This article is a preview from the Winter 2018 edition of New Humanist

Across the world, telegraph operators were electrocuted and, at low latitudes, a blood red aurora borealis appeared, so bright that a newspaper could be read by it at midnight. The Carrington Event, named after Richard Carrington, who noticed a flare on the Sun from south of London at the same time a magnetometer flew off-scale at Kew, changed for ever our ideas about the Sun. Before 1 September 1859, our local star was believed to influence the Earth only through its gravity and the warming effect of sunlight. Afterwards, it was recognised that violent convulsions on the solar surface, or “photosphere”, could fire magnetic missiles at our planet with devastating effect.

In the 1920s, the British astrophysicist Sir Arthur Eddington deduced the internal structure of the Sun and its central temperature of more than 10 million degrees merely by assuming it is a giant ball of gas. Although we now know that the Sun’s heat comes from “nuclear fusion” of hydrogen to helium, the by-product of which is sunlight, remarkably Eddington’s conclusions did not require him to know anything about the source of solar heat. The Sun’s central temperature depends essentially only on its mass, and would be the same for a similar mass of bananas, rusty bicycles or discarded TV sets.

Eddington’s Sun was predictable and dull. However, the fact that it has a magnetic field changes everything. It makes the nearest star an unpredictable, seething, explosive, infinitely surprising laboratory for extreme physics.

Magnetic fields are generated by moving electric charges. In the case of a mundane bar magnet, the movement is only of the electrons inside atoms, and the atoms stay put. The key thing to understand about the Sun is that it is not an ordinary gas – it is an electrically charged gas, or “plasma”, of “ions” and electrons. And, in the solar plasma, the moving charges that create the magnetic fields, unlike the atoms in a bar magnet, are free to move. This movement changes the magnetic field, which affects the movement of the charges, which changes the magnetic field again, and so on. It is this complex interplay between the hot plasma and magnetic field that is behind all the myriad solar magnetic phenomena from the magnetic whorls of sunspots to the mega-detonations of solar flares.

Actually, there is another essential ingredient. The Sun is not a rigid body. Its exterior rotates at a different rate from its interior, and even regions of its exterior at different latitudes rotate at different speeds. Consequently, the magnetic fields in the Sun are continually being twisted and contorted, storing up energy like twisted-up elastic bands. Where loops of magnetic field break through the surface, we see sunspots – nearly always in pairs since a loop that emerges from the Sun at one point must re-enter somewhere else. Where a magnetic field becomes so twisted it snaps, “reconnecting” with other fields, the energy unleashed hurls million-degree plasma tens of thousands of kilometres above the Sun in a solar flare.

Understanding the Sun is more than a mere academic activity. Our very survival may depend on predicting the “space weather” created by the nearest star. Studies of other Sun-like stars reveal that, on rare occasions, they can launch mega flares quite capable of frying a planet like the Earth. More seriously concerning are coronal mass ejections (CMEs). First recognised only in the 1970s, these are missile-like ejections into space of vast amounts of solar plasma and magnetic field: roughly the mass of Mount Everest hurled into space at 500 times the speed of a passenger jet. The Carrington event – the most violent solar event ever recorded – was a CME. In 1859, we had the crude ability to detect a CME but the world was not so dependent on technology that it suffered any serious harm. Not so today.

Changes in the magnetic field across electrical power grids can induce currents big enough to melt equipment. Such “induction” was behind a major power outage on 13 March 1989, which left 6 million people in Quebec in the dark. But the real threat today is to the dense ring of the satellites which girdles our planet and on which our lives now depend. At risk are satellites – weather satellites, Global Positioning satellites – which not only allow us to know our location but play a crucial role in global financial transactions.

In rich countries, efforts have been made to harden infrastructure against a CME. However, it is a sobering thought that the Sun, which has given us life, could in an instant return us to a pre-electrical age.

Marcus Chown is the author, most recently, of "Infinity in the Palm of Your Hand: 50 Wonders That Reveal an Extraordinary Universe" (Michael O'Mara)