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Back in 2015, humanity detected our first gravitational wave event .
GW150914 was the first ever direct detection and proof of the existence of gravitational waves. The waveform, detected by both LIGO observatories, Hanford and Livingston, matched the predictions of general relativity for a gravitational wave emanating from the inward spiral and merger of a pair of black holes of around 36 and 29 solar masses and the subsequent “ringdown” of the single resulting black hole.
Credit : Aurore Simonnet/LIGO Scientific Collaboration
Now, in 2026, we’re up to 390 confirmed detections .
This up-to-date “Eagle plot” of the black holes and neutron stars detected through gravitational wave mergers (orange and blue) and through electromagnetic signals (yellow and red) show the present status of known black holes and neutron stars under 250 solar masses. Just 11 years ago, there were no known gravitational wave events, but today, in June of 2026, there are 390 confirmed events, representing 780 pre-merger and 390 post-merger objects.
Credit : LIGO-Virgo-KAGRA / Aaron Geller / Northwestern
Most correspond to merging pairs of black holes, some at quite high masses.
This illustration shows the results of numerical simulations of the gravitational waves emitted by the inspiral and merger of two black holes. The colored contours around each black hole represent the amplitude of the gravitational radiation; the blue lines represent the orbits of the black holes and the green arrows represent their spins. Even though the two objects orbit one another in the plane, gravitational waves propagate outward in three-dimensions, and the gravitational force falls off as ~1/r².
Credit : C. Henze/NASA Ames Research Center
This poses a theoretical challenge, because stars shouldn’t form such heavy black holes.
This close-up of the stars in Westerlund 1 showcases JWST’s resolution and sensitivity to cool, obscured stars. The wispy red material represents dusty material surrounding the star cluster that hasn’t been fully boiled or evaporated away, while the cluster itself contains between 50,000 and 100,000 solar masses worth of material, along with thousands upon thousands of new stars of all different masses.
Credit : ESA/Webb, NASA & CSA, M. Zamani (ESA/Webb), M. G. Guarcello (INAF-OAPA) and the EWOCS team
When stars form, they come in a variety of masses: following an initial mass function.
When stars are first born, they are sorted into seven different spectral classes: with M-class stars representing the smallest, lowest-mass, reddest, coolest, and longest-lived stars, and O-class stars representing the biggest, most massive, bluest, hottest, and shortest-lived stars. The highest mass stars burn through their fuel the fastest, and will be the first to die in cataclysmic, stellar-life-ending events.
Credit : LucasVB/Wikimedia Commons; Annotations: E. Siegel
Based on that initial mass, they’ll eventually die.
Supernovae types as a function of initial star mass and initial content of elements heavier than Helium (metallicity). Note that the first stars occupy the bottom row of the chart, being metal-free, and that the black areas correspond to direct collapse black holes. For modern stars, we are uncertain as to whether the supernovae that create neutron stars are fundamentally the same or different than the ones that create black holes, and whether there is a ‘mass gap’ present between them in nature. We must also consider that effects other than mass and metallicity (such as the presence of a companion) may indeed play major roles in determining the fate of massive stars, including in whether they can contribute to enriching the interstellar medium.
Credit : Fulvio314 / Wikimedia Commons
The most massive stars can undergo core-collapse,
In the inner regions of a star that undergoes a core-collapse supernova, a neutron star begins to form in the core, while the outer layers crash against it and undergo their own runaway fusion reactions. Neutrons, neutrinos, radiation, and extraordinary amounts of energy are produced, with neutrinos and antineutrinos carrying the majority of the core-collapse supernova’s energy away. Whether the remnant becomes a neutron star or black hole, ultimately, depends on how much mass remains in the core during this process.
Credit : TeraScale Supernova Initiative/Oak Ridge National Lab
detonate in a pair-instability event,
This diagram illustrates the pair production process that astronomers once thought triggered the hypernova event known as SN 2006gy. At core temperatures cresting past 300,000,000 K, high-enough-energy photons are produced, which create electron/positron pairs, which can then cause a pressure drop and a runaway reaction that destroys the star. This event is known as a pair-instability supernova. Peak luminosities of a hypernova, also known as a superluminous supernova, are many times greater than that of any other, ‘normal’ supernova.
Credit : NASA/CXC/M. Weiss
or directly collapse to form a black hole.
The visible/near-IR photos from Hubble show a massive star, at least 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time. The direct collapse of this particular object, while still under investigation, may have been triggered by a stellar companion.
Credit : NASA/ESA/C. Kochanek (OSU)
Then, black hole mergers arise from that population.
When two black holes inspiral and merge, a significant portion of their mass can get converted into energy in one very short time interval. The gravitational radiation that these black holes emit doesn’t propagate across the Universe instantaneously, but rather is limited by the finite speed of propagation: the speed of light.
Credit : NASA’s Goddard Space Flight Center
Our detectors then see them with a distance sensitivity based on mass.
Advanced LIGO’s range for black hole-black hole mergers (purple) is far, far greater than its range for neutron star-neutron star mergers (yellow), owing to the mass dependence of the signal amplitude. A difference by a factor of ~10 in range corresponds to a difference of a factor of ~1000 for volume, so that even though the number density of low-mass black holes far outstrips the higher-mass ones, LIGO and Virgo are more sensitive out to greater distances and can probe larger volumes for higher-mass systems and events.
Credit : LIGO Scientific Collaboration/Beverly Berger, NSF
This scenario can’t explain the abundance of high-mass black hole mergers.
Unlike the more common “Eagle plot” of gravitational wave events, this format showcases instead the population of post-merger black holes, sorted by mass, as detected in gravitational waves. On the left, in orange, neutron stars are indicated, while as we move to the right, blue colors appear, indicating black holes. The least massive post-merger remnant discovered in this fashion is under 3 solar masses, while the most massive one seen so far exceeds 200 solar masses.
Credit : LIGO-Virgo-KAGRA / Aaron Geller / Northwestern
However, hierarchical mergers can help.
At left, an isolated system, formed from two original members of a binary star system, tends to produce black holes aligned with the orbital motion of the original progenitor stars, leading to black hole mergers with aligned spins. However, for dynamically captured black holes, or for black holes arising from trinary (or richer) systems, those rules and tidal effects don’t apply as strongly, producing a closer to 50-50 split of aligned versus anti-aligned spins.
Credit : Shanika Galaudage
After first-generation black holes merge, their remnants can merge again .
This graph shows the differential merger rate (y-axis) as a function of the mass of the heavier black hole (x-axis) involved in the merger of two black holes as seen in gravitational wave events. The orange and blue shaded areas indicate the confidence intervals of two different models, both of which show peaks at 10 and 35 solar masses, and one of which shows an additional peak at around 20 solar masses. This suggests multiple populations and pathways for black hole mergers.
Credit : LIGO/Virgo/KAGRA scientific collaborations, arXiv:2508.18083, 2025
A clue should arise in their spin-alignment with their orbits.
A simulation of the black hole merger GW200129, showing the precession of the orbital plane and the expected gravitational signal. After merging, the final black hole is kicked in the downward direction. These two approximately 30 solar mass black holes merged to form approximately a 60 solar mass black hole: a candidate for a second-generation (or later) hierarchical merger, rather than a first-generation one.
( Credit : V. Varma/Max Planck Institute for Gravitational Physics)
Two independent studies found large spin misalignments in high mass mergers.
These two graphs show five different analyses of the gravitational wave black hole merger data, sorting them by the inspiral spin alignment with the orbital plane (x-axis) as a function of likelihood/probability density (y-axis). In the top graph, first-generation mergers are shown: narrowly peaked around zero. In the bottom scenario, the hierarchical (second-or-later generation) merger scenario is shown, with a random and much broader distribution of spin alignments. The transition appears to occur between 40-45 solar masses.
Credit : S. J. Miller/Caltech
Only low masses are dominated by first-generation mergers.
This five-panel animation shows the evolution of black holes and neutron stars detected from gravitational wave events. The first, sparsest panel shows only the results from O1 and O2, the second and third panels add in the results from the first and second half of the O3 run (O3A and O3B), while the fourth and fifth panels add in the most recent results from O4A and O4B, with O4C still to come. All told, the number of confirmed detection events has now risen to 390, as of June 2026.
Credit : LIGO-Virgo-KAGRA / Aaron Geller / Northwestern
At higher masses, especially above 40-45 solar masses, later-generation mergers occur.
In dense regions of space, such as at the cores of massive star clusters or globular clusters, a large number of initially high mass stars will die, giving rise to a wide variety of black holes. While some of those black holes will form in binary systems and will eventually inspiral and merge together, many heavier, post-at-least-one-merger black hole systems will gravitationally interact with, bind to, and eventually merge with other black holes: a realization of the hierarchical scenario.
Credit : ESA/Hubble, N. Bartmann
This “hierarchical” scenario has long been suspected , but is now supported by the best evidence ever.
These time-frequency spectrographs show how the signal evolves over time (x-axis) and frequency (y-axis) for each merger event thus far. The brightest spots represent the strongest signal strengths, while the “uptick” at the end of each merger corresponds to the chirp that gets made during the final orbits prior to the merger in question. Note how significantly the event rate grew from O2 (orange) to O3 (blue), and from O3 to O4 (in red and violet), showcasing a remarkable increase in scientific power as driven by the upgrades made to the apparatuses and the addition of new detectors.
Credit : R. Nowicki/B. Smith/K. Jani (LIGO-Virgo-KAGRA/Vanderbilt)
As measurements and statistics continually improve, so will our understanding.
This artist concept shows the layout of the LIGO India detector facility, showcasing the flat terrain, laser arms, and science complex at the center. All told, this 174-acre complex expects to begin science operations sometime in 2030, joining the other four active detectors. As the number of detectors, measurement quality of the detectors, and overall statistics continue to improve, so will our understanding of the gravitational wave universe.
Credit : LIGO India/LIGO Collaboration
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This article Big evidence for “second generation” black hole mergers is featured on Big Think .
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