"Then solar neutrinos scatter off electrons in a large vat of liquid scintillator "
Is liquid. Can scintilate under the right circumstances.
As we near the northern winter solstice, the Sun continues to produce a steady power output of 384.6 yottawatts resulting from the fusion of hydrogen into helium in two distinct nuclear reactions. Direct observation of the secondary cycle was published in the journal Nature for the first time yesterday. A team of physicists …
It's a good approximation though. Big bang nucleosynthesis is expected to have made, by mass, about 75% hydrogen, 25% helium-4, 0.01% deuterium and helium-3, some trace amount of lithium and essentially nothing else.
So if you're interested in stars it makes complete sense to look at what they're made of in terms of 'stuff made in the big bang' (which to a really good approximation is hydrogen and helium) and 'stuff made later', where they call the stuff made later 'metal'. Knowing how metallic a star is tells you a lot of interesting things about it.
I think the usage was coined long before we knew about the big bang, that space was expanding or even that there were galaxies, and before we knew atoms had nuclei (remember the plum pudding model?) let alone that nuclei could fuse.
It's probably because of where absorption lines are in the spectrum. So we looked at the sun and saw lines for hydrogen, helium and other "chemical metals". (In fact the helium line was initially though to be a line for sodium - a metal.) It was only later we discovered non-chemical metals were much more abundant.
And it remains the case that most stars are hydrogen, helium and rounding errors. (Yes, that's because of the big bang, but you're looking at it backwards - if the big bang had produced a different pattern of elements it wouldn't matter provided stars had ended up as we see them today.) And when you look outside stars, helium becomes irrelevant; you find ionised hydrogen (HII), atomic hydrogen (HI), molecular hydrogen (H2), free electrons, magnetic fields and pretty-coloured soot.
Way back when I was in college, I was told that an engineering equation was "good enough" if it was accurate to 3 decimal places; a physics equation was good enough if it was within an order of magnitude; and an astrophysics equation was good enough if the units came out correctly.
That was in the days when slide rules ruled....
Just to keep everyone up to speed ln this, in the presence of a large gravity, a neutron can decay into a proton, an electron, and 2 anti-neutrinos. The reason for the particle count is conservation of spin; hence 1 neutron decays into two particles and two anti-particles. Mass+energy are also conserved, as well as momentum. So the decay product particle velocities will reflect any mass defect from the reaction as kinetic energy, and their resulting directions of motion will conserve momentum.
This is why scientists believed the anti-neutrinos actually existed, because they apparently observed a difference between the mass+energy of the neutron vs its decay products, and possibly a difference in total momentum as well [implying the existence of unknown/undetected particles].
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