A red supergiant star cataloged as Westerlund 1-26, or W26, sits inside a young stellar cluster roughly 12,000 light-years from Earth, and its sheer physical size defies easy comparison. With a radius exceeding 1,500 times that of the Sun, W26 is so large that if it replaced our star at the center of the solar system, its outer layers would extend past the orbit of Saturn. That scale places it among the most extreme stellar objects ever measured and keeps it at the center of an ongoing scientific debate about how red supergiants grow, how their sizes are calculated, and whether newer temperature calibrations will shrink the record books.
Why W26’s radius rewrites the scale of stellar size
Stellar radius is not measured with a tape measure. For distant red supergiants, astronomers derive size from two quantities: bolometric luminosity (total energy output) and effective temperature (the temperature of the visible surface). A small shift in either number can change the inferred radius by hundreds of solar radii. That sensitivity is exactly what makes W26’s reported dimensions both striking and contested.
A 2013 study published in Monthly Notices of the Royal Astronomical Society: Letters placed W26’s radius at roughly 1,530 to 1,580 solar radii and described it as one of the largest red supergiants. NASA’s Hubble coverage of the same cluster confirmed the star has a radius over 1,500 times the Sun’s. At that size, the star’s photosphere would stretch to about 7.1 astronomical units (AU) from center, well beyond Saturn’s average orbital distance of 9.5 AU only if one accounts for the full diameter rather than the radius alone. The comparison works because Saturn orbits at roughly 9.5 AU from the Sun, and a sphere with a radius of roughly 1,530 solar radii spans about 7.1 AU, meaning the star’s diameter of roughly 14.2 AU would indeed swallow the planet’s entire orbital path.
A separate contender for the title of largest known star is WOH G64, a red supergiant in the Large Magellanic Cloud. A peer-reviewed study of this luminous supergiant quantified its effective temperature and bolometric luminosity and explicitly posed the question of whether it is the largest star known. The answer depends on which temperature scale and distance assumptions are applied, and those inputs remain under active revision.
Temperature scales and distance uncertainties behind the numbers
The single largest source of systematic error in red supergiant radius estimates is the effective-temperature scale. A foundational 2005 study by Philip Massey and Emily Levesque argued that Galactic red supergiants are not as cool as earlier work assumed. Warmer temperatures, applied to the same luminosity, yield smaller radii. If the revised scale from that work is applied to W26’s spectrum, the star’s inferred radius could contract meaningfully from the 2013 figure, though no published reanalysis has yet produced an updated number specific to W26.
Distance adds another layer of uncertainty. W26 sits inside the massive young cluster Westerlund 1, and pinning down the cluster’s distance has proven difficult because of heavy interstellar dust along the line of sight. A 2022 study in Monthly Notices of the Royal Astronomical Society used the eclipsing binary system W36 inside Westerlund 1 to refine distance and age constraints for the cluster. Better distance values feed directly into luminosity estimates, which in turn change the derived radius. Until the cluster distance is locked down with high-precision parallax or independent geometric methods, W26’s radius carries an irreducible margin of error.
Readers often conflate “largest” with “most massive,” but those are distinct properties. R136a1, a Wolf-Rayet star in the Large Magellanic Cloud, holds the record for the most massive known star. Observations highlighted by the National Science Foundation describe this extreme object as the universe’s most massive known star, but its extreme mass does not translate into extreme physical size because it is far hotter and more compact than any red supergiant. The distinction matters: W26 is enormous in volume, while R136a1 dominates in gravitational heft.
Open questions that could redraw the record
Several gaps in the observational record prevent a definitive ranking of the largest stars. No direct interferometric angular-diameter measurement exists for WOH G64, so its radius still relies on the luminosity–temperature method. For W26, no published time-series monitoring has established whether the reported 1,530 to 1,580 solar-radii value is stable or whether the star’s outer envelope pulsates on timescales that shift the figure by tens or hundreds of solar radii.
The hypothesis that new James Webb Space Telescope (JWST) mid-infrared imaging of W26’s circumstellar nebula will show the star’s current radius is at least 15 percent smaller than the 2013 estimate remains untested. JWST can resolve the thermal structure of the nebula and constrain the dust temperature profile, which in turn tightens the bolometric correction. If the revised effective-temperature scale from Galactic red supergiant studies is combined with a more accurate luminosity from JWST and updated cluster distances, the inferred radius for W26 could move downward enough to put it closer to other well-known giants such as VY Canis Majoris, rather than standing clearly apart at the extreme end of the distribution.
Another open question concerns how much mass W26 has already lost. Red supergiants shed material through powerful stellar winds and episodic outbursts, building dusty nebulae around themselves. That lost mass changes the structure of the outer envelope, potentially inflating or deflating the observable radius over time. Without precise measurements of W26’s current mass and wind properties, models of its internal structure-and therefore its radius-remain poorly constrained.
Ultimately, the race to crown “the largest star” is less important than the physics that underlies these outsized measurements. W26 and WOH G64 sit at the edge of what stellar evolution theory predicts for cool, massive stars near the ends of their lives. Clarifying their true sizes will test how well current models handle convection, mass loss, and the transition toward eventual supernova explosions. As improved temperature scales, distance determinations, and infrared observations accumulate, the numbers attached to W26’s radius may shift, but the star will remain a key laboratory for understanding how big a star can grow before it dies.
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*This article was researched with the help of AI, with human editors creating the final content.
