Have you ever heard of the beautiful story of Tanabata? It is much loved and celebrated in Asian cultures such as the Japanese – and has a lot to do with a recent discovery.

Tanabata is the story of two lovers, Orihime and Kengyu, who can only meet at the seventh day of the seventh month every year. According to the Japanese mythology, the couple is represented by two stars from opposing sides of our Milky Way: Vega (Orihime) and Altair (Kengyu). Like the lovers, both stars are only seen together once a year – in other words,they have a very, very short encounter.

Using some big data tools and the Subaru Telescope from the National Astronomical Observatory of Japan (NAOJ), a team of astronomers was able to spot phenomena that change as fast as the meeting of Orihime and Kengyu. Astronomical phenomena that change that fast – within a day, for example – are called 'transients'. By looking at 20 of them in late 2020, the NAOJ team noticed one in special: MUSSES2020J (AT 2020afay), which looked like a supernova. 

At first, MUSSES2020J looked very dim in the initial images, but got brighter and brighter as the observations went on – astronomers caught it when it was first happening! The team then confirmed that MUSSES2020J evolved much faster and was about 50 times brighter than normal supernovae. Astronomers have seen just a handful of events like these, and the most famous of them is AT 2018cow. 

The team then proposed a new name for events like MUSSES2020J: 'fast blue ultraluminous transients' (FBUT). These events are so powerful and happen so suddenly that astronomers suspect that black holes or neutron stars could be involved – but what happens and how it happens is still a mystery astronomers are looking at.
 

Image: Artist’s impression of possible origins of the fast, high-energy event MUSSES2020J. Credit: Kavli IPMU, University of Tokyo.