Spiral galaxy NGC 1566 in the constellation Dorado as seen by NASA’s Hubble Space Telescope.

This image captured by the Hubble Space Telescope showcases the spiral galaxy NGC 1566, affectionately known as the ‘Spanish Dancer Galaxy.’ NGC 1566 belongs to the category of weakly-barred or intermediate spiral galaxies, characterized by the absence of a clearly defined bar-shaped structure at its center. Its nickname stems from the mesmerizing, vivid swirls of its spiral arms, reminiscent of the graceful movements and colors of a dancer’s form. Situated approximately 60 million light-years from Earth within the constellation Dorado, NGC 1566 is a part of the Dorado galaxy group.

Galaxy groups are assemblies of galaxies bound together by gravitational forces, distinct from larger and more massive galaxy clusters that can contain hundreds of galaxies. Galaxy groups, on the other hand, typically encompass a smaller number, often just several tens of galaxies. Interestingly, they constitute the most common form of galactic gatherings in the universe, housing over 50% of all galaxies. While there is no precise numerical boundary distinguishing a galaxy group from a galaxy cluster, some astronomers propose that collections with less than 80 trillion solar masses should be classified as galaxy groups.

The membership of the Dorado group has undergone fluctuations in recent decades due to revisions in the list of constituent galaxies in scientific literature. This dynamic nature underscores the challenge astronomers face when identifying members of galaxy groups like Dorado. To comprehend this complexity, consider a photograph featuring an adult human and a large oak tree. By knowing the approximate sizes of the person and the tree, if a photo shows them appearing similar in size, we can infer that the person is much closer to the camera than the tree.

Astronomers tackling the identification of galaxy group members confront a similar problem. They don’t necessarily possess exact measurements of individual galaxy sizes but must deduce whether these galaxies are genuinely close together in space or if some of them are at varying distances. This process becomes more manageable with advanced observation techniques, but it remains a significant challenge in the field of astronomy.