Red Giants, White Dwarfs and Planetary Nebulae

This lesson covers the later stages in the life of a Sun-like star — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand what happens when a medium-mass star runs out of hydrogen fuel, and the stages from red giant to white dwarf.

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When the Hydrogen Runs Out

A main sequence star like the Sun fuses hydrogen into helium in its core for billions of years. Eventually, the hydrogen fuel in the core is exhausted. When this happens:

1. Nuclear fusion in the core slows and eventually stops (there is no more hydrogen to fuse in the core).
2. Radiation pressure decreases — the outward push from fusion weakens.
3. Gravity dominates — the core begins to contract (collapse inward) under its own weight.
4. As the core contracts, it heats up (gravitational potential energy is converted to thermal energy).
5. The increased temperature heats the surrounding shell of hydrogen, which begins to fuse — this is called shell burning.

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The Red Giant Stage

As the core contracts and the hydrogen shell burns, the energy output increases. This causes the outer layers of the star to expand enormously and cool.

Key Features of a Red Giant

Feature	Description
Size	Hundreds of times the diameter of the original main sequence star
Surface temperature	Cooler than the main sequence phase (~3,000–4,000 °C) — hence the red colour
Luminosity	Much brighter than a main sequence star of the same mass (due to the huge surface area)
Core	Contracts and heats up; helium fusion begins when core reaches ~100 million °C
Outer layers	Expand and cool significantly

Helium Fusion

Once the core temperature reaches approximately 100 million °C, a new type of fusion begins:

• Helium nuclei fuse to form carbon (and some oxygen).
• This is sometimes called the triple-alpha process (three helium nuclei, also called alpha particles, fuse to make one carbon nucleus).
• This provides a new source of energy, but helium fusion does not last as long as hydrogen fusion.

Size Comparison

To appreciate the scale of a red giant:

Object	Approximate Diameter
Earth	~12,700 km
The Sun (main sequence)	~1.4 million km (109 × Earth)
A red giant	~100–300 million km (up to 200 × the Sun)

When the Sun becomes a red giant in approximately 5 billion years, it will likely expand to engulf the orbits of Mercury and Venus, and possibly reach Earth's orbit.

Exam Tip: Be clear about the cause-and-effect chain: hydrogen fuel exhausted → core contracts → outer layers expand and cool → star becomes a red giant. Examiners want to see this logical sequence.

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The Planetary Nebula Stage

After the red giant phase, the outer layers of the star are only loosely held by gravity. Eventually:

• The outer layers drift away into space, forming an expanding shell of gas and dust.
• This expanding shell is called a planetary nebula (despite the name, it has nothing to do with planets — the name is historical, as early telescopes made them look like planets).
• The planetary nebula glows because the hot remnant core in the centre emits ultraviolet radiation, which ionises the expanding gas and causes it to fluoresce.

What Is Left Behind

After the outer layers have drifted away, all that remains is the hot, dense core of the star. This remnant is called a white dwarf.

Exam Tip: Do not confuse a "planetary nebula" with the "nebula" at the start of a star's life. A nebula (stellar nursery) is where new stars are born from dust and gas. A planetary nebula is the outer layers shed by a dying star. They are completely different stages.

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The White Dwarf Stage

A white dwarf is the final stage in the life of a Sun-like star.

Key Features of a White Dwarf

Feature	Description
Size	Roughly the size of Earth (~12,000 km diameter)
Mass	Roughly the mass of the Sun (huge mass in a tiny volume)
Density	Extremely high — a teaspoon of white dwarf material would weigh several tonnes
Temperature	Very hot initially (up to ~100,000 °C on the surface)
Energy source	No nuclear fusion — the white dwarf simply radiates stored thermal energy
Fate	Gradually cools over billions of years, eventually becoming a cold, dark black dwarf

Why a White Dwarf Does Not Collapse Further

A white dwarf is supported against further gravitational collapse by electron degeneracy pressure — a quantum mechanical effect where electrons resist being compressed any further. At GCSE level, you simply need to know that a white dwarf is stable and does not collapse.

Cooling Over Time

Since a white dwarf has no fusion occurring, it has no new energy source. It slowly radiates its remaining thermal energy into space:

• Over billions of years, it cools from white-hot to yellow, then orange, then red, and eventually becomes too cool to emit visible light.
• The theoretical end state is a black dwarf — though the universe is not yet old enough for any black dwarfs to exist.

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The Sun's Future

The Sun will follow this exact pathway:

Time from Now	Stage
Now	Main sequence star (middle-aged)
~5 billion years	Hydrogen in core exhausted; Sun begins to expand
~5 billion years	Becomes a red giant (engulfing inner planets)
~5.5 billion years	Outer layers shed as a planetary nebula
~5.5 billion years	Core remains as a white dwarf
Many billions more	Gradually cools toward a black dwarf

Exam Tip: The Sun will NOT explode as a supernova. It does not have enough mass. It will end its life quietly as a white dwarf. Only stars much more massive than the Sun die in supernovae.

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