
• Outward radiation pressure balances gravity.
• This is the longest stage of a star’s life.

H–R diagram position:

• Diagonal band from top-left to bottom-right.
• Massive stars are hot and luminous (upper-left).
• Low-mass stars are cool and dim (lower-right).

 Red Giant or Red Supergiant Stage

When hydrogen in the core is exhausted:

• Core contracts and heats up.
• Outer layers expand and cool.
• Luminosity increases greatly.

H–R diagram movement:

• Star moves upward and to the right.
• Becomes a red giant (low-mass star).
• Becomes a red supergiant (high-mass star).

Final Stages of Low-Mass Stars (e.g. Sun)

For stars similar in mass to the Sun:

• Outer layers are ejected.
• Core remains as a white dwarf.

White dwarf properties:

• High temperature
• Low luminosity
• No nuclear fusion

H–R diagram position:

• Bottom-left region

 Final Stages of High-Mass Stars

For stars much more massive than the Sun:

• Fusion continues to heavier elements.
• Ends in a supernova explosion.
• Core collapses into a neutron star or black hole.

H–R diagram relevance:

• Rapid movement off the main sequence.
• Extremely high luminosity before collapse.