The planetary nebulas and the supernova remnants represent debris left over from dying stars. Stars more massive than about 8 Solar masses leave behind a supernova remnant while less massive stars, like the sun, leave behind a planetary nebula.

The above remnants represent what is left of the outer layers of the stars. However, the cores of the stars, where the nuclear burning took place, remain. Stars like the Sun leave behind what are called white dwarfs while more massive stars leave behind neutron stars and sometimes black holes.


The image below shows the relative sizes of these compact stellar cores. Their sizes are shown relative to the Sun which is (still) a normal star. Typically white dwarfs are the size of the Earth, Neutron stars have a radius of about 10 km while black holes are even smaller.