Step by Step
1
Internal and external structure
From the inside out: the core (where fusion occurs), the radiative zone, the convection zone, the photosphere (the visible surface, at 5,778 K), the chromosphere, and the corona — which reaches 1-3 million K, far hotter than the surface below it, a genuine puzzle known as the coronal heating problem.
2
The energy source — the proton-proton chain
The Sun generates energy through the proton-proton chain, converting hydrogen into helium-4 (4H → He-4 + energy), following Einstein's E=mc². This process converts about 4 million tons of mass into energy every second.
3
Solar wind and sunspots
Solar wind is a continuous stream of charged particles flowing outward from the Sun. Sunspots are cooler regions (around 4,000 K) where strong magnetic fields inhibit the normal flow of heat to the surface.
4
The solar cycle and its effects on Earth
The Sun follows an 11-year activity cycle, alternating between sunspot minimum and maximum. Solar flares and coronal mass ejections (CMEs) during active periods can disrupt Earth's magnetosphere, producing auroras and occasionally damaging satellites. The Sun has an estimated 5 billion years of fuel remaining.
Applied Walkthrough
1
Deep in the Sun's core, at temperatures around 15 million K, the proton-proton chain fuses hydrogen into helium, converting about 4 million tons of mass into energy every single second according to E=mc².
2
This energy slowly works its way outward through the radiative zone, then the convection zone, before finally reaching the visible surface (photosphere) at 5,778 K — yet the much thinner corona above the surface is mysteriously far hotter, at 1-3 million K, a genuine unsolved puzzle in solar physics.
3
Over an 11-year cycle, the Sun's activity rises and falls — during solar maximum, more sunspots (cooler, magnetically active regions) appear, along with more frequent solar flares and coronal mass ejections.
4
These CMEs, when directed toward Earth, can disrupt our planet's magnetosphere — producing beautiful auroras, but also occasionally posing a genuine risk to satellites and power grids.
Exam Application
Exams test whether you can correctly order the Sun's internal and external layers, explain the proton-proton chain as its energy source, and understand the 11-year solar cycle and its effects on Earth via solar flares and CMEs.
⚠ Common Trap
The most common trap is assuming temperature decreases steadily outward from the Sun's surface — the corona, despite being much farther from the core than the photosphere, is actually far hotter (1-3 million K vs. 5,778 K), an unresolved puzzle known as the coronal heating problem.
✓ Quick Self-Check
1. What is the correct order of the Sun's layers from the core outward?
Core → radiative zone → convection zone → photosphere → chromosphere → corona.
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2. What process generates the Sun's energy, and what does it convert?
The proton-proton chain, converting hydrogen into helium-4 (with mass converted to energy per E=mc²).
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3. What is the coronal heating problem?
The puzzle of why the corona (1-3 million K) is far hotter than the photosphere below it (5,778 K).
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4. What are sunspots, and why do they appear cooler?
Regions where strong magnetic fields inhibit normal heat flow to the surface, making them cooler (around 4,000 K).
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5. How long is the Sun's activity cycle?
About 11 years.
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