They are called white dwarfs.
its called a super nova.
No, but some stars end their life by becoming a black hole.
The massive stars turn into gas
The most massive stars will end up as black holes. Those are the stars that have more than approximately 3 solar masses at the end of their life - i.e., AFTER the supernova explosion.
Stars end their lives differently based on their mass. Massive stars end in supernova explosions, leaving behind either a neutron star or a black hole. Smaller stars like our Sun end in a less dramatic process called a planetary nebula, eventually becoming a white dwarf. The difference in mass determines the final fate of a star.
Massive stars that are at least eight times more massive than the Sun end their life as a supernova. During the explosion, these stars release a tremendous amount of energy and can briefly outshine an entire galaxy.
A supernova
They are not. A supernova is an explosion of a star. Blue stars usually end their lives in such explosions.
A Protostar.
There are all types of stars in the Milky Way.See related question
I am not aware of any specific element "signaling" this. Towards the end of a stars life, however, relatively large amounts of heavier elements are produced.
There are small hot stars - White dwarfs, neutron stars but by furtue that they are hot, means they are not near the end of their lives. It takes a long long time for all the residual heat to escape into the Universe. So, there are NO hot stars near the end of it's life.