The active Sun

Never look straight at the Sun. It will damage your eyes. But with a safe filter, astronomers can see that its face is not blank.

There are dark spots on it. Each one is a patch that is a little cooler than the rest, and many are bigger than the whole Earth. They come and go over days or weeks.

The number of spots rises and falls. About every eleven years there are many, and in between there are almost none.

When there are many spots the Sun is stormy. Sometimes it hurls a great cloud of gas out into space. If the cloud comes our way, it lights up the sky near the Earth’s poles with moving curtains of green and red. These are the northern and southern lights.

It is all magnetism. The Sun is made of plasma, gas so hot that it carries electric currents, and moving currents make magnetic fields. The Sun does not turn as a solid ball. Its equator goes round in about 25 days and the regions near its poles take about 36. That difference drags the magnetic field round and round, winding it up like elastic until it is tangled and strained.

Sunspots are where bundles of that field break through the surface. The strong field holds back the hot gas rising from below, so the middle of the patch cools to about 3,900 °C while the surface around it is at about 5,500 °C. It is still very bright. It only looks dark beside its surroundings.

The cycle. Over about eleven years the tangle builds up, the spots multiply, and then the whole field collapses and re-forms the other way up, with north and south swapped. Then it starts again. People have counted sunspots since the early 1600s, and the cycles are numbered. The current one, the 25th, began in December 2019 and reached its busiest stretch around 2024.

Flares and eruptions. When strained magnetic field above a group of spots suddenly snaps into a simpler shape, the energy has to go somewhere.

	What it is	Reaches the Earth in	What it does here
Flare	A burst of light, including X-rays	8 minutes	Can black out radio on the daylit side
Coronal mass ejection	A cloud of plasma, often a billion tonnes or more, thrown clear of the Sun	One to three days	Shakes the Earth’s magnetic field: auroras, and trouble for power grids and satellites

Auroras. The Earth’s magnetic field turns most of the Sun’s outflow aside. During a storm, particles are driven down along the field towards the two magnetic poles, and strike the thin air 100 km and more above the ground. Oxygen glows green, and red higher up. Nitrogen adds blue and purple.

When it goes badly. In September 1859 a great storm, now named after the astronomer Richard Carrington who saw the flare, made telegraph wires spark and put auroras in the sky over the tropics. In March 1989 a storm shut down the power grid of Quebec for nine hours. In May 2024 the strongest storm for twenty years brought auroras far further south than usual and did little damage, partly because grid and satellite operators now get warnings and prepare.

The mystery at the top. The Sun’s outer atmosphere, the corona, is at more than a million degrees, hundreds of times hotter than the surface beneath it. That is like the air getting hotter as you walk away from a fire. Magnetic energy must be doing it, but exactly how is still argued over. NASA’s Parker Solar Probe was sent to find out. In December 2024 it passed 6.1 million km above the surface, inside the corona itself, at about 690,000 km an hour: closer and faster than anything people have ever built.