Binary black hole
Two black holes in close orbit, confirmed by gravitational waves.
A binary black hole (BBH) is a system of two black holes orbiting each other closely. Like individual black holes, these binaries come in two main types: stellar-mass, which are the remnants of massive binary star systems or form through dynamic interactions and capture, and supermassive, which are thought to result from galaxy mergers. Stellar-mass binary black holes were directly confirmed in September 2015 through gravitational wave detection. Supermassive binary black hole candidates have been suggested based on indirect evidence, but they have not yet been directly observed.
For many years, proving black holes existed was difficult because they emit no light. However, Einstein’s theory predicted that merging black holes would release immense energy as gravitational waves, with distinct patterns calculable from general relativity. This made binary black holes a key scientific target in the late 20th and early 21st centuries, as they offered a promising way to detect gravitational waves themselves. Mergers of binary black holes are among the universe’s most powerful gravitational wave sources. As the black holes orbit, they emit gravitational waves, causing their orbit to decay and their orbital period to shorten. This slow inspiral continues until the black holes are close enough to merge. In the final fraction of a second, they reach extreme speeds, and the gravitational wave signal peaks. After merging, the single black hole settles into a stable shape, a stage called ringdown, where any remaining distortions radiate away as more gravitational waves.
Stellar-mass binary black holes were finally confirmed when LIGO detected GW150914 in September 2015 (announced February 2016). This signal came from two merging black holes, each about 30 times the Sun’s mass, located roughly 1.3 billion light-years away. In the last 20 milliseconds of their spiral and merger, GW150914 released about three solar masses as gravitational energy, peaking at a power of 3.6×10⁴⁹ watts—more than all the light from every star in the observable universe combined.
Stellar-mass binary black holes have been proven to exist through the detection of GW150914. Supermassive black hole binaries are thought to form when galaxies merge. Some candidate systems include galaxies with double cores that are still far apart, such as NGC 6240.
- First detected
- September 2015
- First event name
- GW150914
- Masses of first event
- around 30 solar masses each
- Distance of first event
- about 1.3 billion light-years away
- Energy released in final 20ms
- around 3 solar masses as gravitational energy
- Peak power of first event
- 3.6×10^49 W
- Orbital period of pks 1302-102
- 1900 days
Lore & Background
For many years, proving the existence of black holes was challenging because they allow no visible light or other electromagnetic radiation to escape. However, it was known from Einstein that if a pair of black holes were to merge, an immense amount of energy would be given off as gravitational wave radiation, with distinctive waveforms that could be calculated using general relativity. During the late 20th and early 21st century, binary black holes became of great interest scientifically as a potential source of gravitational waves—beyond just proving such waves themselves to exist. Binary black hole mergers would be one of the strongest known sources of gravitational waves in the universe, and thus offered a good chance of directly detecting such waves.
The existence of stellar-mass binary black holes (and gravitational waves themselves) was finally confirmed when the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected GW150914, a distinctive gravitational wave signature of two merging stellar-mass black holes of around 30 solar masses each, occurring about 1.3 billion light-years away. In its final 20 ms of spiraling inward and merging, GW150914 released around 3 solar masses as gravitational energy, peaking at a rate of 3.6×10^49 W—more than the combined power of all light radiated by all the stars in the observable universe put together. Supermassive black-hole binaries are believed to form during galaxy mergers, with candidates including NGC 6240, SDSS J104807.74+005543.5, EGSD2 J142033.66 525917.5, OJ287, J0437+2456, and PKS 1302-102.
Reader's Guide
The significance of binary black holes lies in their role as a key source of gravitational waves and as a test of general relativity. The first observation of stellar-mass binary black holes merging, GW150914, was performed by the LIGO detector. As observed from Earth, a pair of black holes with estimated masses around 36 and 29 times that of the Sun spun into each other and merged to form an approximately 62-solar-mass black hole on 14 September 2015, at 09:50 UTC. Three solar masses were converted to gravitational radiation in the final fraction of a second, with a peak power 3.6×10^49 W (200 solar masses per second), which is 50 times the total output power of all the stars in the observable universe. The merger took place 440+160−180 megaparsecs from Earth, between 600 million and 1.8 billion years ago. The observed signal is consistent with the predictions of numerical relativity simulations. The lifecycle of a binary black hole includes the inspiral stage, where the orbit gradually shrinks due to gravitational wave emission and interactions with matter; the merger, where gravitational wave emission peaks; and the ringdown, where the newly formed black hole relaxes to a stable form, emitting damped oscillations that confirm it is a Kerr black hole as predicted by general relativity. The final parsec problem remains an open question for supermassive binaries, with proposed solutions involving additional matter, circumbinary accretion, dark matter, or a third supermassive black hole.
Did You Know?
- The first confirmed stellar-mass binary black hole merger, GW150914, was detected in September 2015 and announced in February 2016.
- Supermassive binary black hole candidates include the quasar PKS 1302-102, which has an orbital period of 1900 days.
- The final parsec problem describes the difficulty of supermassive black holes merging after dynamical friction brings them within a few parsecs, as gravitational waves only become effective at much smaller separations.
More in Binary and Multiple Stars 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced