Astronomy Now - January 2021

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Blue ring is signpost of stellar merger

he mystery of an ultraviolet ring of light around a star 6,300 light years away in the constellation

Hercules has been solved thanks to detective work by astronomers.

First spotted in 2004 around the star TYC 2597-735-1 by NASA’s Galaxy Evolution Explorer
(GALEX) spacecraft, the ring – nicknamed the Blue Ring Nebula – is the result of a red giant star
consuming a smaller neighbouring star. When the stars merged thousands of years ago, they
expelled a cloud of hot gas that was bisected by a disc of gas encircling the smaller star. is resulted
in the cloud forming two cones in opposite directions. Because we are looking at one of the cones
head-on, it appears as a ring.

As the gas cloud continued to expand, it swept up surrounding gas, causing a shockwave that heated
the gas to temperatures high enough for it to radiate at ultraviolet wavelengths. Evidence for this
shockwave was rst seen in 2006 by the Hale Telescope at Palomar Observatory and the Keck
Telescopes in Hawaii.

Ordinarily, such stellar mergers are shrouded in dust, but the Blue Ring Nebula is a rare view into a
merger in the short time frame between the clearing of the dust and the dissipation of the
expanding cloud.

e ndings are published in the 18 November issue of Nature.

The Blue Ring Nebula. The blue is false colour in this ultraviolet-light view. Image: NASA/JPL–Caltech/M. Seibert (Carnegie
Institution for Science)/K. Hoadley (Caltech)/GALEX Team.

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Blue ring is signpost of stellar m...

January 2021

Astronomy Now