Composition of the Sun

Why This Matters

The Sun isn't just a glowing ball in the sky—it's a layered nuclear reactor that powers our entire solar system. When you study solar composition, you're learning about stellar structure, energy transfer mechanisms, and the physics of plasma. These concepts show up repeatedly in questions about stellar evolution, the Hertzsprung-Russell diagram, and how stars generate energy through fusion. Understanding what the Sun is made of and how it's organized helps you answer broader questions about why stars behave the way they do.

You're being tested on your ability to connect chemical composition to physical processes. Why does hydrogen matter? Because it's fusion fuel. Why are there different layers? Because energy moves differently through different densities and temperatures. Don't just memorize percentages and temperatures—know what concept each component illustrates and how the layers work together as a system.

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Chemical Building Blocks

The Sun's composition tells us about both its origin and its ongoing nuclear processes. The relative abundances of elements reflect primordial Big Bang nucleosynthesis plus billions of years of stellar fusion.

Hydrogen

• Comprises ~74% of the Sun's mass—this abundance makes sustained nuclear fusion possible over billions of years
• Primary fusion fuel that converts to helium through the proton-proton chain reaction in the core
• Determines the Sun's main-sequence lifetime—when hydrogen runs out, the Sun will evolve into a red giant

Helium

• Accounts for ~24% of solar mass—the second most abundant element, produced as fusion "ash"
• Byproduct of hydrogen fusion that accumulates in the core over the Sun's lifetime
• Will become fusion fuel later—when the Sun exhausts core hydrogen, helium fusion will begin in the red giant phase

Metals (Heavier Elements)

• Only ~2% of solar mass but includes carbon, oxygen, nitrogen, and iron—astronomers call all elements heavier than helium "metals"
• Inherited from previous stellar generations—these elements were forged in earlier stars and supernovae
• Affect opacity and energy transfer—even trace amounts influence how radiation moves through the Sun's interior

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The State of Solar Matter

Understanding why the Sun behaves as it does requires knowing what state its matter is in. This isn't ordinary gas—it's something far more dynamic.

Plasma State

• Fourth state of matter where electrons are stripped from atoms due to extreme temperatures—the Sun is 100% plasma
• Conducts electricity and responds to magnetic fields—this explains sunspots, flares, and the solar magnetic cycle
• Enables the solar dynamo—moving plasma generates the magnetic fields that drive all solar activity

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Interior Structure: Where Energy Is Made and Moved

The Sun's interior is organized by how energy transfers through different density and temperature conditions. Each zone represents a different dominant energy transport mechanism.

Core

• Site of nuclear fusion where hydrogen converts to helium at temperatures of ~$15 \times 10^6$ K
• Contains only ~25% of the Sun's radius but produces 99% of its energy output
• Density is ~150 g/cm³—about 150 times denser than water, yet still plasma due to extreme temperature

Radiative Zone

• Extends from the core to ~70% of solar radius—energy moves outward through photon absorption and re-emission
• Photons take ~170,000 years to cross this zone because dense plasma constantly absorbs and re-emits them
• Temperature drops from 7 million to 2 million K across this region as energy slowly diffuses outward

Convection Zone

• Outer 30% of solar radius where energy transfers through bulk plasma motion—hot plasma rises, cool plasma sinks
• Creates granulation patterns visible on the photosphere—each granule is a convection cell ~1,000 km across
• Drives the solar dynamo—differential rotation in this zone generates the Sun's magnetic field

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Atmospheric Layers: What We Can Observe

The Sun's atmosphere is where we directly observe solar phenomena. Counterintuitively, temperature increases with distance from the surface in the outer atmosphere—a mystery called the coronal heating problem.

Photosphere

• The "visible surface" at ~5,500°C—this is what we see and photograph; it emits the Sun's light spectrum
• Only ~500 km thick—extremely thin compared to the Sun's 700,000 km radius
• Contains sunspots—cooler regions (~3,500°C) where intense magnetic fields inhibit convection

Chromosphere

• Thin layer above photosphere visible as a reddish glow during eclipses—named for its color (chromos = color)
• Temperature paradoxically rises from 4,000°C to ~20,000°C with increasing altitude
• Features spicules—jets of plasma shooting up at ~20 km/s, lasting only minutes

Corona

• Outermost atmosphere extending millions of kilometers—visible as the pearly white halo during total eclipses
• Mysteriously hot at 1–3 million°C—far hotter than the photosphere below; magnetic heating is the leading explanation
• Source of solar wind—streams of charged particles that create space weather and shape planetary magnetospheres

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Quick Reference Table

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Self-Check Questions

1. Which two solar layers transport energy outward, and what mechanism does each use?

2. The Sun's corona is over 100 times hotter than its photosphere. What term describes this puzzle, and why is it significant for understanding stellar atmospheres?

3. Compare hydrogen and helium in terms of their roles in solar fusion—which is fuel, which is product, and how will this relationship change when the Sun becomes a red giant?

4. A student claims the Sun is made of gas. How would you correct this statement, and why does the distinction matter for understanding solar magnetic activity?

5. If an FRQ asks you to trace energy from its origin to its escape from the Sun, what sequence of layers would you describe, and what changes about energy transport at each boundary?