Edexcel A-Level Physics 9PH0
Working as a Physicist
Mechanics
Electric Circuits
Materials
Waves and Particle Nature of Light
Further Mechanics
Electric and Magnetic Fields
Nuclear and Particle Physics
Thermodynamics
Space
Nuclear Radiation
Gravitational Fields
Oscillations
Space
The life cycle of stars
- 2 things to know
- Papers 2 and 3
Overview
Recall questions on the energy source of a main-sequence star, what keeps it stable, how a Sun-like star moves off the sequence, and why the most massive stars are the shortest lived.
What you need to know
Answer each one before you reveal it.
Two stars have the same luminosity but different surface temperatures. How do their sizes compare?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What is the energy source of a main-sequence star?
Model answer
Nuclear fusion of hydrogen into helium in its core.
- 2
What keeps a main-sequence star at a stable size for most of its life?
Model answer
The outward pressure from the hot radiating core is balanced by the inward gravitational attraction of the star's own mass.
- 3
How does a star begin its life?
Model answer
A cloud of gas and dust collapses under gravity, heating as it contracts until the core is hot and dense enough for hydrogen fusion to begin, at which point the star joins the main sequence.
- 4
Where would a protostar lie relative to the main sequence, and which way does it move?
Model answer
It lies to the right of the main sequence, being cool but luminous because of its size, and it moves leftwards as it contracts and heats up.
- 5
How does a star like the Sun move across the diagram once it leaves the main sequence?
Model answer
As core hydrogen runs out the star swells and its surface cools, so it moves up and to the right into the red giant region.
- 6
What is a planetary nebula?
Model answer
It is the shell of outer gas layers thrown off by a dying red giant, leaving the hot core exposed.
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