# What is proper motion of stars caused by?

B) "Proper velocity" is a concept in Relativity. It is the derivative of outside-observed displacement of a relativistic object with respect to "proper time", which is the time measured by the relativistic object, and can exceed $c$. Note that the use of the word "proper" in this context makes a lot more sense when one realises that it comes from the French "propre", meaning "own".