The explosive supernova of a massive star that should have created a black hole has been found to have created a highly magnetic neutron star instead, perplexing astronomers using the Very Large Telescope (VLT).

The mystery stems from what happens at the end of a starŐs life. As it runs out of its nuclear fuel, gravity causes the core of a star to collapse underneath its own weight. In the case of a Sun-like star, the core collapses into a dense white dwarf, while the outer layers are gently blown away to form a planetary nebula. For stars with masses between 10 and 25 times the mass of the Sun, which explode as a supernova, the gravitational collapse crushes individual protons and electrons together, merging them to form a core of neutrons – a neutron star. In the supernovae of stars greater than 25 solar masses, the mass of the core is so great that gravity crushes the core to practically infinite density: a black hole.

Using the FLAMES (Fibre Large Array Multi Element Spectrograph) instrument on the giant VLT at the European Southern Observatory in Chile, a team led by Simon Clark and Ben Ritchie of the Open University took a careful look at an extremely magnetic neutron star called a magnetar, one of only thirteen known to exist and which have magnetic fields a thousand trillion times more powerful than Earth’s. Exactly how magnetars form is uncertain, but one favoured theory is that they evolve from massive stars that possess strong magnetic fields that then become ‘frozen’ into the neutron star. They seem to be so rare because their magnetic fields decay to normal neutron star levels after only 10,000 years or so.

The extraordinary magnetar studied by Clark and Ritchie’s team has a suitably extraordinary home – the ‘super star cluster’ Westerlund 1, which contains hundreds of very massive stars 16,000 light years away in the Southern Hemisphere constellation of Ara the Altar. It is the closest and most compact super star cluster, with some stars possessing an intrinsic luminosity two million times that of our Sun. More importantly, astronomers have a good idea of how old the cluster is, between 3.5 and five million years old. This puts constraints on the mass of the star that exploded to create the magnetar – the more massive a star is, the shorter its lifetime. For a star to have formed inside the cluster and have exploded within this time frame, it would require a mass of around 40 solar masses – but according to theory a star of this mass should create a black hole, not a magnetar.

To double check, Clark’s team measured the masses of the two stars in the eclipsing binary system W13, which is also located in Westerlund 1, and found that they are of 21 and 33 solar masses respectively. “Because the life-span of a star is directly linked to its mass – the heavier a star, the shorter its life – if we can measure the mass of any surviving stars, we know for sure that the shorter-lived star that became the magnetar must have been even more massive,” says Clark.

The question becomes, why did such a massive star form a magnetar and not a black hole? Clark thinks that it is related to the fact that the progenitor star may have existed in a binary star system before it exploded. As a massive star evolves it expands and begins to throw off mass as it becomes unstable, and in binary systems such as W13 this usually results in stellar mass being transferred from one star to another. However, in the case of the star that bequeathed the magnetar, its outer envelope may have swelled to the point that it totally engulfed its companion. “If this was the case then drag forces on both stars caused them to spiral inwards,” Clark tells Astronomy Now. This would have caused angular momentum to decrease, and the energy be transferred to the envelope of matter around them, causing it to be ejected. In this way, it is possible that the progenitor star lost enough mass that when it did go supernova, it had shrunk down below the 25 solar mass limit. The force of the supernova would also have ejected the companion star from, the system, leaving the magnetar alone.