Heavier Stars Might not Explode as Supernovae, Just Quietly Implode Into Black Holes

Supernovae are a bright end to a huge star. A star's last attempt to shine for a brief time is cut short by cosmic time. Then it collapses on itself. Either a stellar-mass star or a black hole will result. We have always believed that stars with more than ten solar masses would end in a supernova. But, a new study shows that this is not the case.
Large stars experience what is called a core-collapse supernova, and this is in contrast to the Type Ia supernovae. Stars can survive by balancing heat and gravity. A large star must produce heat by fusing heavier elements as more elements fuse. This eventually forms a layer where different elements fuse. This chain cannot be extended to iron. Fusing heavier elements will cost you more energy than it releases. The star is ripped apart when the core collapses.

Not scaled. The onion-skin model for a dying star. R. J. Hall

Core-collapse supernovae are observed in models of large dying stars. They occur for stars with masses above 9 10 solar mass, and up to 40 50 solar mass. Stars above that mass are likely to collapsing into a black hole without ever becoming a supernova. Hypernovae could be produced by stars with extreme mass, 150 solar masses and more. These monsters don't explode due to a core-collapse but rather because of an effect called pair instability. In this process, colliding photons create pairs of electrons or positrons in the core.

The new study suggests that the upper limit of supernovae with core collapse might be lower than previously thought. The team examined the elemental abundances in a pair colliding galaxies called Arp 299. The region is hotbed for supernovae because the galaxies have just collided. The supernova explosions have effected the elements that are found in Arp 299, which means that the Arp 299 elemental abundances will be heavily dependent on them. They also measured the iron-to-oxygen abundance ratio and the neon-magnesium ratio to oxygen ratio. The Ne/O, Mg/O and Fe/O ratios were comparable to the Sun's. However, the Fe/O ratio was lower than the solar levels. Large supernovae are the most efficient at releasing iron into the universe.

A Hubble image of galaxies colliding, known as Arp 299 Credit: NASA, ESA and the Hubble Heritage Collaboration.

Although the ratios observed by the team didn't match core-collapse models, they found that supernova models performed well if the data was excluded from supernovae over 2327 solar masses. This means that models and observations can agree if stars collide into black holes over 27 solar masses.

Although this work does not prove that supernovae have a lower upper mass limit than previously thought, it does suggest that they are more common than we thought. It is possible that supernovae may produce more magnesium and neon than predicted by models. It doesn't matter what, we still have much information to learn about the dying stars of large stars.

Reference: Mao, Junjie, et al. Elemental Abundances in the Hot Atmosphere Luminous Infrared Galaxy Arch 299. The Astrophysical Journal letters 918.1 (2021: L17).