IB Physics Topic E.5.2 Evolution of Stars Revision Notes (SL/HL)

IB Physics Topic E.5.2 Evolution of Stars Revision Notes (SL/HL) | RevisionDojo

Written by official IB examiners

Stellar Stability, Evolution, and End States

Stars are engaged in a constant tug-of-war between two opposing forces: gravity and radiation pressure.
Gravity, driven by the star’s immense mass, pulls it inward, seeking to collapse it.
Meanwhile, nuclear fusion in the star’s core generates energy that creates an outward radiation pressure, pushing against gravity.
When these forces are perfectly balanced, the star remains stable.

Why Does Fusion Create Outward Pressure?

At the core of a star, hydrogen nuclei (protons) fuse to form helium through nuclear fusion.
Fusion releases energy, and this energy radiates outward, generating pressure that pushes back against the inward pull of gravity.
Stable stars maintain equilibrium through the balance of inward gravitational pressure and outward radiation pressure generated by nuclear fusion.

Example

For main sequence stars like our Sun, this balance can persist for billions of years.

Common Mistake

It’s a common misconception that radiation pressure comes from the star’s heat.
In reality, it is the energy released during nuclear fusion that provides the outward force.

Note

Nuclear fusion was covered in article E.5.1 in more detail.

Classifying Stars: The Hertzsprung-Russell (HR) Diagram

The HR diagram is one of the most important tools in astrophysics.
It plots luminosity (brightness) on the vertical axis and surface temperature on the horizontal axis, with temperature increasing to the left.

Note

By placing stars on this diagram, astronomers can classify them and infer their properties.

Main Features of the HR Diagram

Main Sequence:
1. A diagonal band where most stars, including the Sun, are found.
2. These stars are fusing hydrogen into helium in their cores.
Red Giants and Supergiants:
1. Found in the upper-right region of the diagram.

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Stellar Stability, Evolution, and End States

Stars are engaged in a constant tug-of-war between two opposing forces: gravity and radiation pressure.
Gravity, driven by the star’s immense mass, pulls it inward, seeking to collapse it.
Meanwhile, nuclear fusion in the star’s core generates energy that creates an outward radiation pressure, pushing against gravity.
When these forces are perfectly balanced, the star remains stable.

Why Does Fusion Create Outward Pressure?

At the core of a star, hydrogen nuclei (protons) fuse to form helium through nuclear fusion.
Fusion releases energy, and this energy radiates outward, generating pressure that pushes back against the inward pull of gravity.
Stable stars maintain equilibrium through the balance of inward gravitational pressure and outward radiation pressure generated by nuclear fusion.

Example

For main sequence stars like our Sun, this balance can persist for billions of years.

Common Mistake

It’s a common misconception that radiation pressure comes from the star’s heat.
In reality, it is the energy released during nuclear fusion that provides the outward force.

Note

Nuclear fusion was covered in article E.5.1 in more detail.

Classifying Stars: The Hertzsprung-Russell (HR) Diagram

The HR diagram is one of the most important tools in astrophysics.
It plots luminosity (brightness) on the vertical axis and surface temperature on the horizontal axis, with temperature increasing to the left.

Note

By placing stars on this diagram, astronomers can classify them and infer their properties.

Main Features of the HR Diagram

Main Sequence:
1. A diagonal band where most stars, including the Sun, are found.
2. These stars are fusing hydrogen into helium in their cores.
Red Giants and Supergiants:
1. Found in the upper-right region of the diagram.

Definition

Paywall

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Duis aute irure dolor in reprehenderit

Note

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam quis nostrud exercitation.

Excepteur sint occaecat cupidatat non proident

Tip

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris.
Duis aute irure dolor in reprehenderit in voluptate velit esse cillum.

Topic A - Space, time and motion5 subtopics

Topic B - The particulate nature of matter5 subtopics

Topic C - Wave behaviour5 subtopics

Topic D - Fields4 subtopics

Topic E - Nuclear and quantum physics5 subtopics

IB Physics E.5.2 Evolution of Stars Notes

Stellar Stability, Evolution, and End States

Why Does Fusion Create Outward Pressure?

Classifying Stars: The Hertzsprung-Russell (HR) Diagram

Main Features of the HR Diagram

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Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

Unlock the rest of this chapter with a free account

Stellar Stability, Evolution, and End States

Why Does Fusion Create Outward Pressure?

Classifying Stars: The Hertzsprung-Russell (HR) Diagram

Main Features of the HR Diagram

anim nostrud sit dolore minim proident quis fugiat velit et eiusmod nulla quis nulla mollit dolor sunt culpa aliqua

Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

Unlock the rest of this chapter with a free account

Topic A - Space, time and motion5 subtopics

Topic B - The particulate nature of matter5 subtopics

Topic C - Wave behaviour5 subtopics

Topic D - Fields4 subtopics

Topic E - Nuclear and quantum physics5 subtopics