An international group of astronomers has used ESO’s Very Large Telescope at Paranal Observatory in Chile to discover and observe an ionized nebula surrounding the red supergiant star W26.

W26 is located in the southern constellation of Ara about 16,000 light years from Earth. The star belongs to the most massive star cluster in our galaxy – Westerlund 1. The cluster is home to several hundred of thousand stars, and is the closest analogue to some of the truly massive star clusters seen in distant galaxies.

The red supergiant W26 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.

When the astronomers studied the images of Westerlund 1 they spotted something unique. They discovered a huge cloud of glowing hydrogen gas around W26. Such clouds are ionized, meaning that the electrons have been stripped away from the atoms of hydrogen gas. The discovery is described in a paper appearing in the Monthly Notices of the Royal Astronomical Society (arXiv.org).

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 scientists 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 considerably easier to study in the future than if it were not ionized.

W26 will eventually explode as a supernova. The nebula that surrounds it is very similar to the nebula surrounding SN1987A, the remnants of a star that went supernova in 1987. SN1987A was the closest observed supernova to Earth since 1604, and as such it gave astronomers a chance to explore the properties of these explosions.

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Bibliographic information: Nicholas J. Wright et al. The ionized nebula surrounding the red supergiant W26 in Westerlund 1. MNRAS, published online October 16, 2013; doi: 10.1093/mnrasl/slt127