Sounds like you are talking about generic names being trademarked - is that right? If so, then as a general rule, generic words cannot be trademarked.
However, there is an exception to the above rule! If the word acquires a secondary meaning in the eyes of the public then the word can be trademarked. For example, you can't obtain a trademark for Apple used in association with apples. However, you can obtain a trademark for Apple used in association with computers (secondary meaning). However, it's not "easy" to obtain secondary meaning of generic words. A lot of marketing and branding needs to be done to obtain that secondary meaning status in the eyes of the public.
Using your example of "sun." No-one can register the trademark Sun used in association with our earth's star, the sun. However, a third party can register the trademark Sun used in association with newspapers. If you used Sun.com in a different area of business than newspapers, there's no infringement and you won't lose your domain. If you use it in association with newspapers, there will be trademark infringement and you might lose your domain.
Please note the above is simply my own understanding of trademark law. If you need specific advice about a domain, you should consult with a lawyer.
