`\sun` default to `\odot`

[Firestar-Reimu]

https://en.wikipedia.org/wiki/Astronomical_symbols

standard astronomical symbol (used on every textbook and article on astronomy) is U+2609, which look like \odot (and many astronomers simply use \odot to express the sun)

but stix2 use "white sun with rays" to match \sun

[Enivex]

This mapping has nothing to do with the font itself, but unicode-math

As you can see from the symbol list, \sun maps to U+263C (White Sun with Rays)

The astronomical symbol U+2609 is \astrosun, which is presumably what astronomers want to use with fonts that support it.