The Sun
The Sun is the G2 V yellow-dwarf star that holds the solar system together, containing more than 99 percent of its mass. In its core, hydrogen fuses into helium at about 15 million °C; energy takes some 170,000 years to work its way out, and the visible photosphere glows at about 5,500 °C. Around it, an 11-year cycle of magnetic activity rhythmically builds and clears the sunspots, tracked as a numbered series since Solar Cycle 1 in 1755 and now observed by NOAA's Space Weather Prediction Center. Cycle 25 began in December 2019; the 2019 prediction panel expected a peak of 115 sunspots in July 2025, while the smoothed sunspot number has already peaked so far at 160.9 in October 2024. The record of human observation stretches back to the first telescope drawings of 1610, through Schwabe's patient 43-year register and Carrington's 1859 flare, to a fleet of spacecraft — SDO, SOHO, STEREO, Hinode and Wind — joined by Parker Solar Probe, which touched the corona in 2021 and flew within 6.1 million kilometres of the surface on 24 December 2024, and ESA's Solar Orbiter, which returned the first close-up images of the Sun's poles in June 2025. This fact base collects the numbers, dates and quoted evidence behind that story.