Hypergiants or red supergiant stars are the largest stellar bodies known to science. These colossal objects dwarf our Sun, expanding to sizes that challenge astronomical measurement. For years, UY Scuti held the title of the largest star, but a revised understanding of its properties shifted the consensus. The current comparison between UY Scuti and Stephenson 2-18 requires examining their immense sizes and the complex methods scientists use to determine their true scale.
Defining the Hypergiants: UY Scuti and Stephenson 2-18
Both UY Scuti and Stephenson 2-18 are classified as red supergiant or hypergiant stars. These stars are nearing the end of their life cycle, having expanded dramatically after exhausting the hydrogen fuel in their cores. Their enormous size is a result of this evolution, not their initial mass, though they began their lives with many times the mass of the Sun. They are both located in the constellation Scutum within the Milky Way galaxy.
UY Scuti is approximately 5,900 light-years from Earth and is known as a pulsating variable star. This means its brightness and radius fluctuate over a period of about 740 days, a common trait as the outer layers of red supergiants are inherently unstable. Stephenson 2-18, also known as RSGC2-18, resides much farther away, at about 20,000 light-years. It is a likely member of the massive open star cluster Stephenson 2 and is notable for its extreme luminosity, which helps indicate its vast size.
The Current Scientific Consensus on Relative Size
Stephenson 2-18 is currently believed to be the largest star known, with an estimated radius significantly greater than that of UY Scuti. Its radius is calculated to be around 2,150 times the radius of our Sun, giving it a volume approximately 10 billion times greater than the Sun’s. This size places Stephenson 2-18 near the theoretical upper limit for a red supergiant star.
UY Scuti previously held the record, but modern observations have revised its estimated size downward. Older measurements placed its radius at an enormous 1,708 solar radii, but this relied on a distance estimate that proved too large. New data, including measurements from the Gaia mission, demonstrated that UY Scuti is closer to Earth than previously calculated.
This re-evaluation of UY Scuti’s distance and intrinsic luminosity dramatically reduces its inferred size, often cited now around 909 solar radii. This revision is the primary reason Stephenson 2-18 now holds the title of the largest star by a substantial margin. Even with the inherent uncertainties in measuring such distant objects, the scientific consensus strongly favors Stephenson 2-18 as the current record holder.
The Challenges of Measuring Distant Stellar Radii
Determining the exact size of a hypergiant star involves complex astronomical techniques and carries a significant margin of error. The primary method relies on combining a star’s angular diameter with its distance from Earth. Angular diameter, the apparent size of the star as seen from Earth, is measured directly using instruments like stellar interferometers.
The immense distances to these hypergiants, often tens of thousands of light-years away, make distance a major challenge. Astronomers rely on the parallax method, which measures the star’s apparent shift against the background as the Earth orbits the Sun. This technique becomes highly imprecise for very distant objects, meaning an incorrect distance estimate directly translates to a large error in the calculated linear radius.
A further complication is that the surfaces of these stars are not solid, well-defined boundaries like a rocky planet. Red hypergiants possess extremely tenuous atmospheres, and their size measurement is known as the Rosseland radius. This measurement depends on the specific wavelength of light observed, meaning the star’s “edge” is fuzzy and varies. Stellar variability also means the star is constantly expanding and contracting, so a single measurement only captures its size at a specific point in time.
Finally, these stars are often obscured by thick clouds of interstellar dust, particularly because they lie in the crowded plane of the Milky Way galaxy. This dust can scatter and absorb light, making the star appear dimmer than it truly is. This dimming affects calculations of its luminosity and, consequently, its estimated radius.
Visualizing the Immense Scale
To grasp the sheer scale of the largest hypergiants, it is helpful to place them within the familiar context of our own solar system. If the center of Stephenson 2-18 were positioned where our Sun is, its massive photosphere would extend far beyond the orbits of the inner planets. The star’s boundary would engulf Mercury, Venus, Earth, and Mars, swallowing the entire asteroid belt.
The star would also encompass the orbit of Jupiter and reach out past the orbit of Saturn. With an estimated radius of 2,150 solar radii, Stephenson 2-18’s edge would be located somewhere between the orbits of Saturn and Uranus. Its volume is so vast that billions of stars the size of our Sun could fit inside it.