Before 2017, when LIGO captured its first neutron-star merger, everything we knew about neutron stars came from observations of relatively nearby specimens in our own Milky Way galaxy. (Of the 2,500 or so known neutron stars, 18 coexist in orbiting pairs known as binary neutron stars.) GW190425, by contrast, is nearly 5,000 Milky Ways away.
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The first puzzling thing about it is its mass: The new system has a total mass equal to about 3.4 suns. All previously known examples of binary neutron stars weighed the equivalent of about 2.6 suns. LIGO’s first binary-neutron-star pair fell near this lower weight.
But the high combined mass is just the first of the merger’s mysteries. More bewildering still is the inferred abundance of big neutron stars: Based on the recent observation, LIGO scientists estimate that these heavy pairings should be almost as common as the lighter binary-star systems that astronomers have been studying for decades. Big neutron-star pairs should be all over the universe, including in our own Milky Way. Why, then, have none ever been spotted before?
One possibility is that these mergers are hard to detect because they happen so rapidly.
With a telescope that can only see using light—meaning all telescopes until LIGO came along—you have to be looking in the right place at just the right time. A brief flash from a massive neutron star pair might go unnoticed. “If a type of binary merges very quickly, then statistically it’s very unlikely that you can catch one as it happens,” says Salvatore Vitale, an astrophysicist at the Massachusetts Institute of Technology who is part of the LIGO collaboration.
LIGO changes the calculus. It’s an omnidirectional gravitational-wave detector that monitors the entire sky. Vitale and the rest of the team believe that they’ve stumbled across something that was practically invisible before the advent of gravitational-wave astronomy.
The more significant problem with this hidden glut of gigantic neutron stars, however, is that we can’t explain why there should be so many of them.
For starters, if there are as many massive neutron star pairs as there are lighter ones, then we should expect to find as many heavy stars (which create them) as we do lighter stars. But that’s not the case: Astronomers estimate that fewer than 10 percent of all stars are big enough to make such massive neutron stars. “We have confusing evidence coming from very different methods,” says Ramirez-Ruiz.
That’s not where the mystery ends. The best existing computer simulations of stellar evolution simply cannot account for the estimated abundance of these unusually hefty pairs.
Scientists often use computer simulations to model complicated processes over long periods of time. In this case, the authors modeled the life cycle of compact stellar objects over billions of years. “You put in a bunch of stars, and you tell the code how the stars explode,” Vitale says. Then “you let it run for a few million or billion years, and you see what the outcome is.”