Where's the black hole? That's what astronomers are asking as they gaze upon the burned-out remnant of a stellar explosion some 16,000 light-years away in the southern constellation Ara. The defunct star once held at least twice the mass necessary to create a black hole when it exploded as a supernova, yet somehow only an extremely magnetic, asteroid-sized object known as a magnetar remains.
When it comes to stars, astronomers like to say that mass is destiny. Relatively small objects like our sun can live up to 10 billion years before briefly expanding into red giants and then slowly dying as white dwarfs. Bigger stars with masses at least five times greater than the sun's have much shorter life spans—sometimes only a few hundred thousand years—and die spectacularly in huge explosions called supernovae. Current theory also suggests that if the doomed star holds less than 20 times the sun's mass, its explosive death should produce a small but extremely dense remnant called a neutron star. And if its mass tips the scale at 20 or more suns, the supernova should produce a black hole.
That's what makes the object astronomers have found so unusual. Like all magnetars, CXOU J164710.2-455216 is a rare kind of neutron star that for as-yet-unexplained reasons possesses the most powerful magnetic field in the universe. Yet according to detailed measurements of the relative motions of the surrounding stars, the team reports in an upcoming issue of Astronomy & Astrophysics, that like every neighboring star, the mass of the magnetar's progenitor must have been at least 40 times greater than the sun's. And thus it shouldn't be a magnetar at all, but a black hole.
So what's going on? Astronomer Kimberly Weaver of NASA's Goddard Space Flight Center in Greenbelt, Maryland, suggests that we look at CXOU J164710.2-455216's neighbors: "It's surrounded by Wolf-Rayet stars." These are giant blue stars that shine with the light of a million or more suns. All that intense radiation means "there is a ton of mass loss going on among this group," says Weaver, who was not involved in the study.
The characteristics of the surrounding stars suggest that although the magnetar's progenitor probably reached 40 solar masses at one point, it shed its mass so quickly that when the star exploded it fell under the 20-solar-mass limit, thereby creating a magnetar instead of a black hole—and conforming to current theory about stellar evolution. Even so, she adds, "the puzzle of how stars can lose so much mass so quickly is a big deal."
Astronomer Alan Marscher of Boston University agrees. It underscores how "our understanding of the evolution of the most massive stars is hampered by an incomplete knowledge of the processes by which [they] lose mass," he says. "That's why our estimates of the minimum mass [needed for] an isolated star that eventually becomes a black hole are fuzzy."