The word "sun" is actually a very old word, with roots going back to the Proto-Indo-European language, the ancestor of many modern languages. It's likely that the word for "sun" in the Proto-Indo-European language sounded something like *sau̯el-.

The word "sun" has been used to refer to the star at the center of our solar system for as long as we have had language. We don't know exactly why our ancestors chose that particular word, but it's likely that it was chosen because of the sun's bright, fiery appearance.

Here's why it's not a "proper" noun:

* Other stars exist: We now know that the sun is just one of billions of stars in the universe. If we called it a proper noun, it would imply uniqueness, which is not true.

* It's a general term: We use the word "sun" to describe any star that is the center of a solar system. For example, we might say, "The sun in the Alpha Centauri system is similar to our own sun."

Therefore, it's more accurate to consider "sun" as a common noun that refers to any star, rather than a specific name like a proper noun.