Largest Star

When we discuss the "largest" star, we are usually referring to volume (radius) rather than mass. As of 2026, the reigning champion of sheer physical size is Stephenson 2-18 (also known as RSGC2-18), a red hypergiant that defies our current understanding of how large a star can actually become.

1. Mind-Boggling Dimensions

Stephenson 2-18 has an estimated radius of 2,150 times that of our Sun. To put this in perspective, if you replaced the Sun with this star at the center of our solar system, its "surface" (photosphere) would extend past the orbit of Saturn.

While a light beam takes about 14.5 seconds to circle the Sun, it would take nearly nine hours to complete a single trip around the circumference of Stephenson 2-18. It is so vast that roughly 10 billion Suns could fit inside its volume.

2. The "Impossible" Giant

One of the most salient features of this star is that it technically shouldn't exist according to standard stellar models. It sits at the absolute upper limit of the Hertzsprung-Russell diagram, surpassing the Hayashi limit—the theoretical maximum size a star can reach while remaining stable.

Because of this, astronomers are constantly debating its true size. Some argue that its distance (about 19,000 light-years) has been miscalculated, which would mean it is actually smaller, while others suggest it may be in a rare, short-lived transition phase that allows for such extreme expansion.

3. Low Density, High Luminosity

Despite its gargantuan size, Stephenson 2-18 is not the "heaviest" star. While it is about 10 billion times the volume of the Sun, it is estimated to be only 30 to 50 times more massive. This means its outer layers are incredibly thin and "puffy"—less like a solid ball of fire and more like a glowing, ultra-hot vacuum.

However, what it lacks in density, it makes up for in brightness. It is one of the most luminous red supergiants ever discovered, shining with nearly 440,000 times the Sun's total energy output.

4. Recent Observations (2026)

Recent data from the Gaia spacecraft and high-resolution imaging of similar giants like WOH G64 have shown that these "monster" stars are often shrouded in thick clouds of dust that they have shed. This dust can make it difficult to see the star's true edge, leading to ongoing scientific "battles" for the title of the largest star between Stephenson 2-18 and former champions like UY Scuti.