Hoag’s Object, a remarkably distinct ring galaxy, presents a fascinating puzzle with its ancient stars at the core separated from younger, blue stars in its outer ring. Despite being discovered 70 years ago, its formation remains unexplained.

Occasionally, astronomers encounter celestial phenomena that elude comprehension. For decades, the study of various distant galaxies has allowed scientists to categorize them primarily into three groups:

• Spiral galaxies, characterized by sprawling arms populated with stars,
• Elliptical galaxies, where stars orbit a dense central area,
• Irregular galaxies, lacking a defined structure and often resulting from interactions between multiple galaxies.

While spiral and elliptical galaxies are prevalent—spirals thriving in isolated areas and ellipticals dominating galaxy clusters—a singular discovery was made in 1950. Arthur Hoag identified Hoag’s Object, a galaxy defined by its striking ring structure. Yet, even after seven decades, its origin remains a scientific enigma.

Initially, astronomers could only speculate on the formation of ring-like structures, which typically feature a bright core surrounded by a halo. However, none of these models applied to Hoag’s Object. While planetary nebulae like the Ring Nebula consist of dying stars at their centers, Hoag’s Object's nucleus does not conform to this pattern.

The halo reveals a population of vibrant blue stars instead of the expected emission lines, confirming its galactic nature. Hoag even hypothesized that it might be a gravitational lens, a phenomenon that can create a ring-like appearance under specific alignments. However, both the nucleus and halo exhibit identical redshifts, indicating they are part of the same system, not a result of lensing effects.

In his initial publication, Hoag ominously remarked, “Since the object is unique, I suggest that a proper identification would be a worthwhile short-term project.” Over the years, many galaxies have been found with similar features, contributing to a sub-category of irregular galaxies with ring-like structures, yet Hoag’s Object remains difficult to classify.

Typical ring galaxies usually result from the collision of a small galaxy with a larger one at high velocities, generating density waves that push material outward and stimulate star formation along the edges. These interactions are often visually stunning.

The Cartwheel Galaxy exemplifies this phenomenon, formed from a high-speed collision between a gas-rich galaxy and a smaller, rapidly moving one. This event creates shock waves that compress gas, inducing new star formation and resulting in a prominent outer ring, characteristic of classical ring galaxies.

To achieve the observed effects, density waves must travel at incredible speeds—around 100 kilometers per second or more. Additionally, ionized hydrogen must exist long enough to transition back to neutral hydrogen over millions of years, forming the basis of a classic ring galaxy.

In traditional ring galaxies, certain characteristics are evident that are notably absent in Hoag’s Object:

• The existence of a second galaxy with a distinct stellar population,
• Signs of collision remnants that should persist for billions of years,
• A significant relative velocity among various components.

Conversely, upon examining Hoag’s Object closely, we find that the older core has minimal relative velocity to the ring, implying a tranquil formation process. There is no indication of an additional galaxy or remnants, which contradicts the conventional formation theories for ring galaxies. Additionally, the age of the blue stars in the outer ring suggests a younger age than would be expected from a historical collision.

Some scientists have proposed that Hoag’s Object might be linked to spiral galaxies, albeit with instabilities creating gaps between inner and outer components. However, the nucleus is not disk-shaped, a defining trait of spiral galaxies; it has a spheroidal form.

Galaxies can possess a spheroidal bulge along with extended spiral arms, leading to speculation that a galaxy resembling Hoag’s Object could arise from such configurations. Unfortunately, these galaxies lack the necessary components to form a separated ring around a core: specifically, a substantial central bar. While bar instabilities are real, spiral galaxies with bars do not typically feature large spheroidal bulges at their cores, making this explanation unworkable.

Encouragingly, even rare objects tend to appear more frequently when extensive surveys of the Universe are conducted. In 2017, citizen scientists participating in the Galaxy Zoo 2 project, based on data from the Sloan Digital Sky Survey, identified and cataloged 3962 Ringed Galaxies.

Upon closer inspection of these galaxies, most were found to be spiral types with inner or outer rings, indicative of recent interactions or mergers. Typically, these galaxies exhibit star formation in both their inner and outer regions, with many containing central bars. In contrast, fewer than 40 of the nearly 4,000 galaxies displayed classic ring or polar ring characteristics, representing less than 1% of this selective sample.

Nevertheless, a small number of galaxies exhibit remarkable similarities to Hoag’s Object. PGC 1000714, studied in 2016, stands out as perhaps the closest relative. It features an elliptical core encircled by a near-perfect outer ring, containing a younger population of stars compared to the inner core.

A distinct gap devoid of stars and gas lies between the spheroidal core and the outer ring, and there’s no evidence of a barred structure within the inner region, making it the most similar galaxy to Hoag’s Object. The core is estimated to be 5.5 billion years old, while the outer ring appears only 130 million years old, indicating recent formation despite the absence of a known companion.

Other galaxies, including NGC 6028 and UGC 4599, also share features with Hoag’s Object, though they may not be directly related. NGC 6028 possesses a luminous spheroidal core surrounded by a faint outer ring, but the core's elongated shape suggests a potential bar-like feature. The outer ring's asymmetry implies it might result from tightly wound spiral arms, which, while intriguing, do not directly correlate with Hoag’s Object.

UGC 4599 exhibits an elliptical-like core with an outer ring rich in star formation, but its faintness and low gas density in the ring complicate comparisons. Numerous unresolved features persist within these and other analogous systems, leading to ongoing uncertainties regarding the mechanisms responsible for their formation.

Ultimately, the enigma surrounding Hoag’s Object enhances its allure. Detailed observations reveal an ancient star population within a core that resembles an elliptical structure, lacking significant gas and central bar features. A gap exists, nearly devoid of gas and stars, extending for tens of thousands of light-years. Surrounding this is a vast halo of young stars, the origins of which remain uncertain.

It is possible that a low-speed collision occurred billions of years ago, leaving behind this peculiar configuration as a remnant. Alternatively, gas or a series of smaller galaxies may have been absorbed by the larger elliptical core, giving rise to the outer ring. Whatever the reality, Hoag’s Object continues to captivate astronomers and enthusiasts alike, remaining a profound mystery after seven decades of investigation.

Starts With A Bang is now featured on Forbes, with a republished version on Medium after a 7-day delay. Author Ethan has written two books: Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.