Final answer:
White dwarfs are the cores of low-mass stars that have reached the end of their lives, and their formation is accompanied by the production of planetary nebulae. However, there are more white dwarfs than planetary nebulae in the Galaxy due to the longer lifespan of white dwarfs compared to the relatively short-lived planetary nebulae.
Step-by-step explanation:
White dwarfs are the cores of low-mass stars that have reached the end of their lives. These stars have shed their outer layers, leaving behind a dense core composed mainly of carbon, oxygen, and neon. As the core collapses under its own gravity, the electrons in the core become degenerate and exert a pressure that halts further contraction. The end result is a stable, compact object known as a white dwarf, with a diameter determined by the extent of electron degeneracy.
Planetary nebulae, on the other hand, are formed during the late stages of a star's evolution when it ejects its outer layers. The ejected material forms a glowing shell of gas and dust surrounding the remnants of the star, which includes the white dwarf at its core. Therefore, white dwarfs and planetary nebulae are closely related, as the production of a planetary nebula marks the transformation of a star into a white dwarf.
However, there are more white dwarfs than planetary nebulae in the Galaxy because planetary nebulae are short-lived phenomena. They fade away over thousands of years, while white dwarfs can persist for billions of years. As a result, the number of white dwarfs accumulated over the history of the Galaxy is far greater than the number of planetary nebulae currently visible.