What is a Galaxy? Unraveling the Complexity Behind This Fascinating Cosmic Structure

What distinguishes a collection of stars from an entire galaxy? This seemingly simple inquiry poses a significant challenge to astronomers today.

Most casual stargazers view galaxies as distinct entities, akin to completed masterpieces. When peering through a backyard telescope, one can observe billions of stars or even more than 100 billion, radiating in a spiral formation of vibrant light. My experiences as a theoretical particle cosmologist reveal that galaxies are enveloped by an unseen dark matter halo, stretching far beyond the visible sphere. It’s also crucial to note that not all galaxies conform to the spiral design; some take on oval shapes, resembling a flattened sphere, making the classification of galaxies more complex than it appears.

As I’ve noted in previous discussions, our classification of galaxies continues to evolve. Clearly defining an object with a distinct spiral formation and vast star count is straightforward, but what about those that appear spheroidal yet possess millions of stars? Some of these may be categorized as globular clusters—collections of tens to millions of stars bound by gravity within a mere few light years.

These globular clusters are exclusively found within galaxies, indicating a fundamental difference between them and galaxies themselves. Globular clusters exhibit compactness, in contrast to galaxies, which expand more diffusely throughout space. The same holds true for dwarf spheroidal galaxies, which, despite being smaller than our Milky Way, still exhibit a relatively vast reach and often contain a more diverse stellar population. Moreover, while dwarf spheroidals are surrounded by their own dark matter halos, globular clusters typically are not.

Visualize a nested structure of galaxies enveloped by dark matter: at the center lies the Milky Way, encircled by a substantial halo, featuring a small quasi-spheroid and then an even smaller subhalo. This framework assists astronomers in distinguishing between galaxies and globular clusters, as the presence of dark matter identifies an object as a galaxy.

The understanding of these classifications was relatively clear until 2005, when the Sloan Digital Sky Survey (SDSS) released its initial data set. This monumental project aimed to document a quarter of the night sky, revealing numerous celestial entities never before observed. Among these findings was a faint cluster of stars near the Milky Way, dubbed ultra-faint satellites, which blurred the lines separating globular clusters from galaxies.

Further investigations into these enigmatic objects have unveiled some to be galaxies saturated with dark matter, yet the challenge persists, as these satellites remain elusive and hard to detect, living in what is described as the “valley of uncertainty.” A 2018 study by Blair Conn and colleagues noted these peculiar entities—neither clearly galaxies nor definitively not.

Despite our anticipation for data to clarify these obscure classifications, recent sky surveys have unveiled even fainter clusters of celestial objects. We have learned to hesitate before declaring what is or isn’t a galaxy. A recent 2023 study led by Simon Smith from the University of Victoria has reported the discovery of Ursa Major III, labeled as “the least luminous known satellite of the Milky Way.” While this assertion instills confidence, the supporting evidence comprises merely 60 stars—an extraordinarily small number.

Ursa Major III might seem diminutive, yet its implications could resonate widely. A study last year asserted that, if it qualifies as a galaxy, it could help eliminate certain dark matter models. Determining whether Ursa Major III and its counterparts are genuinely galaxies could significantly impact astrophysics, cosmology, and particle physics.

Progress continues on this front. Recently, William Cerny and his team at Yale University published findings from an extensive study on a large group of these mysterious objects. Their conclusion? A diverse array exists among these entities, though further investigation is essential. We may not have definitive answers yet, but we sit at an exciting crossroads—firmly engaging in the pursuit of knowledge and further discoveries.

Your reading list:

Special collection: Poets, particularly Courtney Lamar Charleston and Comeonfune Felix’s Manifesto: Let the poets rule.

What are you currently viewing?

Too Much Alfred Hitchcock!

What are you presently working on?

Prepping for the U.S. release of my book, The End of Space and Time, on April 7th!

Chanda Prescod-Weinstein is an associate professor of physics and astronomy at the University of New Hampshire. She is the author of Turbulent Universe and her forthcoming book The Ends of Space and Time: Particles, Poetry, and the Boogie of Cosmic Dreams.

Unraveling Cosmic Mysteries in Cheshire, England

Join some of the brightest scientific minds for an insightful weekend delving into the universe’s enigmas, inclusive of a tour of the iconic Lovell Telescope.