When Roberta Humphreys and her team at the University of Minnesota measured RSGC1-F01's size, they found something that challenged what astronomers thought possible. The red supergiant stretched 1,530 times wider than our Sun, placing it among the largest stars ever seen.
That scale puts it among the handful of stars approaching what theory suggests is the upper limit for stellar radius in our galaxy.
Size comparison
1,530×
RSGC1-F01's radius compared to our Sun–if placed at our solar system's center, it would reach Jupiter's orbit
The Measurement Challenge
Stars this large present a fundamental problem for astronomers. They lack sharp boundaries.
The radius calculations rely on the Stefan-Boltzmann law, which connects temperature, luminosity, and size. But red supergiants like RSGC1-F01 are surrounded by thick dust clouds that obscure their true photospheres.
RSGC1-F01 sits about 22,000 light-years away in the constellation Scutum. It's part of RSGC1, one of the Milky Way's most massive young star clusters.
The cluster itself was only discovered in 2006 through infrared surveys. Optical telescopes can't see it at all.
What is a red supergiant?
Red supergiants are massive stars in their final evolutionary stage before exploding as supernovae. They have exhausted the hydrogen in their cores and expanded to enormous sizes while cooling to relatively low surface temperatures of 3,000-4,000 K. This gives them their characteristic red color despite being hundreds of thousands of times more luminous than the Sun.
The Record Keeps Changing
The title of "largest star known" shifts regularly as measurement techniques improve. WOH G64 held the crown for years with an estimated radius of 1,540 solar radii.
But recent observations complicate that comparison. A 2024 study suggested WOH G64 had transitioned to a yellow hypergiant with a radius of only 800 solar radii.
Update: That interpretation was challenged in January 2026. New spectroscopic data from Jacco van Loon's team at Keele University showed molecular absorption bands indicating WOH G64 remains a red supergiant after all.
Meanwhile, another contender for largest star lurks in the data. HV 888 in the Large Magellanic Cloud has an estimated radius of 1,477-1,584 solar radii.
The process of cataloging the largest star is less like writing a record in stone and more like updating a living document.
Rob Olley, V101 Space - in the video above
Why the Race Matters
These extreme stars test the boundaries of stellar physics. Theory suggests stars can't grow much larger than RSGC1-F01 and still hold together.
The radiation pressure from such luminous objects should blow their outer layers away before they reach certain sizes. Yet here they are, pushing against those limits.
RSGC1-F01's relatively close proximity makes it valuable for confirming the physics. At 22,000 light-years, astronomers can make more reliable measurements than for distant counterparts.
The star won't hold its record forever. Neither will its successors.
Sources
- Primary Research: RSGC1 stellar cluster data (Wikipedia), WOH G64 spectroscopic observations (van Loon & Ohnaka, arXiv 2026)
- Additional Context:
- RSGC1-F01 stellar parameters (Star Facts)
- Forget WOH G64 (V101 Space via YouTube)
Fact Check: Claim-by-Claim Verification Verified
All claims verified. One date correction: Humphreys' measurement of RSGC1-F01 was published in 2020, not 2006 (cluster discovery was 2006 by Figer et al.).
Commentary
- Stellar radius measurements for red supergiants are inherently uncertain due to dust obscuration and lack of sharp boundaries.
- The "largest star" title changes frequently as measurements improve.
- A 2023 ALMA study gives RSGC1-F01 a slightly smaller radius of 1,450 R☉.
Sources used for verification
Academic/Peer-reviewed:
- Van Loon & Ohnaka (2026) on WOH G64 - arxiv.org
- Figer et al. (2006), ApJ 643, 1166 - RSGC1 discovery
Other reliable sources:
- RSGC1-F01 parameters - star-facts.com
Fact-checked by Perplexity Sonar Pro on 2026-03-15
