What is the major observational difference between a Seyfert galaxy and a normal spiral galaxy?

An active galactic nucleus is created when the supermassive black hole at the center of the galaxy has an accretion disc of gas and dust falling into it; the material entering the accretion disc heats up and releases a lot of electromagnetic radiation. A Seyfert galaxy is a spiral galaxy with an active nucleus.

Extremely bright sources of x-rays, ultraviolet light, and visible light are known as Type I Seyfert galaxies.

Infrared and visible light sources with extreme brightness are known as Type II Seyfert galaxies.

Galaxies that are Type II Seyferts eventually give rise to Type I Seyferts.

Approximately 10% of galaxies are Seyfert galaxies.

The major observational difference between a Seyfert galaxy and a normal spiral galaxy is the presence of a bright, compact nucleus emitting strong and variable radiation in various wavelengths in Seyfert galaxies, particularly in the form of intense emission lines. Normal spiral galaxies typically lack such characteristics in their nuclei.