Designated Stephenson 2 DFK 52, the newly-discovered red supergiant resides in the massive stellar cluster RSGC2.
This image shows the red supergiant star Stephenson 2 DFK 52 and its circumstellar environment. Image credit: ALMA / ESO / NAOJ / NRAO / Siebert et al.
RSGC2 is a cluster of at least 26 red supergiants located at the base of Milky Way’s Scutum-Crux spiral arm at a distance of 5,800 parsecs (18,917 light-years).
Also known as Stephenson 2, the cluster is a site of recent star-forming activity in the region where the arm intersects the Galactic bulge.
Chalmers University of Technology astronomers Mark Siebert and colleagues observed the RSGC2 stars with the Atacama Large Millimeter/submillimeter Array (ALMA).
“What we’re seeing in this photo of Stephenson 2 DFK 52 is actually a red supergiant star expelling a cloud of gas and dust as it nears the end of its life,” they said.
“These nebulae are common around supergiant stars; however, this particular cloud presents an unexpected and considerable mystery for astronomers.”
“This is the largest cloud of ejected material to have been found around a supergiant star, at an enormous 1.4 light-years across.”
“Stephenson 2 DFK 52 is rather similar to Betelgeuse, another famous red supergiant, so they were expecting to see a similar cloud around it.”
“However, if Stephenson 2 DFK 52 was as close to us as Betelgeuse is, the cocoon around it would be as wide in the sky as a third of a full Moon.”
The new ALMA observations allow the astronomers to measure how much material surrounds the star and how fast it is moving.
“The parts that are moving towards us are highlighted in blue, and the sections that are moving away, in red,” they said.
“The data show that about 4,000 years ago the star went through an episode of extreme mass shedding, and then slowed down to its current rate, more similar to that of Betelgeuse.”
According to the team, Stephenson 2 DFK 52 has a mass of between 10 and 15 solar masses, and by now it has already lost 5-10% of its mass.
“It’s still a mystery as to how the star managed to expel so much material in such a short timeframe,” the researchers said.
“Could it be an odd interaction with a companion star? Why is the shape of the cloud so unusually complex? Are there more supergiants like this out there?”
“Deciphering why Stephenson 2 DFK 52 has already shed so much material will help astronomers understand how it will meet its end: a supernova explosion sometime in the next million years.”
The team’s paper will be published in the journal Astronomy & Astrophysics.
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Mark A. Siebert et al. 2025. Stephenson 2 DFK 52: Discovery of an exotic red supergiant in the massive stellar cluster RSGC2. A&A, in press; arXiv: 2507.11609
