The Sun is a ball of gas composed of hydrogen and helium, with nuclear fusion occurring inside it. Understanding its layered structure and activity is the starting point of stellar physics.
Basic Knowledge
From the center outward, the Sun's internal structure consists of three layers: the core (approximately 0–25% of the radius, temperature 15 million K, where nuclear fusion occurs), the radiative zone (25–70%, where energy is transferred as photons by radiation), and the convective zone (70–100%, where energy is carried upward by convection of plasma).
The photosphere is a thin layer that emits visible light (approximately 500 km thick, temperature approximately 5800 K), where sunspots (cool regions whose convection is obstructed by strong magnetic fields, approximately 4000 K) can be seen. Directly above the photosphere is the chromosphere (approximately 2000 km thick, temperature 10,000 K), and above it is the corona (temperature 1–2 million K). The reason the corona is hotter than the photosphere is an unsolved mystery known as the coronal-heating problem.
Solar activity changes in an approximately 11-year cycle as the number of sunspots varies, producing flares, prominences, and coronal mass ejections (CMEs). CMEs reach Earth and cause auroras and magnetic storms.
Core (central region where nuclear fusion occurs; temperature 15 million K and extending to approximately 25% of the radius)
Convective zone (70–100%; transports energy through plasma convection)
Photosphere (surface layer that emits visible light; temperature approximately 5800 K)
Chromosphere (upper layer of the photosphere; temperature approximately 10,000 K; appears pink during a total solar eclipse)
Corona (outermost layer; temperature 1–2 million K; spreads out in white during a total solar eclipse)
Sunspot (dark region where convection is obstructed by strong magnetic fields; approximately 4000 K and varies on an 11-year cycle)
Further Study (Background and Significance)
The solar-neutrino problem was the mystery that fewer neutrinos were observed than predicted by theory. It was solved by the discovery of neutrino oscillation (the 2002 Nobel Prize, awarded to Kajita Takaaki and others).
The Sun is approximately 4.6 billion years old and is currently in the middle of its main-sequence stage. In approximately 5 billion years, its hydrogen will be depleted and it will evolve into a red giant.
The solar wind is a high-speed plasma flow streaming from the Sun. It deforms Earth's magnetosphere (50,000 km on the dayside and more than 600,000 km on the nightside) and forms the heliosphere, which surrounds the entire solar system. Space-weather forecasting is becoming increasingly important for protecting satellites, GPS, and power grids.
- Interior: core → radiative zone → convective zone (from the center outward)
- Surface layers: photosphere (5800 K) → chromosphere → corona (1–2 million K)
- The coronal-heating problem, in which the outer layer is hotter, remains unsolved
- Sunspots are approximately 4000 K and vary on an 11-year cycle
- CME → Earth's magnetic storms and auroras
- Sun's age: approximately 4.6 billion years (lifespan approximately 10 billion years)
- Solar-neutrino problem → solved by neutrino oscillation
Points of Caution (Often Confused)
① Do not reverse the order of the photosphere (visible-light-emitting surface), chromosphere, and corona (outer layers). ② Sunspots are dark and cool regions, but they are dark only compared with their surroundings and actually have a temperature of approximately 4000 K. ③ Distinguish the energy-transport mechanisms of the radiative zone (transfer by photons) and the convective zone (transfer by movement of matter). ④ The 11-year cycle is the cycle of increase and decrease in sunspots; including magnetic-pole reversal, it is 22 years.
Practice
- What is the name of the region where nuclear fusion occurs inside the Sun?
- What is the approximate temperature of the corona in K?
- Approximately how many years is the cycle over which the number of sunspots changes?