The Hubble Tuning Fork, also known as the Hubble Sequence, is a classification system for galaxies developed by the astronomer Edwin Hubble in the early 20th century. This system was introduced in Hubble's 1926 publication, "Extra-galactic Nebulae," and later refined in his book, "The Realm of the Nebulae," published in 1936.
The Hubble Tuning Fork diagram categorizes galaxies based on their visual appearance into three main categories:
Hubble proposed that galaxies could evolve along this sequence, with elliptical galaxies possibly transforming into spirals or barred spirals over time, although this aspect of his model has been subject to debate and revision with modern understanding of galaxy formation.
Before Hubble's work, galaxies were not well understood as separate systems outside of our own Milky Way. His classification was revolutionary as it provided a framework for understanding the diversity of galaxy types observed through telescopes. Hubble's observations were initially made at the Mount Wilson Observatory where he used the 100-inch Hooker telescope, one of the most powerful telescopes of its time.
The Hubble Tuning Fork has been fundamental in:
While the tuning fork diagram simplifies the complexity of galaxy types, it laid the groundwork for later, more detailed classification systems and has been instrumental in the study of galaxy morphology.
With advances in technology and deeper insights into galaxy formation and evolution, astronomers now recognize that the Hubble classification does not fully capture the intricacies of galaxy types, especially regarding the formation processes and the role of dark matter. However, it remains a critical historical and foundational tool in observational astronomy.