Free Astronomy Magazine March-April 2018

26 MARCH-APRIL 2018 SOLAR SYSTEM to perform that kind of analysis because their mineralogical structure has remained substantially unaltered since the birth of the Solar System. And it is precisely through studying the decay products of the two ra- dioisotopes in question that researchers have on several occasions reached an unex- pected conclusion: the 26 Al/ 27 Al ratio, being 1 atom of the first element to every 20,000 atoms of the second element, is 17 times higher than the corresponding average value in the galaxy; but the 60 Fe/ 56 Fe ratio is 1 atom of the first element to every 50 million atoms of the second element, a value much lower than the galactic aver- age. These differences undermine the su- pernova hypothesis as a trigger for the formation of our Solar System. The progen- itor of a supernova, typically a very massive star, produces 26 Al in the outer layers and expels it into space through the powerful winds driven by the pressure of the radia- tion. Radioisotope 60 Fe is, however, pro- duced in the innermost regions and re- leased during the supernova phase. It would, therefore, be reasonable to ex- pect that, after the explosion of a super- nova, a localised surplus of 26 Al would coincide with a surplus of 60 Fe (with respect to the galactic mean value). Instead, 60 Fe seems to be almost absent at the very be- ginning of the Solar System. What if it was not a supernova that trig- gered the birth of the Solar System? Is there A delicate blue nebula trig- gered around 20,000 years ago by the Wolf-Rayet star (WR 31a) visible at its centre. Our Solar System could have been born in a mass concentration like those distributed in the shell of this bubble. [ESA/Hub- ble & NASA, Judy Schmidt]