// add a newly discovered star
public void AddDiscoveredStar(KnownStar s)
{
discoveredStarSystems.Add(s); ;
}
// update last visited time of star
public void UpdateLastVisitedTime(KnownStar s)
{
discoveredStarSystems.Find(x => x.starID == s.starID).lastVisited = s.lastVisited;
}