1 Sep 2021

Conventional theory states that light stars like our Sun gently blow off their layers when they die, while heavy stars explode as a supernova. But for some reason, we are so far failing to find supernovae from stars heavier than eighteen solar masses. Now a team led by SRON astronomers finds a new clue that fuels this apparent mystery. Publication in Astrophysical Journal Letters.

The research team looked at the luminous infrared galaxy Arp 299 using the X-ray telescope XMM-Newton. Their aim was to measure the abundances of several different chemical elements that are normally produced and expelled into space when massive stars explode. They found a mismatch for the heavier elements iron, neon and magnesium compared to existing models for how stars enrich their environment. 'This is another indication that the very heavy stars don't go supernova,' says lead author Junjie Mao (Hiroshima/Strathclyde/SRON). When the researchers compared the measured amounts of iron, neon and magnesium with existing model calculations that describe how stars enrich their environment, the results appeared to be quite different. 'If we remove the expected contribution from supernovae with masses above 23-27 solar masses to the chemical enrichment from the model calculations, the difference between our model and our observations is suddenly a lot smaller.'

Astronomers still don't understand why stars from around eighteen solar masses would disobey the conventional theory of stellar evolution and refuse to go supernova. 'One possible explanation is that they immediately collapse into a black hole, without the explosion,' says co-author Aurora Simionescu (SRON). 'We have now found more evidence that the end of life of massive stars could look very different than we thought so far. It could be more of a quiet passing away than a big cosmic fireworks show.'

Stellar evolution
When a star is born, it consists of mostly hydrogen, the lightest element in the Universe. The immense gravity builds up pressure in the core, igniting nuclear fusion of hydrogen into helium. This continues as a burning shell moving outwards, leaving a core of helium. When this core gets massive enough, gravity does its job again and ignites fusion of helium into carbon and oxygen. Eventually, you get a layered onion structure with increasingly heavier elements towards the center. Stars above eight solar masses get layers of hydrogen, helium, oxygen, carbon, neon, sodium and magnesium, and an iron core. This is how a large part of the heavier atoms in our world are made. Iron doesn't fuse under normal circumstances, so it piles up in the core, until it collapses under its own weight, igniting a chain reaction—a supernova. This should happen to all stars that are massive enough to build up iron at their core; and, generally, the more massive the star, the more chemical elements its supernova explosion should spread out into space, sowing the seeds for new planets. It's still a mystery why astronomers are now finding more and more evidence that this doesn't hold up for stars above eighteen solar masses.