Second Collision of Neutron Stars Detected

The Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo collaborations have detected the collision of the second pair of neutron stars.

This is the first major event of LIGO detector’s third observing run. This is the second observing run to be conducted with the company of the Virgo detector in Italy.

On April 25, 2019, the LIGO Livingston Observatory picked up what appeared to be gravitational ripples from a collision of two neutron stars. LIGO Livingston is part of a gravitational-wave network that includes LIGO (the Laser Interferometer Gravitational-wave Observatory), funded by the National Science Foundation (NSF), and the European Virgo detector. Now, a new study confirms that this event was indeed likely the result of a merger of two neutron stars. This would be only the second time this type of event has ever been observed in gravitational wave.

Neutron Stars that Collided Were Larger Than Previously Known Neutron Stars

The first such observation, which took place in August of 2017, made history for being the first time that both gravitational waves and light were detected from the same cosmic event. The April 25 merger, by contrast, did not result in any light being detected. However, through an analysis of the gravitational-wave data alone, researchers have learned that the collision produced an object with an unusually high mass.

“From conventional observations with light, we already knew of 17 binary neutron star systems in our own galaxy and we have estimated the masses of these stars,” says Ben Farr, a LIGO team member based at the University of Oregon. “What’s surprising is that the combined mass of this binary is much higher than what was expected.”

The source of GW190425 is estimated to be at a distance of 500 million light years from the Earth. It is localized in the sky within an area about 300 times broader than was the case for the BNS observed by LIGO and Virgo in 2017, the famous GW170817, which gave birth to multi-messenger astrophysics. However, unlike GW170817, no counterpart (electromagnetic signals, neutrinos or charged particles) has been found to date.

There are a few explanations for the origin of GW190425. The most likely is the merger of a BNS system. Alternatively, it might have been produced by the merger of a system with a black hole (BH) as one or both components, even if light BHs in the mass-range consistent with GW190425 have not been observed. Yet, on the basis solely of GW data, these exotic scenarios cannot be ruled out. The estimated total mass of the compact binary is 3.4 times the mass of the Sun. Under the hypothesis that GW190425 originated from the merger of a BNS system, the latter would have been considerably different to all known BNS in our galaxy, the total mass range of which is between 2.5 and 2.9 times the mass of the Sun. This indicates that the NS system that originated GW190425 may have formed differently than known galactic BNSs.

Although predicted theoretically, heavy binary systems like those that might have originated GW190425 may be invisible through electromagnetic observations.”

“While we did not observe the object formed by the coalescence, our computer simulations based on general relativity predict that the probability that a BH is formed promptly after the merger is high, about 96%”, says Sebastiano Bernuzzi of the University of Jena, Germany.

43 Gravitational Wave Detectioin Events in Third Run

This observing run started in April 1, 2019 and there have been 43 unretracted alerts of gravitational-wave events in this run. Theere were 10 mergers announced in the catalog of the first two observing runs. This third observing run will continue until April 30, 2020, and a full accounting of the events seen in the first half of the year of observations are expected to come out around April.

The study, submitted to The Astrophysical Journal Letters, is authored by an international team comprised of the LIGO Scientific Collaboration and the Virgo Collaboration, the latter of which is associated with the Virgo gravitational-wave detector in Italy. The results were presented at a press briefing today, January 6, at the 235th meeting of the American Astronomical Society in Honolulu, Hawaii.

Details of the Event

The April 2019 event was first identified in data from the LIGO Livingston detector alone. The LIGO Hanford detector was temporarily offline at the time, and, at a distance of more than 500 million light-years, the event was too faint to be visible in Virgo’s data. Using the Livingston data, combined with information derived from Virgo’s data, the team narrowed the location of the event to a patch of sky more than 8,200 square degrees in size, or about 20 percent of the sky. For comparison, the August 2017 event was narrowed to a region of just 16 square degrees, or 0.04 percent of the sky.

“This is our first published event for a single-observatory detection,” says Caltech’s Anamaria Effler, a scientist who works at LIGO Livingston. “But Virgo made a valuable contribution. We used information about its non-detection to tell us roughly where the signal must have originated from.”

The LIGO data reveal that the combined mass of the merged bodies is about 3.4 times the mass of our sun. In our galaxy, known binary neutron star systems have combined masses up to only 2.9 times that of sun. One possibility for the unusually high mass is that the collision took place not between two neutron stars, but a neutron star and a black hole, since black holes are heavier than neutron stars. But if this were the case, the black hole would have to be exceptionally small for its class. Instead, the scientists believe it is much more likely that LIGO witnessed a shattering of two neutron stars.

14 thoughts on “Second Collision of Neutron Stars Detected”

  1. If neutron star collisions were rare, and were necessary for actinide production, and if radioactive heat is necessary for planets to stay lifebearing, that could be an answer to Fermi’s ‘ Where is everybody ?’ conundrum.

  2. It’s still a very exciting area with a lot of unknowns but:

     From our GW data on GW170817, we get constraints on both the ejecta mass and the NS merger rate, which can then be used to estimate the NS merger contribution to r-process abundances. Assuming that all NS mergers have properties like those inferred from GW170817, we find that if ≳10% of the dynamically ejected mass is converted to r-process elements, NS mergers could account for all the observed r-process abundances. Future observations of GWs from NS mergers will help solve the longstanding mystery of where the majority of r-process elements are created.

    The best estimate I could come to from what I dug up was that we’ve quite probably had somewhere between 10,000 and 100,000 N-N mergers since the Milky Way first formed, with current thought mostly skewed towards the smaller end of the scale.
    This means a lot of dwarf galaxies may have NEVER experienced even a single neutron star collision. Significant because we need radioactives to keep the Earth’s core molten and its surface habitable, and also because iodine, essential to life, may be one of the elements nearly exclusive to N-N merger events.
    Sure hope no one ever travels a hundred thousand light years to settle in one, only to discover part of the table of elements is completely absent, not unless they are true alchemists, at any rate.

  3. Yah, OK. 

    But consider the astrodynamics of a N-N star collision. They appear at least with today’s LIGO evidence, to be rather remarkably rare. We’ve got a sphere perhaps out to [R = 1 G parsec ≈ 3,260,000,000 LY] radius, which in turn is about 1.45×10²⁹ LY³. While I’m not armed with just how many LARGE stars there are, or have been per 1,000,000 LY³, one kind of has to assume the answer is small. A sphere of radius 62 LY.  

    Anyway, some Google searching uncovers that at least in galaxies, and away from the center some, the stellar density is about 0.34 star/parsec³.  Or so. This would be stars of most-every type. From Wikipedia, I found that the fraction f stars that are type [O, B, A and heavy F] … likely to eventually neutron-collapse, or supernova, or whatever is about 1 in 150 stars are in this set. 


    We’ve got the problem that “stars today ≠ stars in the last 13 billion years” problem, which I also can’t answer mathematically well. But its fair to say that a lot of supernovæ happened since the Big Bang.

    Still, when 2 N-N stars collide, its not like they eject very much! A bit, but not much. Mostly grav waves.

    The supernovæ remain my statistical bet. WAY more common, being perhaps one-in–1000 stars in general. We’ve got gazillions of stars, and have had a thousandth that of supernovæ.  

    ⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  4. Heh, while it may or may not be true, calling it far fetched may be a little far fetched.

    Light elements like hydrogen and helium formed during the big bang, and those up to iron are made by fusion in the cores of stars. Some heavier elements like gallium and bromine need something more, such as a supernova. Others—such as gold and uranium, which are the most neutron-rich—require a process called rapid neutron capture. Here, an atomic nucleus is bombarded with neutrons so it swells to an unstable size, but the whole thing happens so fast the element doesn’t have time to split apart.

    Scientists have long suspected that neutron stars, the superdense remnants of burned out suns, are needed for this sort of rapid neutron capture. But until 2 years ago, they had never witnessed such an event. That’s when the GW170817 merger happened. Taking place 140 million light-years away (and imagined above, with strontium in yellow), astronomers first detected it from the gravitational waves generated by the stars crashing together.

    Also which strongly suggests that supernovas can be ruled out as the source of certain heavy elements.

  5. Yah… is the short answer. 


    … ( 1 / exp( –ln(½) × 4.6 Gy ÷ 1.25 Gy-HL ))
    … ⇒ 10.5× 

    And that’d be how much MORE ⁴⁰K was concentrated in the crust after the first few hundred million years. Before most of it decayed. 

    I also made a mistake in the equation for potassium, above.  Its decay rate is ‘only’ 30,000 fairly low energy disintegrations per second per kilogram of lithophillic rock, not 3.9 million. 

    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  6. So if the earliest evidence of life was 4.28 billion years back, what kind of background radiation would the first bugs have had to put up with ? I’m guessing radon from U238 would be double, that from U235 wouldn’t matter because it would decay straight away, but 40K would really be bouncing their DNA around.

  7. Now for the math bit.

    M♁ = 5.97×10²⁴ kg
    A♁ = 5.09×10¹⁴ m²

    ²³⁸U… HL = 4.5×10⁹ a; MeV = 51.5; ppm = 1.1; P = 1.12×10⁻¹⁰ W/kg
    ²³⁵U… HL = 0.7×10⁹ a; MeV = 50.7; ppm = 0.02; P = 1.30×10⁻¹¹ W/kg
    ²³²Th… HL = 14.1×10⁹ a; MeV = 49.3; ppm = 4.50; P = 1.44×10⁻¹⁰ W/kg

    M♁ • ∑( siderophiles, U, Th ) = 1.6×10¹⁵ W, at present;
    1.6×10¹⁵ W / A♁ = 3.15 W/m²


    Since — insofar as the last billion-odd years of geologic evidence shows — the Earth’s interior doesn’t seem to be increasing in temperature, then something on the order of 1 to 5 watts/m² of Earth’s interior heat must be dissipated thru, to the surface, continuously.  

    How about that. 

    Gotta love the Math of Big Numbers. 
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

    PS: instantaneous rate = (1000 / g/mol) × 6.022×10²³ • ( ln( ½ ) / ( HL × 31.558×10⁶ sec/a ) ) • ppm/10⁶

    ²³⁸U = 13.6 decay/sec
    ²³⁵U = 1.61 …
    ²³²Th = 18.2 …

    Since just-about-all ⁴⁰K is crustal exclusively (“lithophilic”, opposite of siderophile), although its decay rate is prodigious (3,900,000 decay/sec per kg of crust!), the crust being such a small mass, hardly contributes at all. 

    Just saying,
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  8. Yep… One really can ignore the after-primary-decay chains of both Uranium and Thorium, whether ²³⁵U, ²³⁸U or ²³³Th. No, not from an energy perspective, but from the fact that each of the decay chain’s isotopes half-lives are so short compared to the primary parent species, that they’re always essentially “in saturation equilibrium”, with the amounts of each type in equilibrium with the parent species decay rate.  

    Still, the energies involved are substantial — especially when adding up the whole decay-to-lead chain for U and Th, ⁴⁰K decays either to ⁴⁰Ca or ⁴⁰Ar, both of which are stable. So… end of chain at 1.3 to 1.5 MeV per, with a 50% decay probability every 1.25 billion years for each atom.

    Uranium … 51.7 MeV/nucleon (whole chain).  
    ²³⁸U, half life of 4.5 billion years.
    ²³⁵U, half life of 0.7 billion years.

    Thorium, … 48 MeV/nucleon, 
    ²³²Th, half life of 14.1 billion years.

    Which is to say, given The Solar system is supposedly some 4.6 billion years young, that 50% of the ²³⁸U has decayed already, that 98.9% of the ²³⁵U has decayed, and only 20.2% of the ²³²Th has decayed.  

    The ORIGINAL abundances … well … can be worked out ‘backwards’, but show definite ‘supernovæ prefer…’ spectra.

    Just saying,
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  9. Well… mebbe, and mebbe not. 

    In particular, the idea of the heavies of proto-Earth (as reflected in present-Earth’s element abundances) having required a neutron-star collision, is fairly far fetched. Any-old-supernova would do. The astronomical (ahem) energies involved with a stellar core-collapse and rebound are plenty sufficient to transmute a substantial fraction of the ‘stuff’ to elements having a Z greater than iron. As in 10% or higher.  In a few minutes.

    The out-flung blast of gaseous bits in turn is moving initially at some 1% to 7% the speed of light. The gravitational well decelerates it quite a bit, so by the time 1000 years has elapsed, the outmost (still-fastest) shock front is zipping along at 0.5% c.  

    Think (and Google/Wikipedia look up) The Crab Nebula. Lots of good numbers and figures there, it being relatively close (6500±1200 Ly distant), and having a reasonably precise local calendar time of its self-immolation. 


    For the expanding SN ejecta to push against, compress, and thus catalyze the gravitational consolidation of local molecular-cloud material into something like the Solar System probably takes on the order of 25,000 to 1,000,000 years. And all that heavy-metal stuff is decaying, but the most-stable bits remain. For to start the Earth consolidation, say. 

    Its just a matter of big enough numbers. Time in kiloyears, distance in light-years, velocity in percents of ‘c’, masses in fractions of one M☉ Sol

    Just saying,
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  10. It is estimated that about 90% of the heat in the Earth’s interior comes from the continuing decay of various isotopes like Potassium 40, Uranium 238, 235, and Thorium 232. The Earth’s core would have long since solidified were it not for this.

    Nearly all of the rest (as in 10%) is heat left over from initial formation (and from the Moon’s impact creation event), and from gravitational heat (caused by internal friction that has not yet dissipated), and latent heat (which is caused by the core expanding as it cools).

    Heat contribution from the tidal force of the Moon is present but negligible.

    Io, being quite small in relation to Jupiter, as well as being subjected to an irregular elliptical orbit due to Europa and Ganymede, does have sufficient tidal energy to be kept molten. But the radiation (never mind the volcanoes) makes it unsuitable for life. Additionally, the electric force caused by its interaction with Jupiter’s magnetic field creates vast lightning storms on Jupiter and ionizes around 1 ton (907 kilograms) per second from the surface of Io. This creates a vast plasma torus, some of which is pulled into Jupiter, and also causes Jupiter’s magnetosphere to be over twice as large as it otherwise would be.

    In other words, Io pays a lethal price to keep its core molten through tidal forces.

    Dr. Manhattan might like it for a second home.

  11. Depends I suppose. In general yes tidal heating will produce heat longer than radioactive decay unless you are talking about a U-Th cycle which could last for a very long time.

  12. It’s fascinating stuff, not least of which because neutron star collisions with each other (or black holes) are probably needed for the rapid neutron capture that the truly heavy elements require for their creation.

    It is thought that a single neutron star collision event not long before the Solar system formed is responsible for most of our heavy elements, especially the radioactive ones, as these have a half-life, yet are needed to keep the core of our planet molten long enough for us to arrive on the scene.

    So that collision had to happen in a relatively tight timeframe, recent enough that the radioactives had not decayed too much, yet before our planet formed and the crust solidified.

    Which also raises a question (one among many). Do planets, even those that receive the timely infusion of radioactives, have a “shelf-life” in which they must develop life or have their cores cool too much to sustain it long enough for something like us to come along?

    It’s especially pertinent as some people have pointed to the long life (trillions of years) of red dwarf stars, by far the most common type, as meaning that their planets have that long in which to begin developing life. If they did have that long (and they probably don’t, for the aforementioned reason, radioactive decay) it might help their chances significantly because the arguments against life around a red dwarf, and it’s sustainment over a multi-billion year time period, are quite substantial.

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