The Sun

The Sun is an ordinary G2V main-sequence star at the center of the Solar System: 1.4 × 10⁶ km in diameter, 2 × 10³⁰ kg in mass, with a luminosity of 3.8 × 10²⁶ W. It is ~73 % hydrogen and 25 % helium by mass.

The Sun has a layered structure. Inside: core (where nuclear fusion happens), radiative zone (energy diffuses outward as photons), and convection zone (rising and sinking gas carries heat the rest of the way). The atmosphere has three layers: the photosphere (the visible "surface," ~5,800 K, ~500 km thick), the chromosphere (hotter, thin), and the corona (very hot, ~1–3 million K, very tenuous, visible during a total solar eclipse). The corona's high temperature remains a puzzle, likely heated by magnetic reconnection.

The solar interior is probed by mathematical models (hydrostatic equilibrium: gas pressure out vs. gravity in), helioseismology (Doppler analysis of surface oscillations), and neutrino detection (neutrinos stream directly from the core).

Sunspots are cooler (~1,500 K below ambient), magnetically dominated regions where convection is suppressed; they appear dark by contrast. They come in pairs linked by magnetic loops, and their numbers rise and fall in an 11-year sunspot cycle (22 years if polarities are counted). Active regions also host solar prominences (gas loops lasting days/weeks) and solar flares (sudden violent explosions releasing huge X-ray and particle bursts in minutes).

The Sun's energy comes from nuclear fusion in the core (~15 million K), via the proton-proton chain: four hydrogen nuclei fuse into one helium-4 nucleus, with the small mass difference released as energy via E = mc². The Sun burns ~600 million tons of hydrogen per second and has ~5 billion years of fuel left.