The Hubble telescope imaged the star two and a half years before it exploded.
https://www.cbc.ca/radio/quirks/may-...nova-1.6025301

Never-before-seen scenario ‘stretches what’s physically possible.’
https://scitechdaily.com/oddball-sup...ally-possible/
“We haven’t seen this scenario before,” said Northwestern’s Charles Kilpatrick, who led the study. “If a star explodes without hydrogen, it should be extremely blue — really, really hot. It’s almost impossible for a star to be this cool without having hydrogen in its outer layer. We looked at every single stellar model that could explain a star like this, and every single model requires that the star had hydrogen, which, from its supernova, we know it did not. It stretches what’s physically possible.”