Just a short one this week since these posts have been getting pretty long lately.
The word sun comes from the Old English sunne, which means sun. As you can see, it had more letters back then, as well as a slightly different pronunciation (it would have been more like sunny). Plus because of how Old English works it would have been a feminine word, which I just like. It comes from the Proto Germanic sunnon, which can be traced back to the Proto Indo European sawel-, the sun.
Solar showed up in the mid fifteenth century, which means it probably came after sun did. It’s from the classical Latin solaris (solar), from sol (sun), which happens to also be from sawel-. So solar and sun happen to be from the same place, just by completely different routes.
Helio- is a prefix we use to things related to the sun, like heliocentric, or the astronomical words anthelion and aphelion. It’s also related to the plant heliotrope and the gas helium. Helio is from Helios, the Greek sun god and is yet another word descended from sawel-. No, I don’t know where the S went.
TL;DR: the old word for sun is everywhere because the sun is pretty important and it makes sense that the word for it wouldn’t change much.
Tony Jebson’s page on the Origins of Old English