The Flame was considered so sacred, that all emperors after their coronation usually made a oath to the Eternal Fire. Senators, Legislators and High Judges had as well to make an oath to the Flame before taking their position.It was said that if the flame was extinguished, the Empire will fall and calamity will shock upon all its inhabitants.
Thats why Hestias was a target for the Union during the wars between both nations. The Union wanted to destroy this symbolic temple to destroy the Imperial might. Although they failed on their ultimate goal, the Flame, which remain largely intact, they severely damage the Planet that soon had to be rebuilt.
The Priestesses were so sacred that they eventually were married to the state and were forbidden to be with any man. Usually born out of bastard unions of the Nobility with Rich non-noble families, sometimes as daughter of high status and even sometimes simple orphans were recruited. That was true when the Union troops assaulted Hestias and raped the Priestesses, some were also killed, while protecting the flame. This was seen as the worst offense ever made and new priestesses were to be recruited fast since they have to be virgin. This led to the recruitment of several orphan women all over the Empire. This started the tradition of recruiting orphans as well.
They dress in white robes with black and gold decorative items, as a representation of the sanctity and the National Colors of the Nation. If they were caught in a relationship or not being virgin, punishment was a death sentence for both or at least for the Priestess if no man was revealed.
Priesthood was usually a livetime service, although in case of force loss of virginity, like an attack and rape, the priestesses get retired of their functions without being killed and receive a state pension and are allowed to live in the Priesthood house for their entire lives.
After relations between the Union and the Empire improved, the Union government travelled to Hestias to present their forgiveness and make an oath to the sacred fire of pardon and repentment.