Wolf-Rayet 104 (WR 104) is a cosmic masterpiece about 8,000 light-years away in the constellation Sagittarius. At its heart lies a dance between two extraordinary celestial partners: a tempestuous Wolf-Rayet star and a radiant O-type main-sequence star.
These stellar companions are locked in a mesmerizing orbital waltz, producing a phenomenon unlike any other in the universe—a luminous, spiraling “pinwheel” structure.
The Wolf-Rayet 104 “pinwheel star” is full of surprises, but a recent study suggests it’s less threatening than once feared. Astronomer Grant Hill and his team at W. M. Keck Observatory have confirmed that WR 104 is a binary star system, where two massive stars orbit each other every 8 months.
As their powerful winds collide, they create a glowing spiral of dust that spins in sync with their orbit, beautifully lighting up the infrared skies. This discovery helps unravel some of the mysteries surrounding this celestial wonder.
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The Wolf-Rayet star is an older, massive star nearing the end of its life. It blows out strong winds filled with carbon and other heavy elements. Its partner, the O-type star, is younger but even larger. Its winds are also powerful but mostly made of hydrogen.
Colliding stellar winds like those in WR 104 can generate hydrocarbons, forming what astronomers call “dust.” When WR 104 was first studied, it gained attention for its stunning spiral structure and its potential to produce a gamma-ray burst (GRB) aimed directly at Earth. Visual models of its “pinwheel” suggested we might be looking straight down at it, like watching someone spin a garden hose overhead.
This raised concerns that the system’s rotational poles could also align toward us. If one of the stars explodes as a supernova, it might generate a GRB beamed along its polar axis. Such an event in our galaxy, with Earth possibly in its sights, earned WR 104 the dramatic nickname of the “Death Star.”
Recent observations using Keck Observatory’s advanced tools—the LRIS, ESI, and NIRSPEC spectrometers—brought WR 104 under closer scrutiny. Through these instruments, researchers analyzed the system’s light spectrum to measure the stars’ velocities, calculate their orbital dynamics, and pinpoint spectral features from the colliding winds. But the real twist came when the data revealed something unexpected, potentially reshaping our understanding of this fascinating system.
“Our view of the pinwheel dust spiral from Earth absolutely looks face-on (spinning in the plane of the sky), and it seemed like a pretty safe assumption that the two stars are orbiting the same way,” says Hill. “When I started this project, I thought the main focus would be the colliding winds, and a face-on orbit was a given. Instead, I found something very unexpected. The orbit is tilted at least 30 or 40 degrees out of the plane of the sky.”
While a relief for those worried about a nearby GRB pointed right at us, this represents a real curveball. How can the dust spiral and the orbit be tilted so much to each other? Is there more physics that needs to be considered when modeling the formation of the dust plume?
“This is such a great example of how with astronomy we often begin a study, and the universe surprises us with mysteries we didn’t expect,” muses Hill.
“We may answer some questions but create more. Ultimately, that is sometimes how we learn more about physics and the universe we live in. In this case, WR 104 is not done surprising us yet.”
Journal Reference:
- G M Hill, Is WR 104 a face-on, colliding-wind binary?, Monthly Notices of the Royal Astronomical Society (2024). DOI: 10.1093/mnras/stae2183
