Stephenson 2-18 (St2-18), a red supergiant star located nearly 20,000 light-years away in the constellation Scutum, is one of the largest single objects known. Its estimated radius is over 2,150 times that of the Sun, resulting in a volume approximately 10 billion times greater. Its size places it near the theoretical maximum for a star, yet the universe holds countless structures that dwarf this stellar giant. The exploration beyond St2-18 leads through a hierarchy of increasing scale, moving from stars to vast assemblies of matter that define the architecture of the cosmos.
Comparing Stellar Giants
Determining the exact size of red supergiant stars like St2-18 presents significant measurement challenges, which is why the ranking of the “largest star” often fluctuates. These stars are enveloped in vast, opaque atmospheres and are often situated at great distances, making their true edges difficult to pinpoint. The faint outer layers of their atmospheres are diffuse, meaning there is no sharp surface boundary like on a planet, further complicating radius estimates.
The current estimate of St2-18’s radius, approximately 2,150 solar radii, places it at the top of the size list, narrowly surpassing other contenders like UY Scuti and WOH G64. UY Scuti, another red supergiant, has a contested size, with estimates ranging from about 800 to over 1,700 solar radii. These measurement variations highlight the inherent uncertainty in quantifying the volume of these distant stellar giants.
The size of St2-18 approaches the theoretical limits for stellar volume. Stars maintain their structure through a balance between the inward pull of gravity and the outward pressure from internal fusion and radiation. For the most massive stars, this outward radiation pressure becomes so strong that it pushes the star’s outer layers away, a phenomenon described by the Eddington limit. St2-18 is near the maximum radius a star can achieve before rapid mass loss reduces its size.
The Immediate Next Step in Scale
Moving beyond individual stars, the next step in cosmic scale introduces structures that occupy a volume far exceeding a single stellar object. Placing St2-18 at the center of our solar system offers a powerful visualization. If the star replaced the Sun, its outer edge would extend past the orbit of Saturn, engulfing all the inner planets and likely Jupiter.
Far greater than this stellar volume are the structures built around supermassive black holes (SMBHs) found at the centers of galaxies. While the event horizon of even the most massive black hole is relatively small, the surrounding structures are vast. The black hole at the core of the quasar TON 618, for example, is one of the most massive known, estimated at 40 to 66 billion solar masses.
The gravitational field of this black hole creates an accretion disk of intensely hot, swirling gas and matter. This luminous, active region, which defines the quasar, can have a diameter in the hundreds of billions of kilometers, or about 82.6 light-years across, a scale that dwarfs St2-18. Furthermore, vast nebulae and molecular clouds represent single clouds of gas and dust that span many light-years. The Tarantula Nebula, for instance, spans over 1,000 light-years in diameter, containing billions of stellar-sized objects.
Structures That Dwarf Stars
The true answer to what is bigger than Stephenson 2-18 lies in the hierarchical clustering of the universe, starting with galaxies. A galaxy is a massive, gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. Our own Milky Way galaxy spans an estimated 100,000 to 200,000 light-years across, a distance that is billions of times larger than the diameter of St2-18.
Even the Milky Way is small compared to the largest galaxies discovered, such as the supergiant elliptical galaxy IC 1101. Situated over a billion light-years away, IC 1101 has a measured diameter ranging from 400,000 to over 6 million light-years, making it one of the largest galaxies known. If IC 1101 were placed in our local space, it would engulf both the Milky Way and our nearest galactic neighbor, the Andromeda galaxy.
Galaxies themselves are organized into clusters and superclusters, representing the largest gravitationally influenced structures. Our Milky Way is a member of the Laniakea Supercluster, which encompasses approximately 100,000 galaxies and stretches roughly 500 million light-years across. The boundaries of these superclusters are defined by the motions of galaxies flowing toward a common gravitational center, like the Great Attractor.
The largest structures of all are the galaxy filaments and voids that form the “cosmic web” across the universe. These filaments are walls of galaxies and galaxy clusters, separated by vast, nearly empty regions known as voids. The Hercules–Corona Borealis Great Wall, a putative galaxy filament, is the largest known structure in the observable universe, spanning an estimated 10 billion light-years in its longest dimension.
The Dynamic Nature of Cosmic Records
The title of “largest object” in the universe is not a fixed one, as it is constantly subject to the limitations of current observational technology. Stellar sizes, in particular, are challenging to pin down accurately due to the distance and the diffuse nature of the stellar atmosphere. St2-18’s ranking as the largest star remains a matter of ongoing refinement, as new instruments like the James Webb Space Telescope continuously provide improved data.
On the grandest scale, the largest structures we identify are constrained by the size of the observable universe itself. The Hercules–Corona Borealis Great Wall challenges the cosmological principle, which suggests the universe should be homogeneous on its largest scales. As technology advances, allowing astronomers to map the cosmic web with greater precision, larger structures may be discovered beyond our current cosmic horizon.