Solved: The Mystery of the Star That Blew Up as Supernova 1987A
Ten years after Supernova 1987A in the Large Magellanic Cloud became the first naked-eye supernova in our skies since 1604, Oxford astronomer Philipp Podsiadlowski says that the mystery about the nature of the star that produced this extremely unusual supernova is probably solved. The star that exploded used to be a member of a close binary pair, but it merged with its partner in an act of stellar cannibalism in the relatively recent past - perhaps some 30,000 years ago. Dr Podsiadlowski will explain how this scenario can account for all SN1987A's oddities when he speaks on Thursday 10th April at the UK's National Astronomy Meeting taking place at the University of Southampton.
One of the main puzzles of SN1987A, recognized soon after the explosion, is the fact that the star that exploded was a BLUE supergiant (with a radius of about 40 times the radius of the Sun), whereas theory had predicted that massive stars end their lives as RED supergiants (more than 1000 times larger than the Sun). One early idea was that this behaviour was somehow connected with the fact that the Large Magellanic Cloud, a satellite galaxy of the Milky Way, has a somewhat lower concentration of heavier chemical elements than the Milky Way, and that the star used to be a red supergiant but turned into a blue supergiant just 30,000 years ago. But the most recent calculations have shown that this idea just cannot be made to work. Even if it did work, it still would not explain the two other main puzzles of the supernova: the fact that a significant fraction of the material from the core of the star seems to have been thoroughly mixed with its outer layers, and the complex nebula surrounding the supernova remnant.
The nebula around the supernova remnant was first discovered with the European Southern Observatory's New Technology Telescope, but is most clearly seen in images taken with the Hubble Space Telescope. The main nebula consists of three rings that appear to float in space. What is left of the supernova is at the centre of the inner ring (with a radius of 0.7 light years), while the two other rings are parallel to the inner ring, but displaced from it by about 1.5 light years above and below. In the north, the nebula is bounded by a structure that has the appearance of Napoleon's hat.
The whole of this nebula was created before the supernova went off and consists of matter that was ejected by the progenitor star (or stars) within the last 30,000 to 60,000 years. It is very noticeable that the nebula is distinctly non-spherical, but nevertheless is symmetrical around an axis. This suggests that the progenitor was a spinning very fast around that axis. The most recent Hubble Space Telescope image of the supernova provides further evidence for this idea, since for the first time it resolves the actual material ejected during the supernova explosion and shows that it is elongated in the direction perpendicular to the inner ring. This is expected if the star that exploded was not spherical but flattened because it was spinning rapidly. But the rules of physics say that no single star, even if it were born with rapid rotation, could still be turning so fast after it has expanded to supergiant dimensions.
However, the conundrum can be resolved if something acted to 'spin up' the progenitor before it exploded. It turns out that a binary companion is capable of doing just that. Indeed, the type of binary system required is quite typical - most stars are members of binary or multiple systems. The only important requirement is that the merger of the two stars takes place after the progenitor has consumed all the helium in its core.
During the merger, which itself takes only a few years or decades to complete, the companion star is completely destroyed and its material is mixed with the envelope and part of the core of the progenitor. This produces a rapidly rotating star with thoroughly mixed outer layers - thereby explaining that observed peculiarity. This star subsequently wants to shrink to become a blue supergiant. However, the merged system is rotating too rapidly to make a blue supergiant and needs to slow down. This it achieves by spinning off matter in a disc around its equator. Once the star has become a blue supergiant, it develops a powerful stellar wind that sweeps the disc outwards. This process forms the observed inner ring. The outer rings may be 'swept-up' parts of a shock region in the shape of a double cone, produced by the interaction of stellar winds from each of the two stars before they merged.
It now seems that a double-star merger scenario is the only way in which all the various anomalies of this very unusual supernova can be understood. This theory predicts particular chemical anomalies, which would have been produced during the merger itself. If these are detected, it would be virtually conclusive evidence that the theory is correct.