It's not.
morning stars have 1 heavy weight at the end like the one in the 1st pic above. Flails have several chains with smaller weights on them. However, any handheld stick/chain/weight combo is a flail including a Morningstar, but not the other way around. You can flail (swing wildly) your arms with any weapon in hand such as a mace. English class over. ;o Edit:Morningstar refers to the spiked ball design and it comes in both a mace and flail design.