Astronomers get rare and surprising before and after shots of a star going supernova

When a supernova appeared in a relatively nearby galaxy in 2019 Astronomers trying to better understand exploding stars got a lucky break. They discovered that the Hubble  telescope had taken a picture of what they think is the same star two years before it blew up.

And to their surprise, the star didn't look very much like what they thought a star heading for a supernova like this should look like. 

When supernova 2019yvr was spotted Maria Drout, an Assistant Professor of Astronomy and Astrophysics at the University of Toronto, and her colleagues checked back on Hubble observations to see if they could see a star in the same location as the supernova. Finding a progenitor star is normally a long shot, but in this case archived Hubble imaging showed a massive star just where they hoped it would be.

Yellow, cool and large

In their new study the team described how the the star defied their expectations. The star exploded in what's called a Type 1b supernova, which is expected to come from hot, blue and compact massive stars. But the progenitor star they identified was yellow, cool and very large. 

Another big surprise was that the star was found to be surrounded by an envelope of hydrogen. After it exploded however, the astronomers were baffled because there wasn't any evidence of that hydrogen. 

Drout explained a couple of theories for the discrepancy. In the years after the star was seen by Hubble, it may have experienced a violent eruption that resulted in the loss of hydrogen. Another possibility is that its envelope of hydrogen was stripped off by another star in its orbit. She said both theories are plausible but amazing in terms of timing as the kind of steller evolution they imply usually happens over centuries or more rather than in a couple of years. .

But the observation may mean astronomers have to rethink models of progenitor stars for this type of supernova.