Since neutrinos interact so rarely with other matter, they can escape from the centre of the sun undisturbed. So if we detect them, they can tell us what’s going on in the interior of the sun: what nuclear fusion reactions are happening at what rate, how much total energy is produced, and even how hot it is. (16 million ºC, ± a few per cent. Yes, we know the temperature at the centre of the sun almost as well as that of the room you’re in right now. 🤯)

Of course, detecting them is hard … (2/8)

Since neutrinos interact so rarely with other matter, we need to build giant detectors to observe a reasonable number of them. My favourite detector? Super-Kamiokande! Just look at these stunning images of its interior: https://www-sk.icrr.u-tokyo.ac.jp/en/sk/experience/gallery/

Now, neutrinos can travel through the whole earth, so Super-K can even observe them at night. And it even sees more solar neutrinos at night!

Why, you wonder? Read on; it’s gonna get weird! 😍 (3/8)

There are actually 3 types: “electron neutrinos” (which prefer to interact with electrons), “muon neutrinos” and “tau neutrinos” (which prefer to interact with the electron’s heavier “sibling particles”, muons and taus, and are less likely to interact with electrons).
These three types can change into each other, back and forth; and they do that in very peculiar ways when going through matter. (Let’s skip the “why?” for now.)

Now, the sun initially produces only electron neutrinos … (4/8)

… but as those electron neutrinos exit the sun, they change their type! After this, only about 32% of them are still electron neutrinos. The other 68% have changed into muon or tau neutrinos—and since our detector Super-K contains pure water (with plenty of electrons, but no muons or taus), those 68% are about 6 times less likely to be detected.

So at daytime, Super-K sees 32% + (1/6 × 68%) ≈ 43% as many solar neutrinos as we’d expect if neutrinos didn’t change type.

But at night … (5/8)