The Sun — Our Nearest Star
Solar structure, nuclear fusion, the solar cycle, and space weather
A Star Up Close
The Sun is, in cosmic terms, a thoroughly ordinary star — a middle-aged, medium-mass, main-sequence star of spectral type G2V. And yet it is the only star we can study at close range, in extraordinary detail. Everything we learn about the Sun we can apply — with appropriate scaling — to the hundreds of billions of other stars in our galaxy. The Sun is our laboratory for stellar physics.
Some basic numbers place it in context: the Sun contains 99.86% of all the mass in the solar system. Its diameter is 109 times that of Earth. Over one million Earths could fit inside it by volume. Its surface temperature is about 5,778 K; its core temperature reaches approximately 15 million Kelvin.
The Structure of the Sun
The Sun is not a solid or uniform body. It is structured in concentric layers, each with distinct physical properties and processes.
Nuclear Fusion — The Engine of the Sun
The Sun’s energy comes from nuclear fusion in its core — specifically, the proton-proton chain, in which hydrogen nuclei (protons) are fused together to form helium. The process releases energy because the helium nucleus that results has slightly less mass than the four protons that went into it. That missing mass is converted directly into energy via Einstein’s famous equation.
The net reaction of the proton-proton chain can be summarised as four hydrogen nuclei fusing to produce one helium-4 nucleus, two positrons, two neutrinos, and a cascade of gamma-ray photons. These photons begin an extraordinary journey outward through the Sun.
A photon generated in the Sun’s core does not simply shoot outward. It is absorbed and re-emitted in a random direction billions of times as it fights through the dense radiative zone. This random walk means the average photon takes approximately 100,000 years to travel from the core to the surface — then just 8.3 minutes to reach Earth.
The Solar Atmosphere — A Temperature Mystery
As you move away from any heat source, you expect the temperature to fall. And moving from the Sun’s core outward, it does — from 15 million K in the core to about 5,778 K at the photosphere. But then something counterintuitive happens: the temperature rises again, reaching 20,000 K in the chromosphere and over 1 million K in the corona.
This coronal heating problem remains one of the great unsolved puzzles in solar physics. Leading hypotheses involve the dissipation of magnetic waves (Alfvén waves) and nanoflare heating — countless tiny magnetic reconnection events releasing bursts of energy. We do not yet have a definitive answer.
The Solar Cycle
The Sun is not a static object. Its magnetic field undergoes a regular cycle of approximately 11 years, swinging from solar minimum (few sunspots, quiet activity) to solar maximum (many sunspots, frequent eruptions) and back again.
Space Weather
Solar activity does not stay on the Sun. Energetic particles and magnetic disturbances from flares and CMEs travel through space and can reach Earth within one to three days. When they interact with Earth’s magnetic field, the results range from the sublime — the aurora borealis and aurora australis — to the disruptive.
When charged particles from solar wind are funnelled toward Earth’s poles by the magnetic field, they collide with atmospheric atoms. Oxygen at high altitudes glows green and red; nitrogen glows blue and purple. The aurora is not just beautiful — it is visible evidence of space weather hitting home.
Severe solar storms can disrupt satellite operations, GPS signals, power grids, and radio communications. The Carrington Event of 1859 — the most powerful solar storm in recorded history — induced currents so strong in telegraph lines that operators received electric shocks and messages continued transmitting even after being disconnected from power. A comparable event today would be catastrophic for modern infrastructure.
Solar Neutrinos
Fusion in the Sun’s core produces not only photons but also vast numbers of neutrinos — nearly massless, electrically neutral particles that interact so weakly with matter that they pass through the entire Sun essentially unimpeded. About 65 billion neutrinos from the Sun pass through every square centimetre of your body every second. The detection and study of solar neutrinos has confirmed our models of fusion in the solar core, and the resolution of the historical solar neutrino problem — why we detected fewer than predicted — led to the discovery that neutrinos oscillate between three flavours, implying they have mass. This earned the 2015 Nobel Prize in Physics.
1. In which layer of the Sun does nuclear fusion take place?
2. The Sun’s corona is far hotter than its surface. Why is this considered a major puzzle?
3. A photon is generated in the Sun’s core. Approximately how long does it take to reach Earth?