The Sun’s Inner Workings: Energy, Structure, and Activity

Posted on Jun 9, 2025 in Physics

The Sun: Our Star’s Energy, Structure, and Activity

Stellar Energy: The Power of Nuclear Fusion

Stars are powered by nuclear reactions involving hydrogen (H) and helium (He).

The source of stellar energy is nuclear fusion: the combining of lighter nuclei into heavier ones. The immense amount of energy released is described by Einstein’s famous equation: E = mc2.

The greater the nuclear binding energy, the more stable the nucleus.

Conditions for Nuclear Fusion

Nuclear fusion only occurs at temperatures greater than 10 million Kelvin (K) and at very high densities. These extreme conditions, achieved through gravitational contraction, allow nuclei to move with high velocities, overcoming the electric repulsion between their positively charged protons.

Fusion requires very high temperatures (greater than 10 million degrees), high density (more than 20 times the density of iron), and consequently, high pressure (greater than 1010 atmospheres).

Stellar Balance: Hydrostatic Equilibrium

A one-solar-mass star can maintain a stable size and structure for approximately 10 billion years, continuously emitting steady power.

Hydrostatic equilibrium describes the stellar balance where gravity and internal pressure are opposite and equal at every point within the star, ensuring its stability.

The Pressure-Temperature Thermostat

This mechanism explains how contracting gases heat up, while expanding gases cool down, regulating the star’s internal temperature and pressure.

The Sun’s Internal Structure and Layers

The Sun is composed of several distinct layers, each with unique characteristics:

  • Core: Approximately 15 million K
  • Radiation Zone: 7 million to 1.5 million K
  • Convection Zone: Approximately 1.5 million K
  • Photosphere: Around 6000 K (visible surface)
  • Chromosphere: 104 to 105 K
  • Corona: Approximately 1 million K (outer atmosphere)
  • Solar Wind: Stream of charged particles flowing outwards

Heat Transfer Mechanisms

Heat transfer within the Sun occurs primarily through two modes:

  • Radiation: At high temperatures, all atoms are ionized, allowing energy to be transported by photons.
  • Convection: At lower temperatures, atoms can absorb radiation, leading to the movement of hot plasma.

The dominant heat transfer mode depends on temperature, thermal and density gradients, and pressure.

Stellar Mass and Internal Structure

The internal structure of a star varies significantly with its mass:

  • Less than 0.4 Solar Masses (M☉): The entire star is convective.
  • Around 1 Solar Mass (M☉): Features three distinct zones: a core, a radiative zone, and a convective zone.
  • Greater than 1 Solar Mass (M☉): The core is convective, and the outer shell is radiative.

Hydrogen Fusion: The Proton-Proton Chain

The proton-proton chain is the primary nuclear fusion process in the Sun, fusing four protons into a helium nucleus and producing most of the Sun’s energy. The process occurs in three main steps:

  1. 1H + 1H → 2H (deuterium) + positron (e+) + neutrino (ν)
  2. 2H + 1H → 3He (Helium-3) + gamma ray photon (γ)
  3. 3He + 3He → 4He (Helium-4) + 1H + 1H

The timescales for these reactions are vastly different:

  • Step 1: On average, a proton waits for approximately 9 × 109 years to fuse with another proton. This long timescale determines the Sun’s life expectancy of about 10 billion (1010) years.
  • Step 2: A deuterium nucleus lives for only about 1 second before forming helium-3.
  • Step 3: This step takes approximately 106 years.

Neutrinos: Messengers from the Sun’s Core

Neutrinos are massless, neutral particles that travel at the speed of light and carry energy. They interact with matter very weakly and can penetrate matter easily, leaving the star immediately. They are common ingredients in cosmic rays.

Solar neutrinos are abundant neutrinos originating from the Sun’s core. An astonishing 1012 neutrinos pass through you every second!

The overall reaction of the proton-proton chain is: 4(1H) → 4He + 2(e+) + 2ν + 2γ rays.

The mass difference between four hydrogen nuclei and one helium nucleus is converted into energy according to E=mc2. Specifically, the total mass of the products is 0.7% lower than the initial four protons.

Observation of solar neutrinos provides direct insight into the nuclear processes occurring deep within the Sun’s core.

Solar Activity and Magnetic Fields

The Sun exhibits various forms of activity, all closely related to its powerful magnetic fields:

  • Sunspots
  • Solar Prominences
  • Solar Flares
  • Coronal Mass Ejections (CMEs)

Sunspots

Sunspots are regions on the Sun’s surface that are cooler than their surroundings and are characterized by extremely strong magnetic fields.

The Zeeman Effect

The Zeeman Effect provides evidence of these strong magnetic fields: a single spectral line observed outside a sunspot splits into three lines when observed within a sunspot, due to the magnetic field’s influence on atomic energy levels.

Charged particles spiral along magnetic field lines, influencing solar phenomena.

Solar Prominences

Strong magnetic activity causes solar prominences, which are large, bright features extending outwards from the Sun’s surface, often in a loop shape.

Solar Flares

Magnetic storms can cause powerful solar flares, which are intense bursts of radiation that send X-rays and charged particles into space.

Coronal Mass Ejections (CMEs)

Coronal Mass Ejections (CMEs) are massive expulsions of plasma and magnetic field from the Sun’s corona, sending bursts of energetic charged particles throughout the solar system.

These charged particles, streaming from the solar wind, can disrupt electrical power grids and disable communications satellites on Earth.

Temperature Anomalies in the Sun’s Atmosphere

The Sun’s outer atmospheric layers exhibit unexpected temperature increases:

  • Chromosphere: Has a higher temperature than the Photosphere. This is thought to occur when tangled or broken magnetic fields explosively reconnect, producing tremendous instantaneous currents that dissipate and heat the plasma to high temperatures.
  • Corona: Has an even higher temperature than the Chromosphere. A small input of energy at the base of the corona (e.g., in the chromosphere) gets amplified as it travels down the decreasing density gradient in the corona. Because energy must be conserved even as the density of the gases diminishes, the gases farthest from the base of the corona must travel at higher and higher speeds, heating the plasma to increasing temperatures. This phenomenon is related to the conservation of energy (or momentum) in a mass-reducing system, leading to higher velocities.

Key Solar Facts and Constants

  • Wien’s Law: Peak wavelength (λmax) = 2.9 × 106 / T(K) nm
  • Sun’s Life Expectancy (Nuclear Potential Energy / Luminosity): Approximately 10 billion years
  • Luminosity of the Sun: 3.846 × 1026 watts
  • Radius of the Sun: 6.9 × 108 meters (109 times that of Earth)
  • Mass of the Sun: 2 × 1030 kg (300,000 times that of Earth)
  • Mass of Earth: 5.972 × 1024 kg
  • Mass of Electron: Approximately 1/1840 of a proton’s mass
  • Speed of Light (c): 3 × 108 m/s