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Why can an early planetary nebula look like a star from Earth?

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Planetary nebula Southern Ring Nebula as captured by the James Webb Space Telescope in the near-infrared (left) and the mid-infrared (right). The dimmer star at the center of this scene has been sending out rings of gas and dust for thousands of years in all directions. The Southern Ring Nebula is approximately 2,500 light-years away.

An early planetary nebula can appear as a star from Earth because it is physically too small to be resolved as a disk by telescopes. While these nebulae are actually expanding shells of gas ejected by dying stars, their compact size at great distances causes them to look like point-like stellar objects when viewed or photographed directly.12

In this early evolutionary stage, the nebula is relatively small and dense, consisting of gas ionized by radiation from the exposed, hot core of the central star. Because the nebula has not yet expanded to a size that subtends a significant angular diameter, it lacks the distinct, extended appearance associated with more mature or closer nebulae. Many planetary nebulae remain difficult to distinguish from stars until they expand sufficiently or are observed with high-resolution imaging.12