11-15-2025, 10:08 AM
Chapter 13 — The Life of the Sun
The Sun is the dominant object in our Solar System.
It provides light, heat, gravity, and the conditions necessary for life on Earth.
Understanding the Sun means understanding the behaviour of billions of similar stars across the universe.
This chapter explores how the Sun formed, how it generates energy, how it evolves, and what its eventual fate will be.
---
13.1 What Is the Sun?
The Sun is a G-type main sequence star composed mostly of hydrogen (74%) and helium (24%).
It contains 99.86% of all mass in the Solar System.
Key properties:
• Diameter: 1.39 million km
• Mass: 330,000 times Earth
• Core temperature: ~15 million °C
• Surface temperature: ~5,500 °C
• Age: ~4.6 billion years
• Distance from Earth: 150 million km (1 AU)
---
13.2 How the Sun Formed
The Sun formed from a collapsing cloud of gas and dust.
Steps of formation:
1. A shock wave triggered the collapse of a nebula
2. Material fell inward, forming a spinning disc
3. The central region grew dense and hot
4. When temperatures reached ~10 million °C, hydrogen fusion began
5. The Sun ignited and stabilised on the main sequence
The remaining disc formed the planets.
---
13.3 How the Sun Produces Energy — Nuclear Fusion
At its core, the Sun fuses hydrogen into helium through the:
Proton–Proton Chain
Fusion converts mass into energy using Einstein’s equation:
E = mc²
This energy:
• Keeps the Sun stable
• Produces light and heat
• Drives the Solar wind
• Supports life on Earth
The Sun emits 3.8 × 10²⁶ watts — equivalent to trillions of nuclear bombs per second.
---
13.4 Internal Structure of the Sun
The Sun has several important layers:
Core:
• Fusion occurs here
• Extremely hot and dense
Radiative Zone:
• Energy travels outward through radiation
• Photons are absorbed and re-emitted millions of times
Convective Zone:
• Hot plasma rises and cool plasma sinks
• Causes granulation on the surface
Photosphere:
• Visible “surface” of the Sun
• Where sunlight escapes
Chromosphere & Corona:
• Upper atmosphere
• Corona is hotter than the surface (a major mystery)
• Source of solar wind and flares
---
13.5 Sunspots, Flares & Solar Activity
The Sun has an 11-year activity cycle.
Sunspots:
• Cooler, darker regions
• Caused by magnetic fields
Solar Flares:
• Sudden energy bursts
• Can disrupt satellites, radio, power grids
Coronal Mass Ejections (CMEs):
• Massive eruptions of plasma
• Can interact with Earth’s magnetic field (auroras)
These processes influence space weather.
---
13.6 The Sun Today — Middle Age
The Sun is in the main sequence stage.
It is:
• stable
• fusing hydrogen
• halfway through its 10-billion-year life
Over time, the Sun slowly brightens — it is about 30% brighter today than when it formed.
---
13.7 The Future of the Sun
The Sun's future follows predictable stages:
1. End of Hydrogen (in ~5 billion years)
• Core contracts
• Outer layers expand
• Sun becomes a red giant
Earth may be engulfed or scorched beyond habitability.
2. Helium Burning
• Core fuses helium → carbon & oxygen
• Sun becomes unstable and pulsates
3. Planetary Nebula Phase
• Sun sheds its outer layers
• Creates a glowing nebula
4. White Dwarf
• Core becomes a dense, Earth-sized remnant
• Slowly cools for trillions of years
5. Black Dwarf (theoretical)
• A cold, inactive stellar corpse
• Takes longer than the current age of the universe to form
---
13.8 Why the Sun Matters
The Sun is essential for:
• climate and weather
• photosynthesis
• Earth’s orbit and stability
• the existence of liquid water
• driving the evolution of life
Studying the Sun also helps us understand:
• stellar physics
• magnetic fields
• fusion energy
• exoplanet habitability
---
Chapter Summary
• The Sun is a middle-aged G-type star formed 4.6 billion years ago.
• It produces energy through nuclear fusion.
• Its structure includes the core, radiative zone, convective zone, and atmosphere.
• Solar activity includes sunspots, flares, and CMEs.
• The Sun will eventually become a red giant and then a white dwarf.
• Its energy makes life on Earth possible.
---
Practice Questions
1. What process powers the Sun’s energy?
2. What are the main layers of the Sun’s interior?
3. What causes sunspots and solar flares?
4. What will happen to the Sun when it leaves the main sequence?
5. Why is studying the Sun important for understanding other stars?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive
The Sun is the dominant object in our Solar System.
It provides light, heat, gravity, and the conditions necessary for life on Earth.
Understanding the Sun means understanding the behaviour of billions of similar stars across the universe.
This chapter explores how the Sun formed, how it generates energy, how it evolves, and what its eventual fate will be.
---
13.1 What Is the Sun?
The Sun is a G-type main sequence star composed mostly of hydrogen (74%) and helium (24%).
It contains 99.86% of all mass in the Solar System.
Key properties:
• Diameter: 1.39 million km
• Mass: 330,000 times Earth
• Core temperature: ~15 million °C
• Surface temperature: ~5,500 °C
• Age: ~4.6 billion years
• Distance from Earth: 150 million km (1 AU)
---
13.2 How the Sun Formed
The Sun formed from a collapsing cloud of gas and dust.
Steps of formation:
1. A shock wave triggered the collapse of a nebula
2. Material fell inward, forming a spinning disc
3. The central region grew dense and hot
4. When temperatures reached ~10 million °C, hydrogen fusion began
5. The Sun ignited and stabilised on the main sequence
The remaining disc formed the planets.
---
13.3 How the Sun Produces Energy — Nuclear Fusion
At its core, the Sun fuses hydrogen into helium through the:
Proton–Proton Chain
Fusion converts mass into energy using Einstein’s equation:
E = mc²
This energy:
• Keeps the Sun stable
• Produces light and heat
• Drives the Solar wind
• Supports life on Earth
The Sun emits 3.8 × 10²⁶ watts — equivalent to trillions of nuclear bombs per second.
---
13.4 Internal Structure of the Sun
The Sun has several important layers:
Core:
• Fusion occurs here
• Extremely hot and dense
Radiative Zone:
• Energy travels outward through radiation
• Photons are absorbed and re-emitted millions of times
Convective Zone:
• Hot plasma rises and cool plasma sinks
• Causes granulation on the surface
Photosphere:
• Visible “surface” of the Sun
• Where sunlight escapes
Chromosphere & Corona:
• Upper atmosphere
• Corona is hotter than the surface (a major mystery)
• Source of solar wind and flares
---
13.5 Sunspots, Flares & Solar Activity
The Sun has an 11-year activity cycle.
Sunspots:
• Cooler, darker regions
• Caused by magnetic fields
Solar Flares:
• Sudden energy bursts
• Can disrupt satellites, radio, power grids
Coronal Mass Ejections (CMEs):
• Massive eruptions of plasma
• Can interact with Earth’s magnetic field (auroras)
These processes influence space weather.
---
13.6 The Sun Today — Middle Age
The Sun is in the main sequence stage.
It is:
• stable
• fusing hydrogen
• halfway through its 10-billion-year life
Over time, the Sun slowly brightens — it is about 30% brighter today than when it formed.
---
13.7 The Future of the Sun
The Sun's future follows predictable stages:
1. End of Hydrogen (in ~5 billion years)
• Core contracts
• Outer layers expand
• Sun becomes a red giant
Earth may be engulfed or scorched beyond habitability.
2. Helium Burning
• Core fuses helium → carbon & oxygen
• Sun becomes unstable and pulsates
3. Planetary Nebula Phase
• Sun sheds its outer layers
• Creates a glowing nebula
4. White Dwarf
• Core becomes a dense, Earth-sized remnant
• Slowly cools for trillions of years
5. Black Dwarf (theoretical)
• A cold, inactive stellar corpse
• Takes longer than the current age of the universe to form
---
13.8 Why the Sun Matters
The Sun is essential for:
• climate and weather
• photosynthesis
• Earth’s orbit and stability
• the existence of liquid water
• driving the evolution of life
Studying the Sun also helps us understand:
• stellar physics
• magnetic fields
• fusion energy
• exoplanet habitability
---
Chapter Summary
• The Sun is a middle-aged G-type star formed 4.6 billion years ago.
• It produces energy through nuclear fusion.
• Its structure includes the core, radiative zone, convective zone, and atmosphere.
• Solar activity includes sunspots, flares, and CMEs.
• The Sun will eventually become a red giant and then a white dwarf.
• Its energy makes life on Earth possible.
---
Practice Questions
1. What process powers the Sun’s energy?
2. What are the main layers of the Sun’s interior?
3. What causes sunspots and solar flares?
4. What will happen to the Sun when it leaves the main sequence?
5. Why is studying the Sun important for understanding other stars?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive