For many years, the red supergiant UY Scuti captured the public imagination as the single largest star known in the universe. Located in the constellation Scutum, its immense radius, often calculated to be over 1,700 times that of the Sun, demonstrated the staggering scale possible within a single star. A star’s radius is generally defined by its photosphere, the boundary where the gas becomes opaque enough for light to escape. While UY Scuti is undeniably massive, astronomical discovery is a constantly evolving field. New measurements and a greater understanding of stellar physics have revealed objects that now exceed UY Scuti’s size.
The Challenges of Defining Stellar Size
Determining the exact size of these stellar behemoths is a complex task, meaning that the measurements for UY Scuti and its larger counterparts are estimates rather than precise figures. The difficulty stems primarily from the nature of these stars, which are classified as red hypergiants or red supergiants. Unlike our Sun, these stars do not possess a sharp, distinct surface; instead, they are surrounded by vast, diffuse atmospheres of gas and dust that extend far into space. The effective radius is typically defined by the Rosseland radius, but this boundary is fuzzy and changes depending on the wavelength of light used for observation.
A star’s calculated size also depends heavily on its assumed distance from Earth, which is a major source of uncertainty. Even small errors in the initial distance measurement, often obtained using the parallax method, lead to enormous differences in the calculated radius. For instance, older estimates for UY Scuti’s radius were based on a greater assumed distance, yielding the massive 1,708 solar radii figure. Newer measurements suggest the star is closer, which significantly reduces the calculated radius to a much smaller figure, possibly around 755 to 909 solar radii. This wide variation highlights why the list of largest stars is constantly being revised as underlying distance data is refined. The immense size of these stars also causes a significant amount of astrometric noise, making direct parallax measurements from missions like Gaia unreliable for the most massive and distant red hypergiants.
Current Contenders Exceeding UY Scuti
Despite the measurement uncertainties, current estimates place several red hypergiants in our galaxy as larger than UY Scuti. The most prominent contender for the title of the largest star is Stephenson 2-18 (St2-18), a red hypergiant located in the Stephenson 2 star cluster in the constellation Scutum. St2-18 is currently estimated to have a radius of approximately 2,150 times that of the Sun, making it the largest star currently known. This colossal size is derived from assumptions about its luminosity and effective temperature, placing it at the theoretical upper limit for a red supergiant. However, astronomers caution that this figure is highly uncertain, as some models suggest a physical limit of around 1,500 solar radii for stars in the Milky Way.
Another strong candidate that exceeds UY Scuti’s revised size is Westerlund 1-26 (W26), located in the massive Westerlund 1 star cluster. W26 is classified as a red hypergiant, and its radius estimates generally range between 1,165 and 1,530 solar radii, depending on the specific measurement technique and distance assumed.
Other stars also challenge the size of UY Scuti, including RSGC1-F01 and WOH G64. RSGC1-F01, a star in the RSGC1 cluster, is estimated to have a radius of up to 1,530 solar radii. WOH G64, located outside our galaxy in the Large Magellanic Cloud, has a well-determined radius of about 1,540 solar radii, making it a very reliable example of a star larger than the updated UY Scuti estimate.
Placing Hypergiant Stars into Perspective
The immense sizes of these hypergiant stars are difficult to grasp without a terrestrial comparison. If the Sun were the size of a standard basketball, the Earth would be a tiny grain of salt orbiting about 70 feet away. The colossal size of UY Scuti, even using the conservative modern estimate of 909 solar radii, means that if it were placed at the center of our solar system, its visible surface would extend past the orbit of Mars. Using the older, larger estimate of 1,708 solar radii, the star’s atmosphere would easily swallow the orbits of all the inner planets and extend well past the orbit of Jupiter.
The sheer magnitude of Stephenson 2-18, with its estimated 2,150 solar radii, places it on a truly incomprehensible scale. If this star replaced the Sun, its outer layers would expand beyond the orbit of Saturn. To put this in terms of volume, nearly 10 billion Suns could fit inside the volume of Stephenson 2-18.
Even light, the fastest thing in the universe, takes only about 14.5 seconds to travel around the circumference of our Sun. A beam of light would take approximately six to seven hours to make a single trip around the circumference of a hypergiant like St2-18. This staggering difference emphasizes the scale of these objects, which represent the upper limit of stellar size in the cosmos.