Final answer:
At the center of a planetary nebula, a white dwarf is typically found, resulting from the outer layers of a star being expelled at the end of its life. Planetary nebulae are comparatively short-lived, explaining why there are more white dwarfs than planetary nebulae in the Galaxy.
Step-by-step explanation:
The center of a planetary nebula is usually occupied by a white dwarf. Planetary nebulae are formed when intermediate-mass stars (like our sun) reach the end of their life cycles. As these stars run out of fuel, they expel their outer layers into space, creating a shell of gas and dust that glows, forming a planetary nebula. The core that remains heats this gas but eventually cools and fades over time. The reason there are more white dwarfs than planetary nebulae in the Galaxy is because planetary nebulae are only visible for a comparatively short time. The gas disperses within tens of thousands of years, whereas white dwarfs can cool for billions of years before finally becoming black dwarfs.