Sun

The Sun is the one object in the solar system you can point to without a telescope and, in a real sense, everything else is a rounding error. This G-type main-sequence star holds 99.86% of all the mass in the system — the eight planets, every moon, asteroid and comet share the remaining 0.14% between them. It is the gravitational anchor that keeps Neptune on its 165-year loop and Earth on its 365-day one, and the furnace whose light, taking 8 minutes and 20 seconds to cross the 150 million km to Earth, powers nearly every living thing on our planet.

For all its familiarity, the Sun is a genuinely extreme place: 696,340 km in radius (about 109 Earths across), roughly 4.6 billion years old, and quietly converting some 600 million tonnes of hydrogen into helium every second in a core hotter than 15 million °C. It is a middle-aged, unremarkable star by galactic standards — and that ordinariness is exactly what makes it the best-studied star in the universe.

A star, not a planet

The Sun is not a burning ball of gas in the campfire sense — nothing is on fire. It shines because its core, compressed by the weight of everything above it to densities greater than lead and temperatures near 15 million °C, fuses hydrogen nuclei into helium. Each fusion event releases a trickle of energy, but with the core running at roughly 600 million tonnes of hydrogen consumed per second, the total is staggering: the outward pressure of that energy is what holds the star up against its own crushing gravity, a standoff that has lasted 4.6 billion years and has about 5 billion left to run.

Composition-wise the Sun is about three-quarters hydrogen and one-quarter helium by mass, with everything else — carbon, oxygen, iron and the rest — making up less than 2%. With a mass of 1.989 × 10³⁰ kg, it is the definition of 'solar' for the whole field of astronomy: stellar masses, luminosities and radii are all measured in multiples of the Sun's.

Layer by layer, from core to corona

A photon born in the core does not simply fly out. It works outward through the radiative zone in a random walk that can take on the order of 100,000 years, before reaching the convective zone, where great cells of hot plasma boil to the surface and sink again like water in a heating pot. The visible 'surface' — the photosphere — is not solid at all but the layer where the gas finally thins enough to let light escape into space, at a comparatively cool 5,500 °C.

Above the photosphere sits the reddish chromosphere and then the corona, the Sun's tenuous outer atmosphere that flares out during a total eclipse as a pearly halo. Strangely, the corona is far hotter than the surface beneath it — over a million degrees — a puzzle called the coronal heating problem that solar physicists are still working to explain. It is from this outer atmosphere that the solar wind streams outward, a constant flow of charged particles that fills the entire solar system and shapes the tails of comets.

Sunspots and the 11-year cycle

The Sun does not rotate as a solid body: its equator turns roughly once every 25 days while its poles take over a month, and this differential spin winds up its magnetic field like a tangling rubber band. The result is an 11-year cycle of activity in which dark, cooler sunspots wax and wane, the magnetic poles eventually flip, and the star lurches between quiet 'solar minimum' and stormy 'solar maximum'.

In October 2024, NASA and NOAA announced that the current Solar Cycle 25 had reached its solar maximum period, though the precise peak month can only be pinned down years later. This cycle ran stronger than forecasters expected — on 3 October 2024 the Sun unleashed an X9.0 flare, the most powerful of the cycle. At maximum, the Sun launches solar flares and coronal mass ejections that drive space weather: geomagnetic storms that light up the aurora far from the poles but can also disrupt satellites, GPS, radio and power grids on Earth.

Touching the Sun: a century of missions

Because we cannot land on it, the Sun is studied from a distance and, increasingly, from within its atmosphere. NASA and ESA's SOHO has watched it continuously since 1995, NASA's Solar Dynamics Observatory has imaged it in extreme ultraviolet since 2010, and ESA's Solar Orbiter, launched in 2020, has returned the first views of the Sun's poles from outside the plane of the planets.

The boldest of all is NASA's Parker Solar Probe, which on 24 December 2024 flew just 6.1 million km above the surface — closer to a star than any spacecraft in history — while hurtling past at about 690,000 km/h, the fastest any human-made object has ever travelled. Shielded by a carbon-composite heat shield, it flew directly through the corona to sample the solar wind at its source and probe why that outer atmosphere is so improbably hot.

How to observe the Sun safely

The Sun is the one object in this catalogue that can permanently blind you, so the rule is absolute: never look at it through any telescope, binoculars or camera without a proper, certified solar filter fitted over the front of the instrument. Ordinary sunglasses, exposed film and smoked glass are not safe.

The safe ways in are certified eclipse glasses (ISO 12312-2) for the naked eye, a dedicated white-light or hydrogen-alpha solar telescope, or the classic pinhole projection method, which casts a small image of the Sun onto a card with no risk at all. With safe equipment during a quiet spell you may spot sunspots drifting across the disc; the grandest view of all, the corona itself, is visible only during the few minutes of a total solar eclipse.