This is a star less than mass of the main sequence star that has exhausted all of its nuclear fuel and has collapsed to a very small size.

White dwarf

Explanation:

These are what stars become when have exhausted their nuclear fuel. When the stars near the end of its nuclear burning stage, they expels most of their outer material thereby creating a planetary nebula in which only the hot core of the star remains.

They are stars which have less than mass of the main sequence star that has exhausted all of its nuclear fuel and has collapsed to a very small size.