A pulsar is a rapidly rotating highly magnetized neutron star, formed in the supernova explosion of a massive star.
When initially formed, pulsars are "born" wil small rotation periods, on the order of 1 to 10 milliseconds, and and large slow down rates, P-dot, on the order of 10-12 seconds per second.
They then evolve initially along lines of constant slow down rate.
Eventually, their slow down rate decreases but their magnetic field also weakens, making their radio emission less powerful. When they are too faint to be detected, we say they have crossed the pulsar "death line" and have become invisible.
