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How does the sun shine? Here’s why we are still a little in the dark

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MEK84H Sun fusion energy and Artificial intelligence technology can help physicists predict hazardous solar flares and warning to protect power grids and com

JIRAROJ PRADITCHAROENKUL/Alamy Stock Photo

ONE of the best things about being a columnist for New Scientist is the readers. I can tell you read my columns closely because I get fantastic emails asking smart questions about them. Last month, I wrote about how fusion works inside the local plasma gas ball, otherwise known as the sun. This resulted in a letter from someone who had been inspired to read in detail about how fusion works and had realised that there are inconsistencies in the scientific literature on this subject.

Now, for many of us, it won’t be news that there are unsolved mysteries associated with the sun. In my last column, I wrote about the coronal heating problem, the fact that one of the outermost layers of the sun is significantly hotter than its surface. We would expect the opposite: that as we go further from the sun’s primary energy source in its central core, the outer regions of the sun would be increasingly cooler. (One of my hopes for 2022 is that this problem will be solved, or at least one of my students will decide to tackle it themselves.)

But the sun doesn’t only have grand one-off mysteries. My correspondent has a point: the basic workings of how the sun burns are complicated and imperfectly understood.

Generally speaking, the reason stars shine is that gravity has pulled a sufficient amount of hydrogen atoms into such close quarters that they start to fuse together into helium. Every star starts this way. When the hydrogen runs out, the helium starts fusing together, and so on, producing heavier and heavier elements.

This is where we humans begin. The majority of the elements we are composed of are made in stars and, during supernovae and kilonovae, the exploding deaths of those massive stars.

This sounds like a simple matter of gluing elements together, but it isn’t: the conditions have to be just right. The hydrogen has to be hot enough and close enough together to fuse. And the fusion happens in stages. The theories that describe how all this happens aren’t the classical Newtonian physics that describes, for example, two football players colliding when they both want to control the ball. Instead, we need quantum mechanics and nuclear physics.

“The majority of the elements we are composed of are made in stars and during supernovae”

One of the biggest challenges to hydrogen fusion occurring at all is that the simplest hydrogen atom, an isotope called protium, has just one positively charged proton and no electron when ionised under the extreme conditions of fusion – and so an overall positive charge. Like charges repel, so two protiums naturally electrically repel each other. Gravity, of course, works against this, pulling them together. We have competing forces.

As if that wasn’t enough, there is another force, the strong nuclear force, that turns on when particles are very, very close to each other and pulls them together. It is this force that ultimately tips the balance; once it is activated, the two protiums can smash together.

The next part of the story is again more complex than popular narratives sometimes admit. Instead of instantly spitting out a helium atom, these two colliding particles actually spit out another type of hydrogen that has a proton and a neutron (deuterium), as well as a positron (the antimatter version of an electron) and another fundamental particle, a neutrino. In this step, yet another fundamental force comes into play, the weak nuclear force.

Catch your breath, because that isn’t the last of it. The newly formed positron is now positioned to annihilate when it inevitably comes into contact with an electron, a collision that produces two photons, or particles of light.

These photons will eventually journey out of the sun and maybe make it all the way to our planet, providing a small fraction of the sunlight that governs our lives. The deuterium also undergoes its own transformation, producing, among other things, another photon, which may reach us on Earth. This sequence, known as the first stage of the proton-proton chain, produces not just helium but also energy in the form of photons and neutrinos that are released out into the universe.

There is, to put it simply, an awful lot going on. Perhaps that goes some way to explaining why there is the odd inconsistency in scientific papers about solar physics.

The question of exactly how much energy is released in these chain reactions, for instance, and how frequently they occur involves calculations in nuclear theory and combining that information with (extremely safe) nuclear experiments on Earth. We are always refining the numbers.

So, to the person who asked me why there are inconsistencies in the literature on solar fusion: the truth is, we are still working out the details.

Chanda’s week

What I’m reading
I’m wrapping up Imani Perry’s new book South to America: A journey below the Mason-Dixon to understand the soul of a nation

What I’m watching
I rewatched all of Big Love recently with my spouse, because he had never seen it

What I’m working on
I’m doing some long-term planning for my research

  • This column appears monthly. Up next week: Graham Lawton

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