A star went supernova with more than twice the mass needed to ultimately collapse into a black hole. But something weirder happened - the star became a magnetar, an asteroid-sized star with the most powerful magnetic field in the universe.
The fates of stars are almost entirely determined by their mass. If a star is anything up to five times the mass of our Sun, it will live for several billion years before expanding into a red giant and then contracting into a degenerate white dwarf. Once you get past that five solar mass threshold, stars don't last nearly long, perhaps just a few hundred thousand years, after which they explode in a supernova.
Now, if the star is less than twenty times the mass of the Sun, the supernova leaves behind a neutron star. Such a star packs in anywhere from 1.5 to 2 times the Sun's mass into a sphere only 15 miles across. As you might imagine, such a densely packed object has incredibly strong gravitational forces, but that still pales compared to when a star is twenty or more times the Sun's mass. At that point, the star collapses in on itself after the supernova, creating a black hole.
All of that should be fairly straightforward, which is why a newly discovered neutron star some 16,000 light-years away is such a cosmic oddity. It's not just any neutron star - it's a magnetar, the most powerfully magnetic object in the known universe. Essentially, the same process that gave the Earth its magnetic field - known as the dynamo mechanism - can sometimes go into overdrive as the neutron star forms, creating a magnetic field a thousand times stronger than seen in an ordinary neutron star. At least, that's our best guess - nobody is quite sure why 1 in 10 neutron stars becomes a magnetar.
The newly discovered magnetar designated (deep breath) CXOU J164710.2-455216 is the remnant of a star that, by all rights, should have become a black hole instead. Astronomical measurements indicate the original star had a mass forty times that of the Sun, putting it way over the lower limit for black hole formation. So how did it end up as a magnetar?
The best explanation is that the star picked the wrong neighbors - it's surrounded by so-called Wolf-Rayet stars, humongous blue stars that are a million times brighter than the Sun. That sort of constant, intense radiation would have been enough to strip the exploding star of its mass way more quickly than normal, taking it from comfortably over to just below the cutoff to become a black hole. And so, with no other options, the star collapsed into a magnetar instead.
Even so, astronomers point out that they may be able to explain this isolated incident, but they still don't know how a star can lose mass so incredibly quickly. As Boston University astronomer Alan Marscher explains:
"Our understanding of the evolution of the most massive stars is hampered by an incomplete knowledge of the processes by which [they] lose mass. That's why our estimates of the minimum mass [needed for] an isolated star that eventually becomes a black hole are fuzzy."