Q: When did astronomers realize that the Milky Way galaxy was composed of stars, and that the Sun was one of them?

A: The idea that the Milky Way was made up of stars was first proposed by the philosopher Immanuel Kant in the 18th century. However, it wasn't until the early 20th century that astronomers realized that the Sun was just one of many stars in the Milky Way.

In 1917, the astronomer Harlow Shapley used observations of globular clusters to estimate the size and shape of the Milky Way. He found that the Milky Way was much larger than previously thought and that the Sun was located far from the center of the galaxy.

Then, in the 1920s, Edwin Hubble used observations of Cepheid variable stars in other galaxies to show that they were much farther away than previously believed. This provided evidence that the universe was much larger than previously thought and that the Milky Way was just one of many galaxies in the universe.

Together, these discoveries helped astronomers to understand that the Milky Way was a vast collection of stars, and that the Sun was just one of many stars in the galaxy.

Q: When was it first realized by modern astronomy that there were multiple galaxies?

A: The idea that there were multiple galaxies beyond our Milky Way was first proposed by the astronomer Immanuel Kant in the 18th century. However, it wasn't until the early 20th century that astronomers began to gather observational evidence to support this idea.

In 1924, the astronomer Edwin Hubble used observations of Cepheid variable stars in the Andromeda Galaxy to show that it was much farther away than previously believed, and therefore much larger than the Milky Way. This provided the first strong evidence that there were other galaxies beyond our own.

In the following years, astronomers continued to study other galaxies and found that they came in many different shapes and sizes, and that there were vast numbers of them spread throughout the universe. This led to a revolution in our understanding of the structure and nature of the universe, and laid the foundation for modern astronomy.