Bosses are enormous pirates that appear during the final round (4-5) in pirate sectors with a level of 4 or more. It Generally has a lot more health than Tank Heads and Ogres and shoots in a horizontal spread of 5 shots. It is recommended to fight bosses with a flight group with engineer class ships.
In level 4 sectors, they will only spawn one at a time. But when you reach Bluetonium sectors (level 5-6), they can spawn 2 at the same time.