What is Stephenson 2-18?
Stephenson 2-18 is a star categorized as a red supergiant, or possibly an extreme red hypergiant. This classification indicates it is a very large, cool, and luminous star nearing the end of its life cycle. Red supergiants like Stephenson 2-18 are stars that began with considerable mass, burning through their hydrogen fuel rapidly. As they deplete this fuel, their outer layers expand dramatically while their core contracts and heats up, causing them to become exceptionally large and bright.
These stars are characterized by their reddish appearance due to their lower surface temperatures, typically ranging from 3,500 to 4,500 Kelvin. Despite being cooler than our Sun, their enormous size makes them incredibly luminous, shining with the brightness of tens to hundreds of thousands of Suns combined. Stephenson 2-18 was first identified in 1990 by astronomer Charles Bruce Stephenson and has since been a subject of interest due to its extreme properties.
Locating Stephenson 2-18
Stephenson 2-18 is located within our own galaxy, the Milky Way. It resides in the constellation Scutum, a region rich with other red supergiant clusters. Its estimated distance from Earth is approximately 18,900 to 20,000 light-years, or about 6,000 parsecs. This star is believed to be situated within the Scutum-Centaurus Arm, one of the Milky Way’s prominent spiral arms.
Astronomers determine the distances to such remote stars using various methods, particularly spectroscopic parallax for stars too far for direct trigonometric parallax measurements. Spectroscopic parallax involves analyzing a star’s light spectrum to ascertain its luminosity, or true brightness. By comparing this intrinsic brightness with how bright the star appears from Earth, scientists can estimate its distance.
The Immense Size of Stephenson 2-18
Stephenson 2-18 stands out as one of the largest stars known to humanity. Its estimated radius is about 2,150 times that of our Sun. To grasp this scale, if Stephenson 2-18 were placed at the center of our solar system, its outer edge would extend beyond the orbit of Saturn, engulfing all inner planets. This means its volume is roughly 10 billion times greater than the Sun’s.
Traveling around the Sun’s circumference at the speed of light would take approximately 14.5 seconds, but circling Stephenson 2-18 would require nearly 9 hours. While its size is extraordinary, current stellar evolution theories suggest a theoretical upper limit for red supergiant radii around 1,500 solar radii, making Stephenson 2-18 an intriguing outlier that challenges existing models. It surpasses previously recognized large stars like UY Scuti, which has an estimated radius of 755 solar radii.