Scientists at the Australian National University (ANU) have discovered evidence for supernova explosions in the interstellar space around our Solar System, from analysing deep-sea sediments. Supernovas (or supernovae) are the super-powerful explosions which mark the death of massive stars. All stars are powered by nuclear fusion reactions, as a consequence of which stars convert hydrogen into ever-heavier elements, which they are able to do because of their density and gravity. 

Massive stars are so dense and have so much gravity that they end up producing iron in their cores. But they cannot burn iron, so the metal accumulates in their cores, killing the star from the inside, finally resulting in a supernova, which blasts all the elements created by that star, including the iron, across space. What is left often, but not always, collapses into a black hole.

The analysis also established that our Solar System, including Earth, has been moving through a faintly radioactive dust cloud for the past 33 000 years. “These clouds could be remnants of previous supernova explosions, a powerful and super bright explosion of a star,” observed ANU nuclear physicist Professor Anton Waller.

The research team analysed several deep-sea sediments from two separate locations using the ANU Heavy Ion Accelerator Facility (HIAF). In particular, they employed the extremely sensitive HIAF mass spectrometer. And they discovered clear traces of iron-60.

Iron-60 is a radioactive isotope that is created in supernova explosions. It decays completely away in 15-million years. But the Earth is 4.6-billion years old. So the supernova or supernovas must have happened long after the Earth came into being. 

In previous research, Waller had found traces of iron-60 in sediments dating back 2.6-million years. And possible traces of the isotope dating back 6-million years. This suggests that the Earth has passed through the fallout clouds of more than one nearby supernova.

The dust and gas cloud through which the Solar System has been passing is known as the local interstellar cloud, and its origin has been a mystery. The discovery of the iron-60 traces support the idea that it was created by a supernova. However, the iron-60 traces are spread across the entire 33 000-year period that was measured. And it appears to be evenly spread throughout our local space. 

These facts raise more questions than they provide answers, as they do not seem to fit the supernova hypothesis. “There are recent papers that suggest iron-60 trapped in dust particles might bounce around in the interstellar medium,” noted Waller. “So the iron-60 could originate from even older supernovae explosions, and what we measure is some kind of echo. More data is required to resolve these details.”

The research team included scientists from the Australian Nuclear Science and Technology Organisation, the Helmholtz-Zentrum Dresden-Rossendorf and the Technical University Berlin. Their results have been published in the Proceedings of the National Academy of Sciences.