Britannica Sources used in this answer

How did George Ellery Hale discover that sunspots have magnetic fields?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Sun Joshua trees at sunset, Joshua Tree National Park, southern California, U.S.

In 1908, American astronomer George Ellery Hale discovered that sunspots possess strong magnetic fields by observing the splitting of their spectral lines into multiple components.12

This phenomenon, known as the Zeeman effect, occurs when a light source is placed within a magnetic field, causing its spectral lines to split into components of slightly different frequencies. By applying this principle to his observations at the Mount Wilson Observatory, Hale was able to demonstrate that sunspots are regions of intense magnetic activity in the Sun's photosphere.324