The Sun is the star nearest to Earth. The nuclear fusion reactions occurring inside it produce enormous energy, supporting life and climate on Earth.
Basic Knowledge
Structure of the Sun (from the inside outward):
Core: Central temperature of about 15 million℃. The site of hydrogen fusion. Four hydrogen nuclei → one helium nucleus + energy (E=mc²).
Radiative zone: Photons produced in the core move by random walks. It takes about 1 million years for light to pass through the radiative zone.
Convection zone: Energy is transported by convection. Granules (each about 1000km across) can be seen on the surface.
Photosphere: Surface temperature of about 6000K. The visible surface of the Sun. It contains sunspots (regions with a lower temperature of about 4000K than their surroundings, where convection is suppressed by strong magnetic fields).
Chromosphere: Outside the photosphere. Several thousand km thick. It appears red during a total solar eclipse.
Corona: Extremely hot plasma in the outermost layer (over 1 million K). It appears as a white halo during a total solar eclipse. It is the origin of the solar wind.
Nuclear fusion (Energy released by converting hydrogen → helium. E=mc²)
Photosphere (The visible surface of the Sun. About 6000K)
Sunspot (A dark spot on the photosphere. About 4000K. Convection is suppressed by a strong magnetic field)
Corona (Extremely hot plasma in the outermost layer. Over 1 million K. Origin of the solar wind)
Solar wind (A flow of plasma blowing outward from the corona. Interacts with Earth's magnetosphere)
Solar flare (An explosive release of energy on the surface of the Sun. Release of magnetic energy)
Further Study (Background and Significance)
The number of sunspots increases and decreases in an approximately 11-year cycle (the solar activity cycle). During periods with many sunspots (solar maximum), flares and solar winds become more active. Magnetic storms may occur, causing auroras to be visible even at low latitudes and disrupting artificial satellites, GPS, and power grids.
Coronal heating paradox: It may seem strange that the corona outside the Sun is over 1 million K even though the solar surface (photosphere) is about 6000K. This appears to contradict the second law of thermodynamics (heat does not naturally move from a low temperature to a high temperature), but energy is thought to be transferred to the corona by magnetic waves (Alfvén waves) and magnetic reconnection.
The Sun is currently burning hydrogen as a main-sequence star. In about 5 billion years, after using up its hydrogen, it is predicted to expand into a red giant, release its outer layers, and become a white dwarf.
- Sun's energy source = nuclear fusion (hydrogen → helium)
- Photosphere temperature = about 6000K
- Sunspot = about 4000K, strong magnetic field, suppressed convection
- Corona = over about 1 million K, outermost layer of the Sun
- Solar activity cycle = about 11 years
- Solar wind = flow of plasma from the corona
- The Sun has about 5 billion years of life remaining
Points to Note (Common Confusions)
① Sunspots are cool (lower in temperature than their surroundings), but they are still extremely hot at about 4000K. ② Do not confuse the corona (outermost, hottest layer) with the chromosphere (inner to the corona, intermediate layer). ③ The solar activity cycle is about 11 years (the cycle of sunspot numbers). ④ The solar wind is a flow of particles; despite being called “wind,” it is not a sound wave.
Practice
- What type of nuclear reaction produces the Sun's energy?
- What are the dark spots seen on the Sun's photosphere called? What is their temperature in K?
- What is the extremely hot plasma layer at the outermost part of the Sun called?