Stephenson 2-18 (St2-18) consistently ranks among the most colossal stellar objects discovered in the Milky Way galaxy. This star represents the extreme upper limit of stellar size. Astronomers classify St2-18 as a red supergiant or even a red hypergiant, a rare and short-lived phase in the lives of the most massive stars. The sheer size of this object offers a profound perspective on the difference between our Sun and the true giants of the cosmos.
The Direct Answer: St2-18’s Size Relative to the Sun
Stephenson 2-18 is approximately 2,150 times the radius of the Sun (\(R_{\odot}\)). This means the star’s diameter stretches across a staggering 3 billion kilometers.
This immense radius estimate is derived from calculations involving the star’s high luminosity and its relatively cool surface temperature. The star’s volume is estimated at nearly 10 billion times the volume of the Sun. This measurement represents a calculated size, which is subject to a high degree of uncertainty due to the star’s distant nature and the difficulties inherent in defining the boundary of such a diffuse object.
Defining the Red Supergiant Stephenson 2-18
St2-18 is a candidate for the red hypergiant classification, an extremely luminous and massive type of star that has evolved off the main sequence. It is located approximately 20,000 light-years away from Earth, situated within the Scutum-Centaurus Arm of the Milky Way galaxy. The star is found near the massive open cluster Stephenson 2, though its membership in the cluster is debated due to its extreme properties.
The star’s effective surface temperature is low, measuring around 3,200 Kelvin compared to the Sun’s 5,778 Kelvin. This low temperature is a characteristic trait of red supergiant stars, which emit most of their energy in the infrared part of the spectrum. St2-18’s luminosity is estimated to be between 440,000 and 630,000 times that of the Sun, marking it as one of the most luminous cool stars known.
A star becomes a red supergiant after exhausting the hydrogen fuel in its core, causing the core to contract and heat up, while the outer layers expand dramatically. Only stars with an initial mass greater than about nine times that of the Sun will enter this relatively brief phase. The expansion increases the star’s size by hundreds or even thousands of times its original radius.
Conceptualizing the Immensity: A Solar System Analogy
The size of Stephenson 2-18 is best understood by imagining it replacing the Sun at the center of our Solar System. If St2-18 were positioned there, its photosphere, or visible surface, would extend well past the orbits of Mars and Jupiter. With a radius of 2,150 \(R_{\odot}\), the star would entirely engulf the orbit of Saturn.
The star’s surface would lie somewhere between the orbits of Saturn and Uranus, depending on the precise radius measurement used. All the inner planets—Mercury, Venus, Earth, and Mars—would be swallowed up deep within the star’s interior. Even the gas giants, Jupiter and Saturn, would orbit within the star’s diffuse atmosphere.
The difference between the radius ratio and the volume ratio illustrates the three-dimensional nature of its size. Light, traveling at 300,000 kilometers per second, would take approximately 2.6 hours to travel a single circumference around the equator of St2-18.
Why Precise Measurement Remains Difficult
Determining the exact size of a distant red supergiant like Stephenson 2-18 presents several scientific hurdles. The primary challenge is the star’s immense distance, which makes accurate measurement of its parallax—the apparent shift in position used to calculate distance—extremely difficult. Since radius is calculated from luminosity and temperature, an uncertain distance directly leads to an uncertain luminosity, and thus, an uncertain size estimate.
The nature of a red supergiant star also complicates the definition of its “surface.” These stars possess diffuse outer atmospheres that gradually fade into the surrounding space. Thick dust clouds and circumstellar material further obscure the true stellar surface, often leading to an overestimation of the star’s size when observed in infrared light.
The calculated radius of 2,150 \(R_{\odot}\) is treated with caution because it exceeds the theoretical maximum size predicted by stellar evolution models, which suggest a limit closer to 1,500 \(R_{\odot}\). Furthermore, the star’s low temperature of 3,200 Kelvin places it below the theoretical boundary known as the Hayashi limit for a star of its luminosity. This highlights the difficulty in accurately characterizing these extreme stellar behemoths, as the estimated parameters may be physically unlikely.