What causes a quasar to die?
A quasar will stop radiating when is runs out of the material which generates the output.
A quasar is thought to be the result of material falling into the accretion disc of a supermassive black hole at the centre of an a distant galaxy.
The huge energy output is due to gravitation stresses and friction on the material falling into the accretion disc caused by the black hole.
If the supply of material falling into the accretion disc becomes exhausted the quasar will stop emitting.
A quasar can be ignited or reignited if additional material becomes available as a result of the black hole's gravitational pull.