An explosion that occurs at the end of a massive star's life is a _________ ?

A ___________ is a large cloud of gas and dust in space.

A low- or medium-mass star becomes a ____________ at the end of its life.

A ___________ appears to emit regular bursts of radio waves.

A ___________ forms from the leftover material from a supernova; it may contain as much as three times the mass of the sun.