When astronomers studied the images of Westerlund 1 they spotted something truly unique. Around one of the stars, known as W26, they saw a huge cloud of glowing hydrogen gas, shown as green in this new image. Such glowing clouds are ionized, meaning that the electrons have been stripped away from the atoms of hydrogen gas.
Clouds of this type are rarely found around massive stars and are even rarer around red supergiant stars such as W26 -- this is the first ionized nebula ever discovered around such a star. 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. The fact that the nebula is ionized will make it considerable easier to study in the future than if it were not ionized.
On investigating the star W26 in more detail the researchers realized that the star is probably the largest star ever discovered, with a radius 1,500 times larger than the Sun and is also one of the most luminous red supergiants known. Such large and luminous massive stars are believed to be highly evolved, all of which suggests that W26 is coming towards the end of its life and will eventually explode as a supernova.
The nebula observed around W26 is very similar to the nebula surrounding SN 1987A, the remnant of a star that exploded as a supernova in 1987. SN 1987A was the closest observed supernova to Earth since 1604 and as such it gave astronomers a chance to better study the properties of these explosions. Studying objects like the new nebula around W26 will help astronomers to understand the mass loss processes around these massive stars, which eventually lead to their explosive demise.