How did Hubble find that the objects that were first called spiral nebulae are instead spiral galaxies?

Hubble found that the objects initially called spiral nebulae are actually spiral galaxies by observing individual stars within these nebulae and measuring their distances. He discovered that some of these "nebulae" were too far away to be part of the Milky Way, indicating that they were separate galaxies. This evidence led Hubble to redefine them as spiral galaxies, contributing to our understanding of the universe's vastness and structure.

Hubble found that the objects initially called spiral nebulae are instead spiral galaxies through careful observation and analysis of their properties, including their distances, apparent sizes, and the characteristics of their individual stars. He used the Cepheid variable stars, whose brightness fluctuates regularly over time, as "standard candles" to measure distances to these objects. By comparing the brightness of Cepheid variables in spiral nebulae to those in nearby galaxies, he determined that spiral nebulae were actually much farther away than previously thought, indicating that they were not part of our own Milky Way galaxy but were separate galaxies in their own right. This discovery revolutionized our understanding of the universe's scale and structure.