What are sunspots and why does the Sun have an 11-year cycle?

Sunspots are dark patches on the Sun's surface where strong magnetic fields hold back the flow of heat from below. They look dark only because they are cooler than their surroundings, and their number rises and falls in a rhythm of about eleven years called the solar cycle. For the current cycle, Solar Cycle 25, NASA and NOAA announced in October 2024 that the Sun had reached its solar maximum period.

Sunspots can be tiny or enormous, from 16 kilometres to 160,000 kilometres across, and the biggest can be seen without a telescope. Thomas Harriot first saw them through a telescope in 1610, but the pattern took longer to spot. Only in 1843 did Samuel Heinrich Schwabe, after 17 years of patient watching, realise that their numbers rise and fall regularly.

That rhythm matters because sunspots mark where the Sun's magnetism is tangled. Most solar flares and coronal mass ejections, the explosions that cause space weather, come from the active regions around sunspot groups. When the spots are plentiful, the Sun is usually stormier; when it is spotless, it is usually quiet.

Cycles are not clockwork. They last about eleven years, but from just under 10 to just over 12, and sunspot numbers rise quickly and fall more slowly. The cycle in progress began in December 2019. Scientists first expected a weak one, but activity has been much stronger than predicted.