Main Sequence Stars and the Sun

This lesson covers the properties of main sequence stars and the Sun in detail — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand how the Sun produces energy, the relationship between a star's mass, temperature, luminosity and lifetime, and have awareness of the Hertzsprung-Russell diagram.

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The Sun — A Typical Main Sequence Star

The Sun is classified as a main sequence star. It is a medium-sized star roughly halfway through its expected lifetime.

Property	Value
Age	Approximately 4.6 billion years old
Expected total lifetime	Approximately 10 billion years
Time remaining	Approximately 5 billion years
Core temperature	Approximately 15 million °C
Surface temperature	Approximately 5,500 °C
Composition	Mainly hydrogen (~73%) and helium (~25%)
Diameter	Approximately 1.4 million km (109 × Earth's diameter)
Mass	Approximately 2 × 10³⁰ kg (330,000 × Earth's mass)

Exam Tip: The Sun is about halfway through its main sequence life. It has been shining for ~4.6 billion years and will continue for another ~5 billion years before it runs out of hydrogen fuel in its core.

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How the Sun Produces Energy

The Sun's energy comes from nuclear fusion in its core:

1. The immense temperature (~15 million °C) and pressure in the core are high enough to overcome the electrostatic repulsion between positively charged hydrogen nuclei (protons).
2. Hydrogen nuclei fuse to form helium nuclei.
3. In this process, a small amount of mass is converted into energy according to Einstein's equation E = mc².
4. This energy is released as electromagnetic radiation (including visible light, infrared, ultraviolet and other wavelengths).
5. The energy travels outward from the core to the surface and is radiated into space.

Why Fusion Requires Extreme Conditions

• Hydrogen nuclei are positively charged (they are protons).
• Like charges repel each other — this is called electrostatic repulsion.
• To overcome this repulsion and get the nuclei close enough to fuse, extremely high temperatures and pressures are needed.
• Only in the core of a star are conditions extreme enough for fusion to occur naturally.

Exam Tip: If asked to explain why nuclear fusion only occurs in the core of a star, mention the extreme temperature and pressure needed to overcome the electrostatic repulsion between positively charged hydrogen nuclei.

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Mass, Luminosity and Temperature

There is a clear relationship between a star's mass and its other properties during the main sequence phase:

Property	Low-Mass Stars	Medium-Mass Stars (like the Sun)	High-Mass Stars
Temperature	Cooler (reddish)	Moderate (yellowish)	Hotter (bluish-white)
Luminosity	Dim	Moderate	Very bright
Lifetime	Very long (tens of billions of years)	~10 billion years	Short (millions of years)
Colour	Red	Yellow	Blue/white

Why More Massive Stars Have Shorter Lifetimes

This seems counterintuitive — you might expect a larger star with more fuel to last longer. However:

• More massive stars have much higher core temperatures and pressures.
• This means fusion occurs much faster — they burn through their hydrogen fuel at a much greater rate.
• Although they have more fuel, they use it up far more quickly.
• A star 10 times the mass of the Sun might only last 20 million years instead of 10 billion years.

Exam Tip: A common exam mistake is saying massive stars live longer because they have more fuel. The opposite is true — they burn fuel much faster, so they have shorter lifetimes.

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The Hertzsprung-Russell (HR) Diagram

The Hertzsprung-Russell diagram is a graph that plots stars according to their luminosity (brightness) against their surface temperature (or spectral class/colour).

Key Features of the HR Diagram

Region	Location on Diagram	Stars Found Here
Main sequence	Diagonal band from top-left (hot, bright) to bottom-right (cool, dim)	Most stars, including the Sun
Red giants/supergiants	Top-right (cool but very bright)	Stars in later stages of life that have expanded
White dwarfs	Bottom-left (hot but very dim)	Small, dense remnants of dead stars

Reading the HR Diagram

• The x-axis shows surface temperature — but note it goes from hot on the left to cool on the right (the scale is reversed).
• The y-axis shows luminosity (brightness) — increasing upward.

What the HR Diagram Tells Us

• Main sequence stars follow a clear pattern: hotter stars are brighter and are positioned towards the top-left; cooler stars are dimmer and are positioned towards the bottom-right.
• Red giants are cool (right side) but very luminous (top) because they are enormous — their huge surface area compensates for their lower temperature.
• White dwarfs are hot (left side) but dim (bottom) because they are very small — their tiny surface area means they emit very little light despite being hot.

graph TD
    A["Hertzsprung-Russell Diagram"] --> B["Main Sequence<br/>(diagonal band)<br/>Hot & bright ↔ Cool & dim"]
    A --> C["Red Giants /<br/>Red Supergiants<br/>(top-right)<br/>Cool but very luminous"]
    A --> D["White Dwarfs<br/>(bottom-left)<br/>Hot but very dim"]

    style A fill:#2c3e50,color:#fff
    style B fill:#f39c12,color:#fff
    style C fill:#c0392b,color:#fff
    style D fill:#bdc3c7,color:#2c3e50

Exam Tip: At GCSE level, you are expected to have an awareness of the HR diagram and be able to identify the three main regions (main sequence, red giants, white dwarfs). You do not need to draw it from memory, but you should be able to interpret one if given it in the exam.

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The Sun's Position on the HR Diagram

The Sun sits roughly in the middle of the main sequence band:

• Surface temperature: ~5,500 °C (moderate — yellow colour).
• Luminosity: moderate (neither the brightest nor the dimmest main sequence star).
• It is a G-type main sequence star (also called a yellow dwarf, although it is actually white when seen from space).

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Star Colours and Temperature