Missing: a large and unstable blue star

A massive and unstable star is missing from a dwarf galaxy, and astronomers are intrigued.

It may just have become less bright and be partially obscured by dust, but that would mean it is eluding some pretty impressive technology. An alternative possible explanation is that it collapsed into a black hole without producing a supernova.

“If true, this would be the first direct detection of such a monster star ending its life in this manner,” says Andrew Allan from Ireland’s Trinity College Dublin, lead author of a paper in Monthly Notices of the Royal Astronomical Society.

The star is/was located in the Kinman Dwarf Galaxy, 75 million light-years away in the constellation of Aquarius.

That’s too far for astronomers to see individual stars, but they can detect some signatures, and between 2001 and 2011 the light from the galaxy consistently showed evidence that it hosted a luminous blue variable star some 2.5 million times brighter than the Sun.

Observations indicated it was in a late stage of its evolution, making it a promising subject for astronomers hoping to understand how such massive stars end their lives. But when Allan and colleagues from Ireland, Chile and the US went looking last year, it wasn’t there to be seen.

They first tried ESPRESSO, which simultaneously uses all four eight-metre telescopes of the European Southern Observatory’s Very Large Telescope (VLT) in Chile’s Atacama Desert. When that drew a blank, they tried the VLT’s X-shooter instrument. Another blank.

They then delved into the ESO Science Archive Facility and found old data collected by a number of telescopes indicating that the star might have been undergoing a strong outburst period that likely ended sometime after 2011.

Luminous blue variable stars are prone to giant outbursts over the course of their lives, causing their rate of mass loss to spike and their luminosity to increase dramatically.

So, the researchers say, the outburst may have resulted in the luminous blue variable being transformed into a less luminous star – or the star may have collapsed into a black hole, without producing a supernova explosion.

We hopefully will get a clearer picture in 2025, when the ESO begins operating its Extremely Large Telescope (ELT), which will be capable of resolving stars in distant galaxies