The universe contains celestial objects of immense scale, including red supergiant and hypergiant stars. These colossal stars are among the largest known, often extending beyond the orbits of planets in our solar system if placed at its center. This article explores two such stars, examining their properties and how astronomers determine their dimensions.
UY Scuti: The Former Record Holder
UY Scuti is a red supergiant star in the constellation Scutum. For many years, it was considered the largest known star. Early estimates placed its radius at approximately 1,708 times that of our Sun, at a distance of about 9,500 light-years from Earth. The star is variable, fluctuating in brightness over a period of about 740 days as it pulsates.
UY Scuti is exceptionally luminous, shining with an energy output hundreds of thousands of times greater than the Sun. Despite its brilliance, its immense distance and location within a dust-obscured region of the Milky Way, known as the Zone of Avoidance, make it appear dim from Earth, invisible to the unaided eye. Recent studies, utilizing Gaia mission data, revised its estimated size to about 909 solar radii and its distance to approximately 5,900 light-years. This adjustment reflects refined measurement techniques and improved understanding.
Stephenson 2-18: The Current Behemoth
Stephenson 2-18 is an even larger red supergiant star, situated in the constellation Scutum within the open cluster Stephenson 2. This star currently holds the record as the largest known, with an estimated radius of about 2,150 times that of the Sun. Its volume is roughly 10 billion times greater than the Sun’s.
Stephenson 2-18 is located approximately 19,000 to 20,000 light-years away from Earth. Like UY Scuti, it is extremely luminous, emitting hundreds of thousands of times more light than the Sun. Its M6 spectral type indicates a relatively cool surface temperature, but its enormous surface area contributes to its immense energy output.
Comparing the Cosmic Giants
Stephenson 2-18 is currently considered the larger of the two stars. UY Scuti’s updated estimated radius is around 909 solar radii, while Stephenson 2-18 has an estimated radius of 2,150 solar radii. This makes Stephenson 2-18 more than twice the size of UY Scuti based on current scientific understanding.
To visualize their scale, if UY Scuti were placed at the center of our solar system, its outer layers would extend past the orbit of Mars, or potentially the asteroid belt. If Stephenson 2-18 replaced the Sun, its photosphere would engulf all inner planets and stretch beyond the orbit of Saturn. This comparison highlights the extraordinary dimensions of these red supergiant stars.
Measuring the Unfathomable
Determining the precise sizes of these distant, massive stars presents significant challenges. Stars are not solid spheres with sharp boundaries; red supergiants have diffuse, pulsating atmospheres. Their dynamic nature means their sizes can vary over time, adding to measurement complexity.
Astronomers primarily rely on indirect methods to estimate stellar radii. One common approach involves measuring a star’s luminosity, temperature, and distance, then applying physical laws like the Stefan-Boltzmann law. Parallax, which measures the apparent shift of a star against background objects as Earth orbits the Sun, determines the distance to closer stars. For more distant stars, other techniques infer distance, which then feeds into size calculations.
Direct measurements of angular size can be achieved through techniques like interferometry, which combines light from multiple telescopes. However, even with these advanced methods, factors such as intervening dust and uncertainties in distance measurements contribute to the margin of error in stellar size estimates. The reported sizes are estimates that can be refined as new data and observational techniques become available.