15.1 The Structure and Composition of the Sun

The Sun's Composition and Structure

The Sun is our nearest star and the engine of our solar system. It's a massive ball of hot plasma, composed mostly of hydrogen and helium, with a layered structure that generates and transports energy from its core outward into space. Understanding how the Sun is built helps explain everything from why it shines to how it affects conditions here on Earth.

Composition of the Sun vs. Earth

The Sun and Earth are made of very different stuff. The Sun is about 74% hydrogen and 24% helium by mass, with heavier elements (astronomers call everything heavier than helium "metals") making up only about 2%. Earth, by contrast, has a much higher concentration of heavy elements like iron and nickel in its core, and oxygen, silicon, and aluminum in its crust.

The physical states are different too. The Sun's extreme temperatures keep its matter in a plasma state, where atoms are stripped of their electrons. Earth, being much cooler, has the solid rock, liquid water, and gaseous atmosphere you're familiar with.

Layers and Functions of the Sun

The Sun has a layered structure, with energy generated deep inside and transported outward through several distinct zones. Think of it as two main regions: the interior (core, radiative zone, convective zone) and the atmosphere (photosphere, chromosphere, corona).

• Core
  • The innermost layer, where nuclear fusion converts hydrogen into helium. This is the Sun's power source.
  • Temperatures reach about 15 million K, and the density is enormous, roughly 150 times that of water.
  • All of the Sun's energy and radiation originates here.
• Radiative Zone
  • Surrounds the core. Energy moves outward here by radiation, meaning photons bounce from particle to particle.
  • This zone is so dense that a single photon can take hundreds of thousands of years to work its way through it, constantly being absorbed and re-emitted.
• Convective Zone
  • The outer layer of the Sun's interior. Here, energy is transported by convection: hot plasma rises toward the surface, cools, and sinks back down.
  • These rising and sinking motions create convective cells that are visible on the surface as granulation.
• Photosphere
  • The visible surface of the Sun, and the layer that emits the light we see.
  • Temperature is about 5,800 K.
  • Features like sunspots and granulation are observed here.
• Chromosphere
  • A thin, reddish layer just above the photosphere. You can sometimes see it as a red rim during a total solar eclipse.
  • Home to spicules (jets of plasma) and prominences (large loops of plasma).
• Corona
  • The outermost layer of the Sun's atmosphere, extending millions of kilometers into space.
  • Temperatures exceed 1 million K, which is paradoxically much hotter than the photosphere below it. This is called the coronal heating problem, and it's still an active area of research.
  • Visible to the naked eye only during total solar eclipses, appearing as a faint, pearly-white halo.

Processes in the Sun's Atmosphere

Each atmospheric layer has its own characteristic activity, much of it driven by the Sun's powerful and complex magnetic fields.

Photosphere

• Granulation is the visible pattern of convective cells on the photosphere. Each granule is roughly 1,000 km across. The bright centers are where hot plasma rises, and the darker edges are where cooler plasma sinks back down. Individual granules last only about 10–20 minutes before being replaced.
• Sunspots are darker, cooler regions (around 3,800 K compared to the surrounding 5,800 K) where intense magnetic fields inhibit convection, preventing hot plasma from reaching the surface. A sunspot has a dark central umbra surrounded by a lighter penumbra.

Chromosphere

• Spicules are narrow jets of plasma that shoot upward from the photosphere into the chromosphere, lasting only a few minutes each. Thousands are active at any given time.
• Prominences are large, loop-like structures of plasma that extend outward from the chromosphere, held in place by magnetic fields. They can persist for days or weeks. When a prominence becomes unstable, it can erupt and release material into space as a solar flare or a coronal mass ejection (CME).

Corona

• The corona is thought to be heated by magnetic reconnection (where tangled magnetic field lines snap and release energy) and by waves propagating through the plasma.
• It is the source of the solar wind, a continuous stream of charged particles flowing outward from the Sun in all directions.
• Coronal holes are regions where the magnetic field lines are open rather than looped back, allowing solar wind to escape more easily and at higher speeds.
• Coronal mass ejections (CMEs) are massive eruptions of plasma and magnetic fields launched from the corona into space. When a CME reaches Earth, it can compress our planet's magnetic field, triggering geomagnetic storms and producing auroras (the northern and southern lights).

Solar Dynamics and Observation

A few key tools and concepts help scientists study the Sun beyond what's visible on the surface:

• Helioseismology is the study of the Sun's interior by analyzing oscillations (vibrations) on its surface. Just as geologists use seismic waves to map Earth's interior, solar physicists use these oscillations to figure out conditions deep inside the Sun.
• Magnetohydrodynamics (MHD) is the branch of physics that describes how electrically conducting fluids, like solar plasma, interact with magnetic fields. It's essential for understanding sunspots, flares, and the solar wind.
• The solar cycle is an approximately 11-year periodic variation in the Sun's activity. During solar maximum, sunspot numbers peak and flares and CMEs are more frequent. During solar minimum, the Sun is relatively quiet. The magnetic polarity of the Sun actually flips every cycle, so the full magnetic cycle is about 22 years.