What illuminates the expanding gas cloud comprising a planetary nebula?
Answer
The extremely hot, dense stellar remnant, specifically a white dwarf, left at the center.
The glowing shell of gas and dust that forms a planetary nebula is made visible because it is illuminated from within by the stellar core that remains after the star has shed its outer layers. This remnant is characterized as being extremely hot and very dense, identifying it specifically as a white dwarf. This central object radiates intensely, causing the surrounding ejected material to glow brightly, allowing astronomers to observe the nebula's structure. The cloud itself is composed of material that was previously the star's outer atmosphere.
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