The distinction between a planetary nebula and a galaxy is primarily determined through spectroscopic analysis and detailed imaging. When an object is mistaken for a planetary nebula, it is usually because it appears as a small, fuzzy, and glowing cloud in early telescope observations. To resolve this confusion, astronomers look for specific chemical signatures and light patterns.
Spectroscopic evidence is the most definitive factor. A planetary nebula is a shell of gas ejected by a single dying star. Its spectrum shows strong emission lines from ionized elements like oxygen and hydrogen, but it lacks the vast range of stellar signatures found in a galaxy. In contrast, a galaxy contains billions of stars. Its spectrum reveals a complex mixture of light, including absorption lines from many different types of stars and a redshift caused by the expansion of the universe.
Morphologically, high resolution imaging allows astronomers to see the structure. A planetary nebula typically shows a symmetric or bipolar shape centered on a single star. A galaxy, however, often reveals a nucleus surrounded by a much more complex distribution of stars, dust lanes, and spiral arms that cannot be explained by a single stellar system.
