Astronomers revealed that the unstable massive star in a dwarf galaxy is missing. They used the European Southern Observatory's Very Large Telescope (VLT) in making this discovery. They think that the massive star's absence indicates that it became less bright and partially obscured by dust.
But an alternative explanation could also be that the star collapsed into a black hole without producing a supernova, Phys.org reports. If proven right, this would be the very first detection of a monster star ending its life in that manner, said team leader and Ph. D. student Andrew Allan of Trinity College in Dublin, Ireland.
A Missing Massive Star in Kinman Dwarf Galaxy
Various teams of astronomers have studied the mysterious massive star located in the Kinman Dwarf galaxy between 2001 and 2011. Their observations point out that the star was already in its late stage of evolution.
Together with his collaborators in Ireland, Chile and the US, Allan tried to find out more about the mysteries of how massive stars end their lives. However, when they pointed out ESO's VLT to Kinman Dwarf galaxy in 2019, the scientists could no longer see any sign of the star.
"Instead, we were surprised to find out that the star had disappeared!" Allan said.
The Kinman Dwarf galaxy lies some 75 million light-years away in the constellation of Aquarius. A very far distance for astronomers to see its stars, but they can detect signs of some of them. Just like from between 2001 to 2011 when they can still detect a 'luminous blue variable' star that shines 2.5 million times brighter than the Sun.
These type of stars are unstable, showing occasional dramatic shifts in their spectra and brightness. But even if they are unstable, luminous blue variables leave specific traces that astronomers can identify. However, they are absent from the data that the team collected in 2019, baffling them of what happened to the star.
"It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion," Allan said.
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Using ESO's Very Large Telescope to Solve the Cosmic Mystery
The team of researchers first turned the ESPRESSO instrument toward the massive star in August 2019, through the VLT's four 8-meter telescopes simultaneously. But they failed to detect any signs of the luminous star.
Then a few months later, they tried the X-shooter instrument, also known as ESO's VLT, but there was still no traces of the star.
They then turned to older data gathered by X-shooter and the UVES instrument on ESO's VLT, located in the Chilean Atacama Desert, other telescopes situated elsewhere. That is when they were able to find and use the data of the massive star collected in 2002 and 2009, according to Andrea Mehner, a staff astronomer at ESO in Chile who participated in the study.
The old data revealed that the massive star could have been undergoing a strong outburst period which would have likely ended sometime after 2011. Luminous blue stars such as the one in Kinman Dwarf galaxy are prone to giant outbursts throughout their life. This causes the rapid mass loss of the star and dramatic increase in their luminosity.
Astronomers concluded that there could be two possible explanations of the star's disappearance, which relates to this outburst. One is that it may have resulted from the transformation of the luminous star into being a less bright star, which could also be partly hidden by dust.
On the other hand, the star may have also collapsed into a black hole, without producing supernova; a rare event since current knowledge on massive stars dictates that most of them end their lives in a supernova.
But researchers still recommend further studies to confirm the fate of the massive star. In 2025, they plan to use ESO's Extremely Large Telescope (ELT) in resolving cosmic mysteries such as this.
