Stephenson 2-18 Compared to the Sun: Unveiling the True Scale of the Largest Star

Imagine our Sun, a colossal star that dictates life within our solar system. Now, picture a star so immense that it makes the Sun look like a mere speck of dust in comparison. This is Stephenson 2-18 (St2-18), a red supergiant star residing in the Scutum constellation, and currently holding the title of the largest known star in the observable universe. To truly grasp the sheer scale of Stephenson 2-18, we need to compare it to our familiar Sun, a comparison that will undoubtedly reshape your perception of cosmic sizes.

What is Stephenson 2-18? A Red Supergiant Unveiled

Stephenson 2-18 is classified as a red supergiant, or possibly even a red hypergiant star. These stellar classifications denote stars at the end stages of their lives, having expanded dramatically as they exhaust their nuclear fuel. Located approximately 18,900 light-years away from Earth within the constellation Scutum, St2-18 is also known under other designations like Stephenson 2-DFK 1 and RSGC2-18. It resides in the same region as the open cluster Stephenson 2, a massive collection of stars that further emphasizes the grandeur of this cosmic neighborhood.

Stephenson 2-18 in infrared, processed through 2MASS surveyA zoomed-in infrared image of the red supergiant star Stephenson 2-18, processed from the 2MASS survey, revealing its immense size and cool temperature. Image credit: 2MASS (CC BY-SA 3.0)

This stellar behemoth is an M6 spectral type red supergiant, placing it among the coolest yet most luminous stars. With an effective temperature of around 3,200 K, significantly cooler than the Sun’s surface temperature, Stephenson 2-18 compensates with its enormous size to achieve a staggering luminosity. It shines with an estimated 436,516 times the luminosity of our Sun, although estimates vary widely, ranging from 90,000 to an incredible 630,000 solar luminosities. This variance highlights the uncertainties in measuring such distant and immense objects, but even the lowest estimates confirm its status as one of the most luminous stars in our Milky Way galaxy.

Size Comparison: Stephenson 2-18 vs. The Sun – A Cosmic David and Goliath

The most striking aspect of Stephenson 2-18 is its colossal size. Its radius is estimated to be around 2,150 times that of the Sun. To put this into perspective, if Stephenson 2-18 were placed at the center of our solar system, it would engulf not only Mercury, Venus, Earth, and Mars, but also Jupiter and Saturn. Its outer edge would extend beyond the orbit of Saturn, which orbits the Sun at a distance of approximately 1.43 billion kilometers (9 Astronomical Units).

A comparative size illustration showcasing Stephenson 2-18 alongside other giant stars and the IC 1101 galaxy, emphasizing its extreme dimensions relative to other celestial objects. Image: Wikimedia Commons/THE COLOSSAL GALAXY NAMED IC1101 (CC BY-SA 4.0)

In terms of volume, Stephenson 2-18 is approximately 10 billion times larger than the Sun. Imagine filling a balloon representing the Sun with air; you would need 10 billion of these balloons to represent the volume of Stephenson 2-18. Traveling around the circumference of Stephenson 2-18 at the speed of light would take nearly 9 hours, a stark contrast to the mere 14.5 seconds it would take to circle the Sun at the same speed. This comparison truly emphasizes the almost incomprehensible scale of this star.

While other stars like MY Cephei and WY Velorum have been considered contenders for the title of the largest star, current estimates place Stephenson 2-18 firmly in the lead. It dwarfs even former record holders like UY Scuti and WOH G64, highlighting its exceptional status among the known stellar giants.

Context: Largest Stars and the Limits of Stellar Size

The immense size of Stephenson 2-18 challenges our understanding of stellar evolution. Current theories suggest a theoretical limit to the size of stars in the Milky Way, estimated to be around 1,500 solar radii. Stars exceeding this limit are predicted to be too unstable to form and sustain themselves. Stephenson 2-18, with its radius of 2,150 solar radii, pushes these theoretical boundaries, prompting astronomers to re-evaluate the factors that govern the maximum size of stars.

The existence of such colossal stars raises questions about the processes of star formation and the mechanisms that allow them to reach such extreme dimensions. It is believed that stars like Stephenson 2-18 are in the final stages of their lives, experiencing significant mass loss and instability. They are likely to eventually end their lives in spectacular supernova explosions, contributing enriched material back into the interstellar medium, which will then form new generations of stars and planets.

Stephenson 2-18 and UY Scuti: A Tale of Stellar Giants

For a long time, UY Scuti held the title of the largest known star. Both Stephenson 2-18 and UY Scuti are red supergiants located in the constellation Scutum, making it a region of the sky rich in stellar giants. However, more recent and accurate measurements, particularly from the Gaia mission, have revised the estimated size of UY Scuti downwards.

A size comparison chart depicting the Sun, UY Scuti, and Stephenson 2-18, illustrating the significant difference in scale between these stars, particularly highlighting Stephenson 2-18’s immense size. Image: Wikimedia Commons/THE COLOSSAL GALAXY NAMED IC1101 (CC BY-SA 4.0)

While earlier estimates placed UY Scuti’s radius at around 1,708 solar radii, Gaia data suggests a much smaller radius of about 755 solar radii. This revision places UY Scuti in a size range comparable to stars like Antares and Betelgeuse, still significantly larger than the Sun, but considerably smaller than Stephenson 2-18. Stephenson 2-18 has now definitively surpassed UY Scuti to claim the title of the largest star, demonstrating the dynamic nature of astronomical discoveries and the continuous refinement of our understanding of the cosmos.

Stephenson 2 Cluster and the Discovery of St2-18

Stephenson 2-18 is associated with the open cluster Stephenson 2, a massive and relatively young cluster of stars. This cluster was discovered by astronomer Charles Bruce Stephenson in 1990 during an infrared survey of the Milky Way. Stephenson identified a group of faint, red stars in this region, suspecting them to be red supergiants.

Further studies confirmed the presence of a significant population of red supergiants within Stephenson 2, making it one of the most massive open clusters known in our galaxy. Stephenson 2-18 itself was identified as a member of this cluster, although its exact membership is still debated. The cluster is estimated to be around 17 million years old and contains dozens of red supergiants, making it a valuable target for studying the evolution of massive stars.

Location and Constellation: Finding Stephenson 2-18 in Scutum

Stephenson 2-18 resides within the constellation Scutum, a small and faint constellation in the summer sky. Scutum, Latin for “shield,” was named by astronomer Johannes Hevelius in the 17th century. While Scutum itself is not visually prominent, lacking bright stars visible to the naked eye, it holds significant astronomical interest due to the presence of massive star clusters like Stephenson 2 and other notable objects like UY Scuti and the Wild Duck Cluster (Messier 11).

An image from Wikisky illustrating the location of Stephenson 2 within the constellation Scutum, aiding in visualizing its position in the night sky. Image: Wikisky

Located in the region between the brighter constellations of Alpha and Beta Scuti, Stephenson 2 and its giant star Stephenson 2-18 are obscured by dust and gas, making them invisible in visible light. However, they become readily apparent in infrared wavelengths, allowing astronomers to study these distant and fascinating objects.

Key Facts About Stephenson 2-18:

Conclusion: A Cosmic Perspective

Comparing Stephenson 2-18 to the Sun is not just an exercise in astronomical measurements; it’s a journey into understanding the vastness and diversity of the universe. Stephenson 2-18, the largest star known to us, dwarfs our Sun to an almost unimaginable degree, forcing us to reconsider our cosmic perspective. This stellar giant serves as a powerful reminder of the incredible scales at play in the cosmos and the ongoing discoveries that continue to expand our knowledge of the universe. As we continue to explore the depths of space, undoubtedly, more wonders and perhaps even larger stars await discovery, further enriching our understanding of these celestial titans.