We are made of “star stuff,” a term popularized by astronomer Carl Sagan that encapsulates the mind-boggling reality that our bodies are literally made of elements forged in the cores of stars. As it happens, we are also made of dead star stuff, according to a new study about a weird pair of distant neutron stars, which are the dense corpses of dearly departed stars.
Neutron stars are created when massive stars blow up and collapse into gnarly, roiling spheres that can pack about two times the mass of the Sun into a 12-mile-wide ball. Sometimes, two neutron stars end up merging together to form a black hole, a union that creates a stupendous amount of light and energy, as well as ripples in spacetime known as gravitational waves.
Scientists led by Robert Ferdman, a physicist at the University of East Anglia, have been closely examining one of these double neutron star systems, called PSR J1913+1102, since it was first discovered in 2012. The reason being: it’s one of the strangest neutron star pairs yet, and it could shed light on a host of open mysteries ranging from the origins of the elements that make up our bodies to the expansion rate of the entire universe.
Most of these systems include two comparably sized neutron stars, but this one contains neutron stars with masses that are 1.62 and 1.27 times the mass of the Sun, making it “the most asymmetric merging system reported so far,” according to the team’s paper, published on Wednesday in Nature.
“Myself and my colleagues would very much like to know even more precisely the population of these asymmetric systems,” Ferdman said in an email. “To do this, we will continue to survey the galaxy for double neutron star systems in the hopes of increasing the known population of these types of unique systems.”
To understand why these systems are so important, we’ll need to wind the clock back to August 17, 2017. On that date, scientists detected a gravitational wave called GW170817, which was created by a neutron star collision that occurred 130 million light years away.
The 2017 merger was the centerpiece of one of the biggest astronomical breakthroughs in recent history. After the wave was flagged by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo detectors, astronomers scanned the skies looking for any explosions of light that this union may have emitted.
Within a day, multiple telescopes had spotted energetic gamma ray bursts that were eventually confirmed as an “optical counterpart” to GW170817. This discovery was extremely significant because it enabled scientists to study different lines of evidence—in this case, gravitational waves and light—from the same event, a technique known as “multi-messenger astronomy.”
One big application of this new field is to pinpoint the exact expansion rate of the universe.
“The gravitational wave signals (detected by ground-based detectors like LIGO and Virgo) are able to measure a distance to the merger site, independent of any other method,” Ferdman explained. “Measurement of the host galaxy or environment with, e.g., optical telescopes, can measure the velocity at which the system is receding from us.”
“Combined, this gives us a unique and independent measurement of the Hubble constant—the rate at which the universe is currently expanding,” he added.
The Hubble constant has been measured before, using flashing stars and the oldest observable light in the universe. But much to the consternation of the worldwide astronomy community, the figures from those experiments do not match: One says the universe is expanding at about 46,200 miles per hour per million light-years while the other says it is about 50,400 mph per million light-years.
It’s not a big discrepancy, but it’s enough to keep scientists up at night. A third independent method of clocking the Hubble constant could resolve that tension, and Ferdman and his colleagues hope that these asymmetric mergers may be the key to that challenge.
While the total mass of the 2017 merger turned out to be about 2.8 times the mass of the Sun, the individual masses of the two neutron stars are not known. There are many scenarios in which two neutron stars of comparable mass could create a bright optical counterpart, but it’s also possible that the unexpected lumosity was due to a merger of objects with very different masses.
The new study estimates that asymmetric systems, like PSR J1913+1102, could represent one in 10 of the total population of neutron star mergers, so it’s not far-fetched to imagine that we will witness many of these bright events.
Indeed, because these asymmetric mergers are predicted to produce luminous fireworks, they may be compelling candidates to look for as a means to refine estimates of the Hubble constant. It would take about 20 multi-messenger detections of mergers to produce the “tiebreaker” measurement of the Hubble constant” Ferdman said, which “will hopefully be a defining moment in resolving a significant mystery currently faced by the astronomical community.”
Asymmetric systems could also reveal new insights about the role that neutron star mergers play in sowing the universe with heavy elements. Exploding stars create a lot of enriched elements in the cosmos, which have been crucial to the development of life on Earth and possibly elsewhere.
However, neutron star mergers may be one important source of some of the heaviest elements, such as gold. The bright kilonovae created by these mergers—especially asymmetric ones—may allow scientists to spot exotic matter inside these dead stars, which could help resolve this enigma.
Last but not least, Ferdman and his colleagues hope to use PSR J1913+1102 to probe the underlying nature of gravity and spacetime.
“Einstein’s theory is the best description we currently have of gravity, but because of its incompatibility with the other fundamental forces of physics, we know it is not the final word,” Ferdman said. “It’s therefore really important to continue to test the boundaries of the theory in order to find where it may break down.”
“This system provides a unique way to do this with general relativity and alternative theories, thanks to the mass asymmetry,” he concluded. But that’s a story for the team’s next paper, which is already in the works.