A Massive Star Appears To Have Vanished Without A Trace

This illustration shows what the luminous blue variable star in the Kinman Dwarf galaxy could have looked like before its mysterious disappearance. ESO/L. Calçada

About 75 million light-years away in the Kinman Dwarf galaxy there was a star. It was a very special star, a "luminous blue variable". It was about 2.5 million times brighter than the Sun and had an estimated mass tens of times that of our star. Astronomers observed it from 2001 to 2011 as it was undergoing an outburst but recent observations are the reason the star is being talked of in the past tense: the star is no longer visible. It appears to have disappeared.

The observations, or lack thereof, are reported in the Monthly Notices of the Royal Astronomical Society. The disappearance is very unusual. Luminous blue variables are massive evolved stars that can change their luminosity wildly, but if they are on the edge of death, they usually go supernova. However, no such signature to indicate that was spotted for this object, giving the researchers two possibilities for what may have happened.

The first one is that the star might have dramatically dimmed, turning into a less luminous star, and dust and material released from the star during the earlier outburst are now shrouding this peculiar object in a veil of darkness. On the other hand, if the star is truly gone, it might have simply collapsed into a black hole.

“It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion," lead author Andrew Allan of Trinity College Dublin, said in a statement. "If true, this would be the first direct detection of such a monster star ending its life in this manner.”

The black hole scenario would be unusual and unlikely, but it is not unheard of. There is a process known as a "failed supernova" and has been considered a possibility for why different types of stars, including two red supergiant stars in other galaxies, have been discovered missing. In a failed supernova, the star brightens as it would in the early stages of supernova, but then doesn't increase to the full extent as a supernova would. While similar in result, the process behind it is unclear.

Current instruments cannot see individual stars in the Kinman Dwarf galaxy as it is too far away, so we cannot work out what actually happened to this luminous blue variable. But future observatories, like the European Southern Observatory's Extremely Large Telescope being built in Chile, will have the capabilities to identify individual stars and could allow a resolution for this and other mysteries.

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