The star left at the core of a planetary nebula is called a white dwarf.

Here's a breakdown:

* Planetary nebulae are formed when a star like our Sun runs out of fuel and sheds its outer layers. These layers expand and cool, creating a beautiful, colorful nebula.

* White dwarfs are the dense, hot remnants of the core of the original star. They are about the size of the Earth but contain about the mass of our Sun.

So, when a star like our Sun dies, it doesn't disappear completely. It leaves behind a beautiful nebula and a very dense, hot white dwarf star.