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Stephenson 2-18
Details
Stephenson 2-18 is a very cool, luminous red supergiant candidate in the direction of the Stephenson 2 stellar association. It has been given one of the largest published stellar size estimates, but its exact diameter remains uncertain because the star is distant, dust-affected, and difficult to separate cleanly from its surrounding region.
Stephenson 2-18 is famous because it seems to offer a simple superlative: the largest known star. But the better story is less tidy. Also cataloged as Stephenson 2 DFK 1, it sits near the uncertain upper edge of red-supergiant size estimates, where a star no longer behaves like a clean sphere with an easy boundary.
The often quoted diameter depends on assumptions about the star's distance, luminosity, temperature, and whether it truly belongs to the Stephenson 2 region. Change those assumptions and the record changes with them. That makes the number useful as a warning rather than a trophy: at this scale, the edge of a star is partly an observational and model-dependent question.
That is why Stephenson 2-18 belongs on the scale. It is not here as a settled champion, but as one of the most famous disputed candidates for the largest known star. It helps turn a popular size-comparison object into a more honest lesson about how astronomy handles extreme, dusty, unstable stars.
1.31billion km
Visual creditscaleofspace.org / CC BY-NC 4.0
Key facts
- Category
- Stars
- Object class
- Extreme red supergiant candidate
- Temperature
- ~3,200 K surface
- Scale fact
- ~2.99 billion kmdiameter
- Composition
- Hydrogen and helium
Scale context
Where Stephenson 2-18 sits on the full axis
By size on the journey, Stephenson 2-18 sits between VY Canis Majoris and Solar System. The band below uses nearby Stars objects for context.
Shared physical scale
2.29billion km
Together, these objects make the size change around Stephenson 2-18 easy to compare at a glance.
Sources
References for Stephenson 2-18
Measurements and descriptive context are compiled by the Scale of Space team from the references below. If you find an error, please let us know.
- Stephenson 2-18 — Wikipedia
- Visual: scaleofspace.org / CC BY-NC 4.0
Between the smallest and the largest, perspective is everything.
About
Scale of Space is a scroll-based journey through the universe, placing objects on a single logarithmic scale so you can compare size across an unbroken range.
Guides turn parts of that scale into curated essays, while focused views let you explore the same range through specific groups of objects.