15.1 The Structure and Composition of the Sun

The Sun, our nearest star, is a complex celestial body with a unique composition and structure. Unlike Earth, it's primarily made of hydrogen and helium, existing in a plasma state due to extreme temperatures. Its layers, from the core to the corona, each play crucial roles in generating and transporting energy.

The Sun's atmosphere is a dynamic environment, featuring phenomena like granulation, sunspots, and solar flares. These processes, driven by magnetic fields and plasma movements, influence space weather and can impact Earth. Understanding the Sun's composition and structure is key to grasping its effects on our planet and the solar system.

The Sun's Composition and Structure

Composition of Sun vs Earth

• Sun composed primarily of hydrogen (74%) and helium (24%), while Earth has more diverse composition
  • Earth's atmosphere mostly nitrogen (78%) and oxygen (21%)
  • Earth's crust primarily oxygen, silicon, aluminum, and iron
• Sun has very low concentration of heavy elements compared to Earth
  • Elements heavier than helium only about 2% of Sun's mass
  • Earth much higher concentration of heavy elements, especially in core and crust (iron, nickel)
• Sun in plasma state due to high temperature, while Earth has solid, liquid, and gaseous states (rocks, oceans, atmosphere)

Layers and functions of Sun

• Core
  • Innermost layer where nuclear fusion occurs, converting hydrogen into helium
  • Temperatures reach about 15 million K
  • Generates Sun's energy and radiation
• Radiative Zone
  • Surrounds core, energy transported outward by radiation
  • Photons take thousands of years to travel through this dense layer
• Convective Zone
  • Outer layer of Sun's interior where energy transported by convection
  • Hot plasma rises, cools, and sinks back down in convective cells (granulation)
• Photosphere
  • Visible surface of Sun, where light emitted into space
  • Temperatures around 5,800 K
  • Sunspots and granulation are visible features
• Chromosphere
  • Thin, reddish layer above photosphere
  • Characterized by spicules and prominences (plasma loops)
• Corona
  • Outermost, extremely hot, and tenuous layer of Sun's atmosphere
  • Temperatures exceed 1 million K
  • Visible during total solar eclipses

Processes in Sun's atmosphere

• Photosphere
  • Granulation: visible pattern of convective cells on photosphere
    • Bright centers where hot plasma rises and dark edges where cooler plasma sinks
  • Sunspots: darker, cooler regions with strong magnetic fields that inhibit convection
    • Consist of dark umbra and lighter penumbra
• Chromosphere
  • Spicules: jets of plasma that rise from photosphere into chromosphere
  • Prominences: large, loop-like structures of plasma extending outward from chromosphere
    • Can erupt and release material into space as solar flares or coronal mass ejections (CMEs)
• Corona
  • Heated by magnetic reconnection and wave interactions
  • Source of solar wind, continuous stream of charged particles flowing outward from Sun
  • Coronal holes: regions where magnetic field lines are open, allowing solar wind to escape more easily
  • CMEs: massive eruptions of plasma and magnetic fields from corona into space
    • Can cause geomagnetic storms and auroras when interacting with Earth's magnetic field (northern lights)

Solar Dynamics and Observation

• Magnetohydrodynamics: study of electrically conducting fluids (like solar plasma) and their interaction with magnetic fields
• Helioseismology: study of the Sun's interior structure through analysis of its surface oscillations
• Solar cycle: approximately 11-year periodic variation in the Sun's activity, including changes in sunspot numbers and magnetic field strength