The Sun: A Garden-Variety Star

Chapter Outline

  • 15.1 The Structure and Composition of the Sun
  • 15.2 The Solar Cycle
  • 15.3 Solar Activity above the Photosphere
  • 15.4 Space Weather

15.1 The Structure and Composition of the Sun

  • Learning Objectives

    • Explain how the composition of the Sun differs from that of Earth.
    • Describe the various layers of the Sun and their functions.
    • Explain what happens in the different parts of the Sun’s atmosphere.

  • Basic Characteristics of the Sun

    • Enormous size: can fit about 10^9 Earths across its diameter.
    • Composed of extremely hot, largely ionized gases.
    • Structure discussed in layers: core, radiative zone, convective zone, photosphere, chromosphere, corona.

  • Composition of the Sun:

    • Elemental Makeup:
    • Hydrogen: 73% of mass
    • Helium: 25% of mass
    • Other elements (C, N, O, etc.): about 2% of mass
    • The Sun's elemental composition is typical for most stars, primarily hydrogen and helium.

  • Layers of the Sun:

    • Core: where nuclear fusion occurs, about 15 million K.
    • Radiative Zone: energy is transported outwards, about 25-70% of the Sun's radius.
    • Convective Zone: outermost layer of the solar interior, energy transported by convection.
    • Photosphere: visible surface of the Sun, typically around 5800 K.
    • Chromosphere: layer above the photosphere, about 10,000 K.
    • Transition Region: temperature increases rapidly from the chromosphere to the corona.
    • Corona: outer atmosphere, reaches several million K temperatures.

  • Photosphere:

    • Marked boundary where light emerges, around 400 km thick, granulation pattern caused by convection.

  • Granule Lifetime: 5 to 10 minutes.

  • Magnetic Features: Sunspots appear darker due to being cooler than surrounding areas (typically 3800 K).

15.2 The Solar Cycle

  • Learning Objectives

    • Describe the sunspot cycle and, more generally, the solar cycle.
    • Explain how magnetism is the source of solar activity.

  • Sunspot Activity:

    • Observed since ancient times; populations vary in cycles.
    • Sun rotates at different rates depending on latitude (differential rotation).
    • Average Sunspot Cycle: lasts approximately 11 years.

  • Magnetic Field:

    • Drives solar activity; characteristics observed through the Zeeman effect.
    • Pairs of sunspots with opposite polarities reflect the magnetic field's configuration.

  • Solar Activity:

    • Increased activity during solar max (more sunspots, flares, and activity) vs. solar min.

15.3 Solar Activity above the Photosphere

  • Learning Objectives

    • Describe how the solar activity cycle manifests:
    • Flares: brief eruptions releasing energy comparable to millions of hydrogen bombs.
    • Coronal Mass Ejections (CMEs): large expulsions of plasma from the solar corona.
    • Prominences: large, bright features extending from the Sun’s surface.
    • Plages: bright areas in the chromosphere associated with sunspots.

  • Flares and CMEs:

    • Flares can last 5-10 minutes with temperatures up to 10 million K.
    • CMEs can eject massive amounts of matter that impact Earth’s magnetosphere and atmospheric systems.

15.4 Space Weather

  • Learning Objectives

    • Explain what space weather is and how it affects Earth.

  • Space Weather Phenomena:

    • Related to solar activity such as solar flares, CMEs, and coronal holes.
    • Impact on Earth: Can induce geomagnetic storms affecting satellites, communication, and power infrastructure.

  • Historical Context:

    • Carrington Event of 1859: First significant evidence of solar effects on Earth, revealing the connection between solar activity and terrestrial phenomena.

  • Modern Implications:

    • Increased awareness and study of predicting solar effects are critical for technology reliance.
    • NASA and NOAA work on forecasting space weather to mitigate technological disruptions.

  • Key Terms:

    • Active region, corona, solar wind, sunspot cycle, etc.