How does a protostar differ from a star?
It doesn't differ from a star, it is a star and it is the second stage in a stars life after the nebula stage.
When a protostar rises to 10000000 degrees celsius will a protostar become a star before or after nuclear fusion?
a low mass protostar is the begining formation of a low mass star. a low mass protostar is formed by a nebula (stellar nursery), like a cloud in space, where all stares come from. the definition of protostar is: a collection of gas, and dust who`s gravitational pull is causing it to collapse on itself & form a star. a LOW MASS protostar just has a LOWER MASS then a HIGH MASS protostar. Your Welcome:)