There's a lot that astronomers have figured out about the universean amazing amount, considering that all we have to work with is what the sky hands us. You can't do experiments in astrophysics. But we have figured out the basic structures of the universe in stars and galaxies and various clusters at different scales, and we have figured out the energy sources in stars and black holes and supernovae, and have a good idea how the different chemical elements came to be and where they came from.
But we don't understand cosmic rays. There are competing, partial theories. But especially for the most energetic cosmic rays, we have no good theories about where they come from and also no good ideas how they last as long as they do without giving up their energy to the very tenuous gas that fills interstellar space.
When it comes to the sun, we thought we understood all the basics about how it works. The core is a hydrogen bomb, millions of degrees. At the surface, the part of the sun we see has a temperature 'only' thousands of degrees.
But the sun's corona (atmosphere) is another story. It is millions of degrees. We have models for how it gets to be so hot based on magnetic heating, but I'm not sure we would have predicted such high temperatures if we hadn't discovered the hot corona first, by looking at it.
Now there's a new mystery. There are gamma rays coming from the sun, and some of them have really high energies. With energies like that, you could make cosmic rays out of them without too much troublejust add a proton, and there are plenty of those around. But no one expected the sun to be producing gamma rays (or cosmic rays) of its own, especially at the high energy end. These gamma rays are so energetic that if we translate to a temperature, it would be trillions of degrees! That's far too hot to be plausible. There must be some kind of magnetic engine that is shooting these particles out with such impressive energy.
There's a mystery here and also an opportunity. If we can figure out how the sun generates such high energy photons, maybe other stars are doing the same thing"
Article by Natalie Wolchover at Quanta Magazine