Cassiopeia boasted that she was more beautiful than the Nereids who complained to Poseidon. Poseidon sent a sea monster and demanded that Andromeda, Cassiopeia's daughter, be sacrificed to it. Luckily, Perseus came along and saved Andromeda. Cassiopeia was made a constellation.
It sould also be noted that Cassiopea worshipped the ground her daughter walked and openly bragged about Andromeda's beauty in front of the Gods. This ended in Andromeda being offered to the Kraken as a sacrifice in order to beg Hera for forgiveness.