Giant star creates a massive, mysterious bubble of gas
08-04-2025

Giant star creates a massive, mysterious bubble of gas

The star labeled DFK 52 sits in the massive Stephenson 2 cluster nearly 19,000 light years away in Scutum. This red supergiant lies inside a sphere of dust and gas so vast that it strains the limits of current stellar physics.

At roughly 50,000 astronomical units (about 4.6 trillion miles) across, the bubble forms a circumstellar medium that dwarfs every previously cataloged stellar outflow. No established model explains how a single star could shed so much material so quickly.

Red giants shed mass in late life

“We have no idea how you can throw off this much material in that amount of time,” said Mark Siebert of the Chalmers University of Technology who led the research.

The team used the Atacama Large Millimeter/submillimeter Array (ALMA) to map the faint millimeter wave glow from the expelled material. 

A red supergiant forms when a star that began its life with roughly eight or more times the Sun’s mass exhausts core hydrogen and swells to planetary scale. During this brief late phase, many such stars lose gas at rates between one ten millionth and one ten fourth of a solar mass per year.

Even the celebrity hypergiant VY CMa, famous for dusty clumps, launches winds that reach only about 11,000 AU from the star. Compared with that benchmark, DFK 52’s halo is roughly five times larger.

ALMA data show parts of the shell racing outward at 27 kilometers per second, pointing to an explosive episode around 4,000 years ago. The outer layers now hold as much as a tenth of the Sun’s mass, a surprising haul for a star that itself glows only modestly.

DFK 52’s stunning gas bubble

Earlier optical and infrared surveys saw only a tame wind around DFK 52. ALMA’s sensitivity to cool molecules revealed the hidden structure, tangled filaments, nested loops, and a ring-like bar halfway out.

That bar expands at nearly 19 miles per second, consistent with a single blast that flung material outward in one go. Because the star’s present brightness is far too low to power that blast, astronomers suspect an extra energy source.

One idea is a short lived supernova preparatory superwind that can drive mass loss rates above a thousandth of a solar mass per year, just as late stage models propose. The timing seems off, though, because DFK 52 has already settled back into a gentle breeze.

Another possibility involves a close companion star plunging through the bloated envelope, injecting kinetic energy and dredging up gas. Such interactions can sculpt disks, tori, or lopsided shells, but the irregular shape of DFK 52 hints that if a partner exists, the dance was messy.

ALMA reveals new details of DFK 52

ALMA’s 37 antennas resolved details smaller than 0.2 arcseconds, about 1,200 AU at the cluster’s distance. Carbon monoxide molecules trace the shell’s full reach, confirming that dust and gas share the same sprawling domain.

Near the center, strong silicon monoxide emission reveals ongoing but much slower mass loss. That contrast signals a sudden downshift in activity after the ancient outburst.

Within the bar region, velocities vary smoothly with position, forming what the team interprets as an equatorial disk seen edge on. The disk’s mass alone approaches five hundredths of a solar mass, dwarfing many planetary nebulae.

Beyond the disk, wispy arcs spiral outward like frozen smoke strands. Their twisted geometry defeats spherical wind models, the mainstay of stellar evolution codes.

The outer ring mystery

The ring halfway through the bubble suggests a confined, directional blast rather than a symmetric puff. If wave heating deep in the convective envelope drove the eruption, the energy must have surfaced startlingly fast.

Dense, compact shells like this appear in early spectra of several type II supernovae, hinting that many red supergiants dump matter in violent spurts shortly before death. DFK 52 offers a frozen snapshot of what such a spurt might look like a few millennia later.

Because the outburst happened thousands of years ago, the shell has cooled enough for molecules to survive, giving astronomers a rare chance to probe chemistry normally erased by intense radiation.

Cause behind the gas outburst

Siebert’s team plans deeper ALMA runs to map faint isotopologues that betray temperature and density, plus a hunt for any hidden companion whose gravity might be the missing energy source. Optical surveys will monitor the star’s brightness in case it starts another outburst.

Whether the next mass ejection event arrives in decades or millennia, DFK 52 already forces theorists to widen the menu of late stage stellar behaviors. It reminds us that even familiar stellar giants can spring outsized surprises.

The study is published in the journal Astronomy & Astrophysics.

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