How heavy can neutron stars get? Astrophysicists have long wondered how massive these stellar corpses could be without collapsing under their own gravity to form a black hole. Last year's blockbuster observations of two neutron stars merging revealed a collapse as it happened, enabling four different groups to converge on the maximum mass—about 2.2 times that of the sun.
"I'm encouraged that they all agree," says James Lattimer, a nuclear astrophysicist at the State University of New York in Stony Brook. A solid mass limit for neutron stars will help theorists understand these mysterious objects. "Of all the characteristics of a neutron star, the two most important are the maximum mass and the radius," Lattimer says.
A dying star can have one of three afterlives. A lightweight star shrinks into a white dwarf, an Earth-size sphere of carbon. A heavy star explodes when its massive core collapses to an infinitesimal point: a black hole. A star in the middle range—8 to 25 solar masses—also explodes, but leaves behind a fantastically dense sphere of nearly pure neutrons measuring a couple of dozen kilometers across: a neutron star.
As the neutron stars spiraled into each other, gravitational-wave detectors in the United States and Italy sensed ripples in space generated by the whirling bodies. The waves allowed physicists to peg their combined mass at 2.73 solar masses. Two seconds after the gravitational waves, orbiting telescopes detected a powerful, short gamma ray burst. Telescopes on Earth spotted the event's afterglow, which faded over several days from bright blue to dimmer red.
Together, the clues suggest the merger first produced a spinning, overweight neutron star momentarily propped up by centrifugal force. The afterglow shows that the merger spewed between 0.1 and 0.2 solar masses of newly formed radioactive elements into space, more than could have escaped from a black hole. The ejected material's initial blue tint shows that at first, it lacked heavy elements called lanthanides. A flux of particles called neutrinos presumably slowed those elements' formation, and a neutron star radiates copious neutrinos. The short gamma ray burst, the supposed birth cry of a black hole, indicates that the merged neutron star collapsed in seconds.
To derive their mass limits, the teams dove into the details of the spinning neutron star. They generally argue that at first the outer layers of the merged neutron star likely spun faster than its center. Then it flung off material and slowed to form a rigid spinning body whose mass researchers could calculate from the masses of the original neutron stars minus the ejected material. The fact that this spinning neutron star survived only momentarily suggests that its mass was close to the limit for such a spinner.
That last inference is essential, Rezzolla says. Theory suggests that the mass of a rigidly spinning neutron star can exceed that of a stationary one by up to 18%, he says. That scaling allows researchers to infer the maximum mass of a stationary, stable neutron star. The whole argument works because the initial neutron stars weren't so massive that they immediately produced a black hole or so light that they produced a spinning neutron star that lingered longer, Shibata says. "This was a very lucky event," he says.
The analyses are persuasive, Lattimer says, although he quibbles with the precision implied in numbers such as 2.17 solar masses. "If you say 2.2 plus or minus a 10th, I would think it gets the same message across."