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Galactic Archaeology

Wednesday, 06. April 2016

Media information No 47/2016

Scientists research when supernovae have exploded in the vicinity of Earth

An unresolved question among astrophysicists has finally been answered: when and where have stars exploded close to our solar system in the recent past? In cooperation with scientists from the Centre for Astronomy at the University of Heidelberg and the Department of Mathematics at the University of Evora, a team of researchers from the Centre for Astronomy and Astrophysics at the Technische Universität Berlin (TU Berlin) was able to show – using complex model calculations – that 16 such supernovae occurred near Earth over the last 13 million years. The researchers under the direction of Prof Dr Dieter Breitschwerdt (TU Berlin) used 60Fe in their investigations – a radioactive isotope of iron that is only created in giant stars and supernovae through a process of fusion. The isotope acts as an indicator for the distance and time of the explosions. These findings have now appeared in a letter in the issue dated 7th April 2016 of the scientific journal Nature, together with another article written by a second international research group from the Australian National University, under the leadership of Dr Anton Wallner – who performed the precise measurements of 60Fe in multiple samples from the ocean floor.

Verteilung des Eisenisotops 60Fe, das vor 2,2 Millionen Jahren durch mehrfache Supernova-Explosionen in der Umgebung der Erde in den interstellaren Raum geschleudert wurde. Die Erde ist im Bild nicht maßstabsgetreu wiedergegeben. Die „Lokale Blase“, in di

At the end of their lives, massive stars produce lots of new elements, including long-life isotopes that take millions of years to decay. One such isotope is 60Fe. This iron isotope has a half-life of 2.6 million years and almost never occurs naturally on Earth. When these stars with more than eight times the mass of our Sun finally die, they do so in a violent explosion that is called a supernova. This releases so much energy in the form of radiation and particles that all the stars in a galaxy are outshone for a short while. In the first few weeks, a supernova in our proximity shines as brightly as a full moon and can even be seen in the daytime sky. Massive stars are the chemical factories of the universe; they synthesise all the elements that are heavier than helium by way of nuclear fusion. In the supernova, the radioactive iron isotope 60Fe is also thrown into interstellar space. If this happens close enough to our solar system, some of these particles can also reach Earth.

Secrets from the depths of the oceans

In 1999, scientists were already able to provide evidence of extra-terrestrial 60Fe on our planet – in low concentrations in the manganese crusts on the bed of the Pacific Ocean. These manganese nodules undergo change very slowly. Like tree rings, the accumulated layers reflect the chronological distribution of 60Fe. In 2004, a precise measurement gave a very clear indication that its origin lay 2.2 million years in the past.

The research team comprising Prof Dr Dieter Breitschwerdt and his scientific colleagues Dr Jenny Feige and Dr Michael Schulreich, as well as Prof Dr Miguel Avillez (Evora), Christian Dettbarn and Prof Dr Burkhard Fuchs (Heidelberg) has been occupied for many years with the origin of what is referred to as the ‘Local Bubble’. This concerns a region of the Milky Way in which our solar system is located; it is filled with diffuse hot gas and emits soft x-ray radiation. In this bubble which spans around 600 x 600 x 1,200 lightyears, temperatures range between 100,000 and several million degrees. The x-rays which reach Earth are already absorbed by the upper layers of the atmosphere and therefore pose no danger at all.

In the study which is now published in Nature, the first ever quantitative connection is drawn between the origin of the Local Bubble through supernovae and the 60Fe found on the ocean floor. The researchers have now been able to use the data from the astrometry satellite Hipparcos and a catalogue for radial velocities, compiled at the ARI (Astronomisches Rechen-Institut), to calculate the complete spatial motion of all stars within a volume of 1,200 lightyears in diameter. Using this method, they found a motion group of stars in which supernovae occurred in the last 13 million years.

Solar explosions leave their traces on Earth

For this purpose, the scientists used a law empirically derived from observations which enables the computation of exploded stars and their mass, based on the number of stars that are still present in the group. Moreover, it can be asserted that stars within such a system emerged together and are hence the same age. The age of the star constellation itself can be calculated from calculations of the evolution of these types of star; the age of the individual stars correlates with their mass, which allowed the researchers to work out when the supernovae exploded.

The locations of the supernovae were identified using the spatial motion (taking into consideration the measurement inaccuracy in the Hipparcos data) of the motion group, traced back into the past from its current location which lies in the Scorpius Centaurus constellation. This formed the starting point of several years of research for the team, which led to both the analytical as well as highly accurate, numerical calculations for the formation of the Local Bubble as well as the transportation of the 60Fe from its original location of the respective supernova to Earth. Particularly in the complex numerical simulations which also included the emergence of the neighbouring bubble Loop I as well as a complete interstellar medium with a three-dimensional expanse spanning almost 10,000 lightyears with more distant supernovae, a huge amount of details could be calculated for the formation of the Local Bubble and the path of 60Fe to Earth.

The researchers were able to show that around 16 supernovae in the last 13 million years produced the Local Bubble. They are also responsible for the 60Fe that was found on the ocean floor. The simulations demonstrated that around a half of the measured 60Fe came from two supernovae, which took place respectively 2.3 and 1.5 million years ago in the present constellations of Lupus and Libra. The other half can be attributed to the other 14 solar explosions which occurred further away. The two proximate supernovae which had around nine times the mass of the Sun, happened 270 and 300 lightyears away respectively – far enough that no direct harm came to the biosphere.

The second Nature article which was published by the international team of researchers from Australia, Germany, Austria, Israel and Japan also showed that the measured 60Fe originates from several supernovae events. The team, directed by physicist Dr Anton Wallner at the Australian National University, thereby examined the isotope content and age of multiple deep-sea samples of sediments, manganese nodules and crusts from the Pacific, South Atlantic and Indian Ocean. It could be demonstrated in all these deep-sea archives that the 60Fe isotopes are found in certain aged layers. In turn, the age of the layers was determined with the help of the terrestrial radioisotopes 10Be and 26Al. Several layers corresponding to age held 60Fe atoms – namely layers aged 1.7 to 3.2 million years and layers aged between 6.5 and 8.7 million years. The co-author of both publications was TU Berlin scientist Dr Jenny Feige.

According to Professor Dieter Breitschwerdt, it is possible to infer that 60Fe can be found around the world as it has been measured in samples from various different sites in the oceans. Both publications have primarily demonstrated that galactic archaeology for the region of our solar system can be conducted using precise laboratory measurements of long-life radioactive isotopes and theoretical model calculations.

The two publications:

  • D. Breitschwerdt, J. Feige, M. M. Schulreich, M. A. de. Avillez, C. Dettbarn, B. Fuchs
    “The locations of recent supernovae near the Sun by modeling 60Fe transport.”
  • A. Wallner, J. Feige, N. Kinoshita, M. Paul, L.K. Fifield, R. Golser, M. Honda, U. Linnemann, H. Matsuzaki, S. Merchel, G. Rugel, S.G. Tims, P. Steier, T. Yamagata, S.R. Winkler
    „Recent near-Earth supernovae probed by global deposition of interstellar radioactive 60Fe“
    (DOI: 10.1038/nature17196)

For additional information please contact:

Prof Dr Dieter Breitschwerdt | Dr Jenny Feige | Dr Michael Schulreich
Centre for Astronomy and Astrophysics, TU Berlin
Tel.: +49 30 314 25462 | +49 30 314 22092 | +49 30 314 22093

Dr Anton Wallner
Department of Nuclear Physics
Australian National University
Tel.: +61 2 612 52074

Dr Silke Merchel | Dr Georg Rugel
Helmholtz Institute Freiberg for Resource Technology at HZDR
Tel.: +49 351 260 2802 | Tel.: +49 351 260 3296


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