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Escape
Many sea stars can self-amputate their limbs if attacked by a predator -- it's less a matter of an arm being ripped off and more a matter of it being "given up" by the sea star. This allows the animal to escape its predator relatively unharmed.
Regeneration
Instead of brains, sea stars have a complex system of nerves that send signals up and down their bodies. Once a sea star's arm is lost, that nervous system can regenerate a new limb, and sometimes multiple limbs. This occurs when a limb is amputated along a portion of the sea star's central disk, the area in which its arms or "rays" meet. In fact, if you cut a sea star into pieces along its center, each segment will have enough of the animal's nervous system to regrow an entirely new body. Regeneration is a slow process, however; it may take up to a year for one short limb to grow back.
Arm Stumps
There are instances when a sea star's limb will not regenerate. This commonly happens if the amputated limb did not contain any part of the animal's central disc. The tissue around the wound will heal, fold over and close, leaving a stump. Sadly, this alteration to the sea star's body can make it more difficult for the animal to eat and find a mate.
Sea Stars and Cloning
It's a sea star's stem cells that allows the animal to regenerate its parts; sea stars ostensibly duplicate themselves when they form new bodies from old tissue. As such, this animal and its unique ability are widely studied by those researchers looking to better understand, apply and perfect the art of human cloning.