Introduction
The light arriving at your eyes from the Sun right now left the solar core about 100,000 years ago — longer ago than modern humans have existed — yet the neutrinos produced in that same fusion reaction arrived eight minutes after they were made.
The Sun is the overwhelmingly dominant body in our Solar System, containing 99.86 % of all its mass (2 × 10³⁰ kg) and emitting the radiant energy that makes Earth habitable. Yet despite centuries of observation, the physical processes that power the Sun and extend its influence across interplanetary space were not understood until the 20th century. The discovery that nuclear fusion — the fusion of hydrogen nuclei into helium deep within the solar core — releases the Sun's luminosity resolved the century-long mystery of how the Sun could have been shining for billions of years without exhausting any known energy source. The subsequent detection of solar neutrinos produced in the core, and the eventual resolution of the Solar Neutrino Problem through the discovery of neutrino oscillation, stand among the most important achievements in 20th-century physics.
The Sun is also a dynamic, magnetically active star whose explosive events shape the space environment all the way out to the outer edges of the heliosphere — the vast bubble of solar influence ~120 astronomical units (AU) in diameter. The continuous outflow of charged particles known as the solar wind fills this bubble, deflects cometary tails, shapes planetary magnetospheres, and drives space weather effects on Earth. Coronal mass ejections — billion-tonne plasma clouds hurled at thousands of kilometres per second — can disrupt power grids, satellite communications, and navigation systems, illustrating that the Sun is not merely a passive heat source but an active participant in the near-Earth environment.
Understanding solar structure and the solar wind is foundational for planetary science. The very compositions and present-day conditions of the planets were shaped during the Sun's formation and early activity: the early intense solar wind may have stripped away the primordial atmospheres of the inner planets; the Sun's gradual brightening over 4.6 billion years has influenced atmospheric evolution; and future solar evolution — including the Sun's eventual expansion into a red giant — will determine the ultimate fate of the Solar System's inner worlds.
Key Terms
The dominant nuclear fusion pathway in stars of solar mass and below, in which four hydrogen nuclei (protons) are ultimately converted to one helium-4 nucleus, two positrons, two electron neutrinos, and energy. The first step — p + p → ²H + e⁺ + νₑ — is mediated by the weak nuclear force and is so improbable that a given proton in the solar core waits an average of ~10 billion years before fusing; the enormous number of protons in the core (roughly 10⁵⁷) makes the overall rate sufficient to power the Sun. The net energy release is 26.7 MeV per helium nucleus formed, with ~2 % carried away invisibly by neutrinos.
An electron neutrino produced in the solar core by the proton-proton chain, primarily in the first step (p + p → ²H + e⁺ + νₑ). Because neutrinos interact only via the weak nuclear force and gravity, they escape the solar interior in approximately 2 seconds (versus ~100,000 years for photons) and arrive at Earth about 8 minutes after production, carrying direct information about conditions in the solar core. The Solar Neutrino Problem — that early experiments detected only one-third of the predicted flux — was resolved in 2002 when the SNO experiment demonstrated that the "missing" neutrinos had oscillated into muon and tau neutrino flavours during transit, confirming that neutrinos have non-zero rest mass.
A continuous supersonic outflow of charged particles — primarily protons and electrons, plus alpha particles (He²⁺) — from the solar corona into interplanetary space. The solar wind has two components: a slow wind (~400 km/s, ~8 particles/cm³ at 1 AU) originating from helmet streamers and equatorial regions, and a fast wind (~750 km/s, ~3 particles/cm³ at 1 AU) emanating from coronal holes where magnetic field lines are open. The solar wind carries away ~10⁹ kg/s — negligible in terms of solar mass but sufficient to sculpt the entire heliosphere and dominate the magnetic environment of all planets.
A large-scale eruption of magnetised plasma from the solar corona, releasing up to 10¹³ kg of material at velocities of 1,000–3,000 km/s. CMEs occur roughly once per day at solar maximum and once per week at solar minimum. When an Earth-directed CME arrives (~1–3 days after eruption), it compresses Earth's magnetosphere and drives geomagnetic storms. The induced time-varying magnetic fields cause geomagnetically induced currents (GICs) in long conducting systems — power lines, pipelines, telegraph wires — which can overload transformers and cause grid failure. The most powerful recorded geomagnetic storm was the Carrington Event of September 1859.
The vast bubble of solar influence surrounding the Sun and extending to ~120 AU, where the pressure of the solar wind approximately balances the pressure of the interstellar medium. Within the heliosphere, the structure from inside out is: (1) the supersonic solar wind (~1–85 AU); (2) the termination shock (~85 AU) where the solar wind abruptly decelerates to subsonic speeds; (3) the heliosheath, a turbulent compressed region; and (4) the heliopause, the outermost boundary. Voyager 1 crossed the heliopause in August 2012 at ~121 AU, becoming the first human-made object to enter interstellar space. The shape of the heliosphere may be elongated in the direction of the Sun's motion through the local interstellar medium.