WR 104

Lore & Background

Discovered as part of the Keck Aperture Masking Experiment, WR 104 is a triple system. The inner spectroscopic pair consists of a Wolf–Rayet star and a B0.5 main sequence star in a nearly circular orbit separated by about 2 AU, with an orbital period of 241.5 days. The Wolf–Rayet star is visually 0.3 magnitudes fainter than its companion but dominates the spectrum and is more luminous. A third, visually resolved companion lies nearly one arc-second away and is 1.5 magnitudes fainter; it is thought to be a hot main sequence star, though orbital motion has not been observed.

The system is surrounded by a dusty Wolf–Rayet nebula over 200 AU in diameter, formed by the interaction of stellar winds from the two inner stars. The rotation of the system creates a spiral pattern, leading to the name Pinwheel Nebula.

The round appearance of the spiral indicates the system is seen nearly pole-on, with an inclination estimated between 0 and 16 degrees. Photometric variability was reported in 1997, and large-amplitude quasi-periodic variability was reported in 2002, leading to the variable star designation V5097 Sagittarii in 2003. The star shows frequent eclipse events and other irregular brightness variations, believed to be caused by dust formed from expelled material rather than by the companion star.

Reader's Guide

WR 104's significance lies in its potential as a nearby supernova progenitor and the public interest in whether it could threaten Earth. The Wolf–Rayet star is expected to undergo a core-collapse supernova within the next few hundred thousand years, with a small chance of producing a long-duration gamma-ray burst. The rotational axis of the binary system is directed approximately toward Earth, with an estimated inclination of 0 to 16 degrees, though spectrographic observations suggest a larger angle of 30°–40°, possibly up to 45°. The opening angle of any gamma-ray burst jet is estimated at 2 to 20 degrees, meaning Earth would only be in the path if the star's axis is within half that angle.

Australian astronomer Peter Tuthill has noted that for WR 104 to pose a danger, an extraordinary string of events would be required: the Wolf–Rayet star would need to generate a gamma-ray burst (which has not been observed in the Milky Way and is considered unlikely by some astronomers), its rotational axis would need to point toward Earth, and the jet would need to reach far enough. Tuthill tentatively estimates the probability of any GRB event at around one percent but cautions more research is needed. Recent studies suggest the danger to Earth is highly unlikely. The system remains an important object for studying Wolf–Rayet stars, colliding winds, and dust formation in binary systems.