Why do my units uncloak when I attack in strarcraft?

ok so Im playing starcraft against my friend and I'm relatively new to the games so I send in a ghost ( im terran ) with cloak on to attack his base and there are no detectors there because I can freely roam the base without beinted but when I attack this blue specs sparkle over my ghost and instantly I the ghost is shot to death so I just ume that you cant attack while cloaked. But next round I play protoss my friend plays terran and he sends a ghost in cloaked and it shoots the crap out of my base until I get detectors and shoot him down what did he do differently thank you very much to anyone who answers