The sun is an amazing celestial object. Even though it looks about the same size as our moon when viewed from Earth’s surface it’s almost 400 times further away which should give you an idea of just how unfathomably large the sun is. It also heavily influences nearly every aspect of our Earth, providing nearly all of the energy that we, and all other lifeforms on this planet, consume on a daily basis. You’d be forgiven for thinking that we understood it completely however as whilst nearly anyone would be able to tell you that the sun is powered by fusion we, funnily enough, didn’t actually have proof of this.
That is, until now.
It sounds silly right? The theory of the sun being a giant ball of fusion has been around for 75 years and is pretty much established as a scientific fact. Indeed many of the observations that we’ve made of the sun support that theory and the small scale replicas we’ve made also seem to exhibit similar properties. However the surface of the sun, as we see it, doesn’t really tell us the whole story. Indeed the light emitted from the surface of the sun is hundreds of thousands of years old, spending most of its life worming its way out of the deeper layers of the sun. Should we want to verify that for sure we need to observe the products of fusion reactions happening now and, bar venturing into the sun itself, there’s only one way to do that: by observing one of our universe;’s most elusive particles, the neutrinos.
Specifically the neutrinos are called PP neutrinos, those which arise from the fusion of two protons to form helium. A fusion reactor on the scale of the sun generates countless numbers of these particles every second and, thanks to their near massless nature, they rush out unimpeded directly from the sun’s core. However the same properties which allow them to move at such great velocity away from the sun also prevents them being easily detected. Combine this with the fact that PP neutrinos carry less energy than regular neutrinos do you can see why definitive proof of fusion happening within the sun as eluded us for so long. Researchers in Italy though crafted an experiment to capture these ever elusive particles and their research has finally bore fruit.
The Borexino experiment uses a large device called a scintillator, essentially a large array of light detecting devices immersed in ultrapure water. It’s then buried deep underground (about 1.4KM) in order to shield it from cosmic rays and other stray radiation. This experiment was specifically designed to verify the solar output of neutrinos against the standard solar model in order to verify that fusion was indeed occurring within our sun. It began collecting data about 7 years ago and at the beginning of this year they had enough data to submit their final report. The results line up perfectly with what the standard solar model predicts which, for the first time, verifies that fusion is indeed occurring within our sun and has been for a very long time.
It may seem like a silly thing to do but verifying things like this is the key to ensuring that our understanding of the universe is in line with reality. We might have known that fusion was going on the sun for decades but without definitive proof we just had a good model that matched some of the observed behaviours. Now we know for sure and that means that our standard solar model is far more robust than it was previously. Thus, with this new information at hand, we can dive even deeper into the model, probing the various curiosities and figuring out just what makes our sun tick. We might not ever know everything about it but part of the fun of science is finding out what you don’t know and then trying to figure it out.