What visual characteristic distinguishes an active comet from sharp, point-like stars and asteroids?

Lack of a sharp outline (fuzzy/hazy appearance).

The defining visual characteristic of an active comet, particularly when it nears the Sun, is its lack of a crisp, defined edge. Unlike stars or asteroids which appear as sharp points of light even through telescopes, a comet possesses a nucleus composed of rock, dust, and ices. As this icy body approaches the Sun, these ices sublimate, releasing gas that forms a vast, tenuous atmosphere called the coma. This coma diffuses the reflected sunlight, making the entire head of the object appear distinctly fuzzy or hazy. This contrasts sharply with the crisp appearance of non-volatile bodies like asteroids, which maintain sharp outlines regardless of viewing magnification.