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Year in Review

This year’s neutron star collision unlocks cosmic mysteries

Discovery offers clues to heavy element formation, universe’s expansion and more

8:31am, December 13, 2017
illustration of neutron star collision

CRASH AND WAVE   In a galaxy 130 million light-years away, two neutron stars collided. This year, in the first detection of its kind, observatories caught the resulting gravitational waves and light show (illustrated).

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Thousands of astronomers and physicists. Hundreds of hours of telescope observations. Dozens of scientific papers. Two dead stars uniting into one.

In 2017, scientists went all in on a never-before-seen astronomical event of astounding proportions: a head-on collision between two neutron stars, the ultradense remnants of exploded stars.

The smashup sent shivers of gravitational waves through Earth, and the light show that followed sent shivers of excitement down astronomers’ spines. A real-life scientific drama quickly unfolded as night after night, astronomers around the world raced the sunrise, capturing every possible bit of data before day broke at their observatories.

Scientists had long fantasized about using light together with gravitational waves to forge a new kind of astronomy capable of unlocking otherwise unknowable secrets of the cosmos. “Now we’re finally here,” says Vicky Kalogera, an astrophysicist at Northwestern University in Evanston, Ill.

Almost overnight, the discovery vanquished some theories and vindicated others. It has had implications for the origins of the universe’s heaviest elements, the mysterious dark energy that makes up about 70 percent of the cosmos and the source of brilliant-but-brief flashes of high-energy light known as short gamma-ray bursts. In the last two decades of astronomy, the detection “is more significant than any other event,” says theoretical astrophysicist Avi Loeb of Harvard University.

So frenzied was the excitement over the find that researchers had trouble keeping their discovery under wraps as they raced to analyze data. Social media buzzed with rumors. When a Twitter account that regularly announces targets of the Hubble Space Telescope reported one named “BNS-Merger” (presumably for “binary neutron star”), corners of the internet exploded with speculation (SN Online: 8/25/17). Astronomy enthusiasts mined the publicly available logs of astronomical observatories for hints of what telescopes were zeroing in on.

The two neutron stars converged in the galaxy NGC 4993, 130 million light-years from Earth, emitting gravitational waves in the process (SN: 11/11/17, p. 6). Those waves, predicted by Einstein’s general theory of relativity, stretched and compressed spacetime, traveling outward like ripples on a pond. As the waves began their outbound journey 130 million years ago, Earth was in the midst of the Cretaceous Period: Dinosaurs were large and in charge. Life capable of building the complex detectors necessary for spotting the gravitational rumbles wouldn’t arise for many millions of years.

But these spacetime ripples arrived at Earth at an opportune moment, when detectors were finally prepared to spot them. Decades in the making, these highly-sensitive devices can register spacetime shifts a fraction of the size of a proton. On August 17, 2017, the instruments — the two detectors of the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, in Livingston, La., and Hanford, Wash., and Advanced Virgo near Pisa, Italy — spotted the waves and joined forces to triangulate their source. Gamma-ray space telescopes recorded a high-energy light burst less than two seconds later.

A worldwide network of telescopes sprang into action to look for light in the region of the sky where LIGO and Virgo predicted the waves came from. Less than 11 hours after the gravitational waves appeared, astronomers spotted a new point of visible light in the sky. That finding kicked off an astronomical free-for-all: “Basically every telescope pointed at this thing,” says LIGO member Daniel Holz of the University of Chicago. As a result, he says, “there’s just this wealth of information.”

Telescopes captured visible, infrared and ultraviolet light, followed by X-rays and radio waves days later. Each observation was precious: The light faded rapidly and changed colors over time, says Josh Simon, an astronomer with Carnegie Observatories in Pasadena, Calif. “This is a fairly rare occurrence in astronomy,” he says. “Most things we look at don’t change over time at all.”

Researchers announced October 16 that Advanced LIGO (the Laser Interferometer Gravitational-Wave Observatory) and its sister experiment, Advanced Virgo, had detected gravitational waves from colliding neutron stars — a cosmic crash also observed by more than 70 observatories around the world.

Observations revealed a previously theorized process dubbed a “kilonova” — thought to be a source of heavy elements like gold, silver, platinum and uranium — which could form as neutron-rich material is ejected from the stars. Light from the collision directly confirmed this hunch for the first time. “My wedding band emerged from a neutron star merger,” Loeb marvels.

The intensity of observations outstripped all previous astronomical finds, said astronomer Edo Berger of Harvard University at an October 16 news conference in Washington, D.C. “I don’t think there has been anything like this before.” Roughly 15 percent of all astronomers were involved in the discovery, estimates astronomer Bryan Gaensler of the University of Toronto. One of the many papers announcing results, published in Astrophysical Journal Letters, boasted over 3,000 authors. Additional papers appeared in that journal, as well as in Science, Nature and Physical Review Letters, among others.

Already, this event has ruled out hundreds of theories that provide alternatives to dark energy, a perplexing facet of our universe that is the most common explanation for why the cosmos is expanding faster and faster. Some of these theories aren’t consistent with the near-simultaneous detection of light and gravitational waves, several teams reported online October 17 and 18 at (SN: 11/25/17, p. 10).

Researchers also made a new measurement of how fast the universe is expanding, a number that could help solve a lingering puzzle. Observations of supernovas suggest that the universe is expanding at 73 kilometers per second for each megaparsec (about 3.3 million light-years). That’s significantly faster than measurements made using the cosmic microwave background, ancient light from the early years of the universe (SN: 8/6/16, p. 10), which peg the expansion rate at 67 km/s per megaparsec. The new measurement is right in between the previous two, at 70 km/s per megaparsec, researchers reported online in Nature. Resolving the impasse will require catching additional neutron star mergers.

Astrophysicists are confident they’ll get that chance. “This event is just the first of many that will be discovered in the future,” Loeb says. Additional mergers could also tell scientists more about the properties of neutron stars, like how “squishy” their extremely dense matter is, a property known as the equation of state (SN: 12/23/17, p. 7). And pinning down how often such collisions occur will help determine whether neutron stars can explain the abundances of heavy elements observed in the cosmos.

Gravitational waves also topped Science News’ list of discoveries in 2016. That honor marked the first direct detection of gravitational waves (SN: 12/24/16, p. 17), which the LIGO team captured in the aftermath of a merger of two black holes. This year, three pioneers of LIGO, Rainer Weiss of MIT and Kip Thorne and Barry Barish, both of Caltech, received the Nobel Prize in physics for that discovery (SN: 10/28/17, p. 6).

With the neutron star collision, gravitational wave hunters continue their streak of successes. The sighting came just days after a detection of two merging black holes — LIGO’s fourth and the first made in conjunction with Virgo (SN: 10/28/17, p. 8). A fifth black hole merger was reported in November (SN Online: 11/16/17). LIGO and Virgo are now shut off for upgrades until the fall of 2018. Additional gravitational wave detectors, like KAGRA in Japan and LIGO-India, are planned for the future, filling out a global network for monitoring the heavens’ temblors.

Both the fourth black hole collision and the neutron star crash appeared during a short window of less than a month when all three existing gravitational wave detectors were simultaneously operational. That was a stroke of amazing luck: Three detectors are needed to narrow down the location of a neutron star collision and pinpoint its light.

“It’s a little unreasonable how lucky we are,” Holz says. “It really was this gift — just a gift.”


B.P. Abbott et al. GW170817: Observation of gravitational waves from a binary neutron star inspiralPhysical Review Letters. Published online October 16, 2017. doi: 10.1103/PhysRevLett.119.161101.

B.P. Abbott et al. Multi-messenger observations of a binary neutron star mergerAstrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa91c9.

I. Arcavi et al. Optical follow-up of gravitational-wave events with Las Cumbres ObservatoryAstrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa910f.

J. Hjorth et al. The distance to NGC 4993: The host galaxy of the gravitational-wave event GW170817Astrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa9110.

M.C. Diaz et al. Observations of the first electromagnetic counterpart to a gravitational-wave source by the TOROS collaborationAstrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa9060.

B.P. Abbott et al. Gravitational waves and gamma-rays from a binary neutron star merger: GW170817 and GRB 170817AAstrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa920c.

E. Troja et al. The X-ray counterpart to the gravitational-wave event GW170817. Nature. Published online October 16, 2017. doi: 10.1038/nature24290.

I. Arcavi et al. Optical emission from a kilonova following a gravitational-wave-detected neutron-star mergerNature. Published online October 16, 2017. doi: 10.1038/nature24291.

The LIGO Scientific Collaboration and The Virgo Collaboration, The 1M2H Collaboration, The Dark Energy Camera GW-EM Collaboration and the DES Collaboration, The DLT40 Collaboration, The Las Cumbres Observatory Collaboration, The VINROUGE Collaboration & The MASTER Collaboration. A gravitational-wave standard siren measurement of the Hubble constantNature. Published online October 16, 2017. doi: 10.1038/nature24471.

D. Kasen et al. Origin of the heavy elements in binary neutron star mergers from a gravitational wave eventNature. Published online October 16, 2017. doi:1038/nature24453.

S.J. Smartt et al. A kilonova as the electromagnetic counterpart to a gravitational wave sourceNature. Published online October 16, 2017. doi: 10.1038/nature24303.

E. Pian et al. Spectroscopic identification of r-process nucleosynthesis in a double neutron-star mergerNature. Published online October 16, 2017. doi: 10.1038/nature24298.

S. Covino et al. The unpolarized macronova associated with the gravitational wave event GW 170817Nature Astronomy. Published online October 16, 2017. doi:10.1038/s41550-017-0285-z.

M.M. Kasliwal et al. Illuminating gravitational waves: A concordant picture of photons from a neutron star mergerScience. Published online October 16, 2017. doi: 10.1126/science.aap9455.

P.A. Evans et al. Swift and NuSTAR observations of GW170817: Detection of a blue kilonovaScience. Published online October 16, 2017. doi: 10.1126/science.aap9580.

G. Hallinan et al. A radio counterpart to a neutron star mergerScience. Published online October 16, 2017. doi: 10.1126/science.aap9855.

D.A. Coulter et al. Swope Supernova Survey 2017a (SSS17a), the optical counterpart to a gravitational wave sourceScience. Published online October 16, 2017. doi: 10.1126/science.aap9811.

M.R. Drout et al. Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesisScience Published online October 16, 2017. doi: 10.1126/science.aaq0049.

B.J. Shappee et al. Early spectra of the gravitational wave source GW170817: Evolution of a neutron star mergerScience. Published online October 16, 2017. doi: 10.1126/science.aaq0186.

C.D. Kilpatrick et al. Electromagnetic evidence that SSS17a is the result of a binary neutron star mergerScience. Published online October 16, 2017. doi: 10.1126/science.aaq0073.

P.S. Cowperthwaite et al. The electromagnetic counterpart of the binary neutron star merger LIGO/Virgo GW170817. II. UV, optical, and near-infrared light curves and comparison to kilonova models. Astrophysical Journal Letters. Vol. 848, October 20, 2017, p. L17. doi: 10.3847/2041-8213/aa8fc7.

R. Chornock et al. The electromagnetic counterpart of the binary neutron star merger LIGO/Virgo GW170817. IV. Detection of near-infrared signatures of r-process nucleosynthesis with Gemini-South. Astrophysical Journal Letters. Vol. 848, October 20, 2017, p. L19. doi: 10.3847/2041-8213/aa905c.

R. Margutti et al. The electromagnetic counterpart of the binary neutron star merger LIGO/Virgo GW170817. V. Rising X-ray emission from an off-axis jet. Astrophysical Journal Letters. Vol. 848, October 20, 2017, p. L20. doi: 10.3847/2041-8213/aa9057.

Further Reading

L. Grossman. What detecting gravitational waves means for the expansion of the universe. Science News. Vol. 192, November 25, 2017, p. 10.

E. Conover. Neutron star collision showers the universe with a wealth of discoveries. Science News. Vol. 192, November 11, 2017, p. 6.

E. Conover. Trio of detectors tracks gravitational waves to their home. Science News. Vol. 192, October 28, 2017, p. 6.

E. Conover. Rumors swirl that LIGO snagged gravitational waves from a neutron star collision. Science News Online, August 25, 2017.

E. Conover. Year in review: Gravitational waves offer new cosmic views. Science News. Vol. 190, December 24, 2016, p. 17.

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