Stephenson 2-18 (S2-18) is one of the largest stars ever discovered. This massive star is in a late, unstable phase, burning through its nuclear fuel at an astonishing rate. The sheer scale and volatility of S2-18 make its eventual demise a subject of intense scientific interest. Understanding when this stellar titan will meet its end requires examining its physical state and the complex physics driving its final evolutionary stages. The question is not if S2-18 will explode in a supernova, but when this dramatic event will occur.
The Immense Scale of Stephenson 2-18
S2-18 is classified as a red supergiant, a massive, luminous star nearing the end of its life. It resides approximately 20,000 light-years away in the constellation Scutum, within the star cluster Stephenson 2. Measuring its distance precisely is challenging due to dense dust clouds in that region of the Milky Way, which obscure visible light.
The star’s radius is estimated to be around 2,150 times that of the Sun, placing it among the most immense stars known. If S2-18 were placed at the center of our solar system, its outer edge would extend past the orbit of Saturn. Its volume is so vast that it could contain roughly 10 billion stars the size of our Sun.
Despite its enormous size, S2-18 is relatively cool, with a surface temperature around 3,200 Kelvin, contributing to its deep red color. This low temperature, paired with its immense surface area, allows it to radiate energy with extraordinary luminosity, shining approximately 440,000 times brighter than the Sun. Its extreme size and brightness indicate it is operating near the theoretical upper limit for a red supergiant, suggesting inherent instability.
The Final Stages of Red Supergiant Life
The short lifespan of massive stars like S2-18 is dictated by nuclear fusion processes occurring deep within their core. When the star was younger, it spent millions of years fusing hydrogen into helium, which provided the outward pressure to counteract gravity. As the hydrogen supply was exhausted, gravity caused the core to contract and heat up, triggering the next stage of fusion.
This contraction initiated the burning of helium into carbon, causing the star’s outer layers to swell dramatically, turning it into a red supergiant. Following helium depletion, the core further contracts and heats, allowing progressively heavier elements to fuse in concentric shells. This process proceeds through the burning of carbon, followed by neon, oxygen, and finally silicon.
Each successive stage of fusion yields less energy and is sustained for a significantly shorter time. While hydrogen fusion lasted millions of years, the final stages of silicon fusion into iron are incredibly brief, sometimes lasting only a few days to a week. The formation of an iron core signals impending collapse, as fusing iron requires an input of energy rather than releasing it. The star can then no longer support itself against gravity.
Predicting the Supernova Event
Pinpointing the exact moment S2-18 will explode is currently impossible, as stellar evolution models offer a broad range of possibilities. The star is already a red supergiant, indicating it has exhausted its primary hydrogen fuel and is rapidly burning through heavier elements. This places its remaining lifespan on a cosmic timescale that is extremely short, measured in thousands or perhaps a few hundred thousand years.
The difficulty in prediction stems from the speed of the final burning stages, particularly the silicon-to-iron fusion, which lasts mere days. Since S2-18 is approximately 20,000 light-years away, any changes in its luminosity would take 20,000 years to reach Earth. Consequently, S2-18 may have already gone supernova, and we have not yet received the light from the explosion.
The star’s extreme size and high rate of mass loss indicate instability, suggesting it is consuming fuel at a prodigious rate. Astronomers estimate that stars in this late phase have a remaining life expectancy that is not within a human lifespan, but is imminent on a galactic scale. The current scientific consensus suggests the event will occur within the next few hundred thousand years, but S2-18’s unstable nature means it could happen much sooner.
What Happens When S2-18 Dies
The death of S2-18 will be a spectacular core-collapse supernova, likely a Type II event, triggered by the sudden gravitational collapse of its iron core. Once the iron core forms, gravity overwhelms the internal pressure, causing the core to implode in milliseconds. This rapid implosion rebounds violently off the super-dense core, sending a powerful shockwave outward.
This shockwave will blast the star’s outer layers into space at tremendous velocity, releasing energy equivalent to the total output of the Sun over its 10-billion-year lifespan. For a brief period, the supernova will shine with the brightness of an entire galaxy. Despite S2-18’s distance, it is expected to be visible from Earth, likely appearing as a brilliant, new star in the night sky, possibly shining as brightly as the Moon for several weeks.
The ultimate fate of the collapsed core depends on the star’s final mass at the moment of explosion. If the remaining core mass falls within a certain range, the pressure will crush the material into an ultra-dense sphere of neutrons, forming a neutron star. If S2-18 retains enough mass, the gravitational force will be so overwhelming that the core will collapse past the point of no return, forming a stellar-mass black hole.