At 1,500-times wider than our sun, red supergiant W26 is the biggest known star in the Universe. But new observations reveal W26 is dying. Observing how it dies could help reveal how dying superstars supply the cosmos with ingredients necessary for planetary formation โ€“ and possibly even life.

Above: The stars of Westerlund 1 glimmer in gold, the cluster's ionized nebulae exposed by their greenish glow.

W26 counts itself among the hundreds of thousands of stars comprising the nearby superstar cluster known as Westerlund 1 โ€“ the largest congregation of stars in the Milky Way. At just 16,000 light-years from Earth, Westerlund 1 is a stone's throw away in cosmic terms, making it a fantastic analog for other, more distant massive star-clusters that have been observed in neighboring galaxies.


It was while investigating Westerlund 1 that scientists led by Royal Astronomical Society researcher Nick Wright made an unexpected discovery: Surrounding W26 is an enormous cloud of ionized hydrogen gas (i.e., hydrogen atoms stripped of their electrons) that presents as green and glowing in images like the one below. W26's emerald aura sets it apart from the majority of the other stars in the cluster, which below appear reddish gold.

Ionized nebulae stand out clearly in green in this image with the nebula surrounding W26 indicated by the white lines. Image and Caption via Wright et al.

In the paper describing their findings, to be published in a forthcoming issue of the Monthly Notices of the Royal Astronomical Society, the researchers write that "the nebula consists of a circumstellar shell or ring... and a triangular nebula," the latter glowing really bright green in the image above. Here's another view (click to enlarge):


According to the researchers, this is the first ionized nebula ever discovered around a red supergiant.

"W26 itself would be too cool to make the gas glow; the astronomers speculate that the source of the ionizing radiation may be either hot blue stars elsewhere in the cluster, or possibly a fainter, but much hotter, companion star to W26," RAS officials said in a statement. "The fact that the nebula is ionized will make it considerably easier to study in the future than if it were not ionized."


Followup investigations that observe the nebula with high-resolution, narrow band imaging could provide astronomers with an unprecedented look at how W26 and other potential supernovae resupply the interstellar medium with the enriched materials necessary for forming rocky planets like Earth, and possibly giving rise to life itself.

The researchers' findings are published in the latest issue of the Monthly Notices of the Royal Astronomical Society, and appear free of charge in the arXiv. All images via ESO/VPHAS+ Survey/N. Wright