There are few things so familiar yet so alien as the sun. Every day it soars above the horizon and dominates our sky with its dazzling brilliance. A cloudless summer day often sends people into a frenzy. Out come the flip-flops and ice creams as people flock to parks and beaches to make the most of its rays. Every year we celebrate completing another lap of the sun with candles and cake. But how often do you stop to think about what’s really going on? That there is a giant nuclear reactor out there in the darkness of space flooding our planet with light?
The sun is so powerful that it can burn your skin from 150 million km away. From that distance it may seem small to us, but it’s big enough to fit more than a million Earths inside. Almost all of the sun’s eight planets could sit side-by-side in its core alone. Its gravity is so strong that deep inside that core the pressure is 100 billion times greater than atmospheric pressure here on Earth. Material in the core is eight times denser than gold. A single tablespoon weighs as much as a newborn baby. An Olympic-sized swimming pool’s worth weighs seven times more than the Titanic when it set sail fully loaded with passengers and cargo. It is here that the sun’s colossal energy reserves are churned into light. Every second 620 million tonnes of hydrogen get squeezed into 616 million tonnes of helium. The missing four million tonnes is turned into sunshine.
But that is only the very first step in the epic journey of sunlight. The sun is so jam-packed that a packet of light can only travel a millimetre before it careens into an obstacle. Imagine trying to make the 700,000km journey to the surface of the sun when you’re only allowed to proceed using these tiny steps. There is no guarantee either that a step won’t send you back towards the core rather than upwards towards space. This unenviable obstacle course means that it takes an average of 170,000 years for light to make it out of the sun. Some sunlight takes a million years to escape. Now unhindered, it’ll then arrive at the Earth just over eight minutes later. So next time you feel the sun warm your face, know that you are being hit by light that was created before early humans first migrated out of Africa. Some of it predates our species entirely. Remarkably, it takes light considerably less time to travel to you from the surface of the furthest star you can see in the night sky than it does to get here from the core of our own sun.
We’ve only known the true extent of this helter-skelter ride of sunlight since the 1990s when the first dedicated space telescopes were sent to scrutinise our nearest star. Yet there is so much more we have left to learn. Solar activity – in the form of dark patches called sunspots – rises and falls every 11 years or so, but we cannot yet predict the onset of the next cycle with any great accuracy.
The corona
Perhaps the sun’s most confounding mystery is why its atmosphere is so incredibly hot. The temperature drops from nearly 16 million degrees Celsius in the core to close to 6,000 degrees at its visible surface. Bizarrely it then climbs again to 3 million degrees in the corona – the sun’s tenuous, outermost layer. Imagine walking away from a campfire and suddenly getting hotter. Astronomers have known about this eccentric quirk since the 1930s, but no one has yet been able to provide a definitive answer to the riddle.
We’re so desperate for the answer that the next generation of solar observatories is now swinging into action. Just over a year ago Nasa launched the Parker Solar Probe. In October 2018 it broke the record for the closest a spacecraft has ever got to the sun – 43 million km from its surface (or 3.5 times closer than Earth). Eventually it will get within six million km. If the distance between the Earth and sun were reduced to the size of a soccer pitch, with the sun on one of the goal lines, the Parker Solar Probe will be inside the six-yard box. From this unprecedented vantage point, astronomers hope it will give us answers about what is causing the corona to get so hot. Soon Parker will be joined and complemented by the European Space Agency’s Solar Orbiter, which is due for launch in February.
Ejected debris
Getting up close and personal with our star has a more pointed purpose. The sun has the ferocity and sheer power to set us back centuries. When solar magnetic fields get tangled and twisted they can reconfigure and snap, unleashing a brutal barrage of ejected debris known as a Coronal Mass Ejection (CME). CMEs travel outwards at nearly 3,000km per second – fast enough to get from Paris to Moscow in less time than it takes for your heart to beat once. The material contained within them weighs more than Mount Everest. Soon they spread out to be bigger than the sun itself. If the Earth happens to be in the firing line then the consequences can be devastating.
Electric currents flood through our atmosphere, overwhelming power grids and causing widespread blackouts and communication failures. In 2012 we narrowly avoided being hit by a CME. Had it struck, estimates suggest it would have caused up to $2.6 trillion worth of damage. Declassified records have recently revealed that a CME triggered the unwanted explosion of sea mines during the Vietnam War.
These events provide a much needed and stark reminder that the giant nuclear reactor in the sky does a lot more than tan our skin on a warm day. It is an unpredictable and untameable beast that we must continue to explore if we want our modern technological world to continue.
Rebel Star: Our Quest to Solve the Great Mysteries of the sun by Colin Stuart is published by Michael O’Mara, at £16.99
