Solar cycle

Lore & Background

D. Maunder and Edward Walter Maunder. In the second half of the nineteenth century, Richard Carrington and Spörer independently noted sunspots appearing at different heliographic latitudes at different parts of the cycle. Alfred Harrison Joy later described how the tilt of sunspots grows with latitude.

Reader's Guide

The solar cycle is a fundamental periodic phenomenon governing solar activity, with major ramifications for space weather and technologies on Earth. Understanding and predicting the solar cycle remains one of the grand challenges in astrophysics, as it impacts space- and ground-based technologies, the Earth's atmosphere, and possibly climate fluctuations on scales of centuries and longer. The current scientific consensus on climate change is that solar variations only play a marginal role in driving global climate change, since the measured magnitude of recent solar variation is much smaller than the forcing due to greenhouse gases. The cycle's physical basis, elucidated by Hale and the Babcocks, reveals a 22-year magnetic cycle underlying the 11-year sunspot cycle.

Did You Know?

• The Sun's magnetic field flips during each solar cycle, completing a full Hale cycle every 22 years.

The Engine at the Core

The Sun is essentially a colossal sphere of superheated plasma whose inner core sustains nuclear fusion, converting matter into radiant energy. Every single second, roughly 600 billion kilograms of hydrogen are fused into helium within that core, and in the process about four billion kilograms of matter are transformed directly into energy. This energy escapes outward and is radiated from the surface predominantly as visible light and infrared radiation, with roughly ten percent emerging at ultraviolet wavelengths. The photosphere, the layer we perceive as the visible surface, is composed primarily of hydrogen at about 73 percent and helium at about 25 percent, with trace amounts of heavier elements such as oxygen, carbon, neon, and iron. Classified as a G2V main-sequence star, the Sun holds approximately 99.86 percent of the total mass in the Solar System, dwarfing every planet, moon, and asteroid combined. Its absolute magnitude of +4.83 places it brighter than roughly 85 percent of all stars in the Milky Way, most of which are dimmer red dwarfs.

Birth from a Molecular Cloud

Around 4.6 billion years ago, a vast molecular cloud began to collapse under its own gravity, concentrating the bulk of its material toward a central point while the remainder spread into a rotating, orbiting disk that would eventually give rise to the planets and other bodies of the Solar System. The central concentration grew so hot and dense that nuclear fusion was ignited, marking the birth of the Sun. Evidence suggests this collapse may have been triggered by shockwaves from one or more nearby supernovae. The Sun is classified as a Population I star, meaning it is rich in heavy elements compared to older, metal-poor Population II stars. The relatively high abundances of elements like gold and uranium in the Solar System point to their production either through endothermic nuclear reactions during a supernova event or via neutron-capture transmutation inside a massive second-generation star. The Sun now orbits the Galactic Center at a distance of roughly 24,000 to 28,000 light-years, and its mean separation from Earth, about 1.496 × 10⁸ kilometres or eight light-minutes, was once used to define the astronomical unit of length.

The Long Road to a Black Dwarf

In roughly four to seven billion years, the Sun's core will exhaust enough hydrogen that hydrostatic equilibrium can no longer be maintained. The core will then contract, growing markedly denser and hotter, while the outer layers swell dramatically, transforming the star into a red giant. Once that phase concludes, models indicate the Sun will shed its outer envelope entirely, leaving behind a dense, cooling remnant known as a white dwarf. This object will no longer generate energy through fusion but will continue to glow and radiate stored heat for an estimated trillions of years. Eventually, after that long thermal decay, it is theorised to cool further into a black dwarf, an extremely dense stellar corpse emitting only negligible energy. This multi-stage decline stands in stark contrast to the Sun's present state, in which it steadily fuses hydrogen and serves as the principal energy source sustaining life on Earth, a role it has filled since its formation nearly five billion years ago.

Names, Symbols, and the Language of the Sun

The English word sun descends from Old English sunne and is part of a broader Germanic family that includes Dutch zon, German Sonne, Old Norse sunna, and Gothic sunnō, all traceable to Proto-Germanic *sunnōn. These terms are ultimately linked to the sun-words of other Indo-European branches, though those more often preserve an l-stem, as seen in Latin sōl, Greek hēlios, Welsh haul, and Czech slunce. Proto-Germanic itself retained an l-stem form, *sōwelan, which produced Gothic sauil and the modern Scandinavian words Swedish and Danish sol. In English, the adjective solar derives from Latin sol, while heliac comes from Greek helios. In poetry, Helios and Sol serve as personified figures, and in science fiction Sol distinguishes our Sun from other stars. Planetary astronomers use lowercase sol to denote a solar day on another world, such as Mars. The astronomical symbol for the Sun is a circle with a central dot (☉), and the scientific discipline devoted to studying it is called heliology.