TESS and Swift are just two of myriad survey scopes out there, looking through large sections of the sky each night or each orbit, as astronomers look for supernovae, asteroids, and other rare events like microlensed starlight. Anytime you notice a lot of comets with similar names, like NEOWISE, ATLAS, or Pan-STARRS, what you’re hearing are the names of survey scopes that happen to find new comets at a regular cadence.

The Pan-STARRS telescope on Haleakalā in Hawaii is used by a number of different surveys, including the Young Supernova Experiment that works to catch the first light of relatively nearby exploding stars. In 2019, it spotted one such explosion in the spiral galaxy NGC 4666. This galaxy is a regular target for both professional and amateur astronomers, and it turned out that not only was their survey data on this object, but for unrelated reasons, the Hubble Space Telescope had observed the location of this supernova, named 2019yvr, 2.5 years prior to its explosion.

These images provided astronomers the rare chance to study exactly how a star looks prior to its explosion. In this case, the star they observed was a yellow star that should not produce the kind of a supernova they observed. Which is perplexing.

Basically, supernova 2019yvr was observed to be a perfectly normal, hydrogen-free supernova. The Hubble images, taken just 2.5 years before the star’s explosion, however, show a yellow star, which means the star had a hydrogen outer layer. So where did the hydrogen go?

This work appears in Monthly Notices of the Royal Astronomical Society (MNRAS) and is led by Charles Kilpatrick, who explains: 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.

So again we ask, where did the hydrogen go? According to the press release: Several months after the explosion, however, Kilpatrick and his team discovered that the material ejected in the star’s final explosion seemed to collide with a large mass of hydrogen. This led the team to hypothesize that the progenitor star might have expelled the hydrogen within a few years before its death. 

Kilpatrick goes on to further explain: Astronomers have suspected that stars undergo violent eruptions or death throes in the years before we see supernovae. This star’s discovery provides some of the most direct evidence ever found that stars experience catastrophic eruptions, which cause them to lose mass before an explosion.”

One additional possible explanation also exists. It could be there is an invisible, hydrogen-thieving, companion star involved, a star that stole the hydrogen in the couple years before the supernova and after the Hubble image. Maybe. We don’t know for sure, but as astronomers like Kilpatrick watch the supernova evolve, we may be able to figure it out in the next few years. When we know more, we’ll bring it to you here on the Daily Space.

More Information

Northwestern press release

RAS press release

“A cool and inflated progenitor candidate for the Type Ib supernova 2019yvr at 2.6 yr before explosion,” Charles D Kilpatrick et al., 2021 March 30, Monthly Notices of the Royal Astronomical Society