July 17, 2018

The life and fate of our mortal Sun

This story is also available in Spanish.

Today I woke up on the wrong side of the clouds. Having been graced with unusually genial sunshine for more than a month, today looks particularly grim.

“Return, alas! return, O radiance dear!

And drive from me that foul, consuming Fear”

pleads Bradamante in 16th century “Orlando Furioso”.

This got me thinking about our 4.6 billion year-old beast and her glittering head. She bewitchingly promises warmth and cheer and when she’s beclouded our moods flop like a wet towel. But what’s going on in that head of hers? At times she scoffs and flares up, spewing fiery flames. She is spotty and certainly has her moods. Last month a scientist even suggested she may be having a midlife crisis. But we’re certainly lucky to be around when she’s at her best, because in a few billion years, she will unintentionally melt our planet into a sticky blob before meeting her final demise. It’s all quite dazzling, to say the least.

Hot and fiery

A powerful furnace, her core is 15 million degrees. Things cool off away from the core to 5500 degrees Celsius at her visible surface which we call the photosphere. This is the layer where parcels of light, or photons, escape freely to outer space. One would expect things to keep cooling down the farther one is from the Sun but in the atmosphere, something very odd happens. The temperature starts to increase and peaks to more than 8000 degrees Celsius. Even higher up above the solar atmosphere, called the corona, temperature soars to a million degrees! This is because the solar magnetic field affects the way energy is carried around and dissipated through her very sparse and diffuse atmosphere. Despite the searing temperature, it’s not really “hot” because the density is very low. It’s like getting in a car on a hot sunny day. While the dashboard may burn if touched, sitting in the car wouldn’t really burn you. But hey, I’m not suggesting you can go sit in the corona.

The Sun is not idle, she spins. Because she’s gaseous, she spins faster at the equator than at the poles. Rotating once every 25 days at the equator, she takes more than 30 days to rotate once at the poles. So her magnetic field, bundling in tubes just below the surface, gets twisted and tangled and bursts out through the surface at certain spots. This slows down the emergence of sizzling material from the inner parts, making these spots cooler, thus look darker compared to their surroundings. We call those the sun spots. Sometimes the magnetic field gets drastically distorted so its lines get sheared off causing the most violent eruptions in our solar system, called solar flares. These flares are often accompanied by streams of supersonic winds and produce as much energy as a billion hydrogen bombs. When the solar wind reaches Earth a few days later, it can sneak into the Earth’s natural shield through its magnetic funnel near the poles. The energetic particles of the wind blast the atoms and molecules in the air, exciting them into that magnificent spectacle, the northern lights.

A midlife crisis?

It has long been known that the Sun’s spinning and her magnetic field have a very intimate relationship. Remember the streams pouring out of the surface that I just talked about? The particles swimming in those streams follow the magnetic field lines like ants on a pheromone trail. Upon reaching a large enough distance, they break free and carry away the rotational momentum or oomph they’ve stolen from the Sun. This gradually slows the Sun down, and is aptly called magnetic braking.

However, this picture seems to have changed only last month. Turns out that the Kepler Space Telescope has been spying on thousands of stars, and a few clusters too. It revealed that in certain clusters young members are very well-behaved. However, the middle aged members, particularly those of the exact type as our Sun, were not acting their age! They were in fact spinning too fast for their old age. They don’t seem to be slowing down either.

Their brakes seem to have gone faulty, and scientists suspect this has to do with the motion lurking just underneath the stellar surface, or convection. Things may very well be more complicated than we had thought!

The Inescapable demise

She will shine normally for the next several billion years but for us earthlings, life will be far from normal. Earth’s temperature will steadily rise making life more challenging before the final cataclysm arrives. In about 1 or 2 billion years it will be hot enough to boil most of our water and seriously destabilise our biosphere, turning our planet into an arid desert. A few billion years later, whatever has remained of our oceans will evaporate and Earth will be a barren lifeless planet, like Venus.

Well perhaps that’s for the best, because what’s coming afterwards is a hellish ordeal no human would wish to witness. In about 6 billion years, the sun’s core runs out of hydrogen and she will swell up into an angry fiery giant, swallowing Mercury and Venus. She will melt our planet into a blob of magma, even Mars will not be habitable, and distant icy worlds like Uranus and Neptune will start to defrost.

Things will only continue hell-ward from there. Eventually the helium in her core, the product of burnt hydrogen, will run out too. Like a desperado with nothing left to lose, she’ll rage into a giant once again and that’s the point of no return. Ripping herself apart in despair, she will tear away her layers into glowing rings of gas and dust, or planetary nebula, that drift away in the wind. This tragedy will only spare her Earth-size beating heart, an abandoned naked inanimate object called a white dwarf. Left to its fate, it will cool and fade away into the dark as the solar system turns into a chilly, forsaken place.

Thus the tragedy unfurls and the rest is silence..

The dawn has now cracked and its light is streaming in through my window. I shall soon stroll to the beach and bask in the golden shimmer of our Sun, before it’s too late.

Image credit: The Economist


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