Black holes are regions of spacetime that form when massive stars collapse at the end of their life cycles and can continue to grow by absorbing stars and merging with other black holes. This interaction allows scientists to identify their presence, as electromagnetic radiation is given off as visible light across space. In this way, astronomers have identified numerous stellar black hole candidates in binary systems and established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.
But 800 million light years away, a black hole devoured an unidentified object and the resulting cosmic merger released enough energy to wrinkle the fabric of spacetime.
These gravitational waves travelled through the universe and eventually over Earth on August 14, 2019.
Three detectors in the US sensitive enough to measure such minuscule action recorded the activity, but as scientists decoded the information they were left scratching their heads.
Vicky Kalogera of Northwestern University, who coordinated the report on the merger, said: “We haven’t seen, with confidence, anything like this before.”
Published in Astrophysical Journal Letters in June, researchers said the collision, called GW190814, stands out from the dozens of cosmic mergers detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO).
For millions or perhaps billions of years, the two objects orbited one another, spiralling closer and closer until they finally collided.
Astronomers determined that one of those objects was a black hole with as much mass as 23 Suns and the other mystery object was roughly 2.6 solar masses.
The unknown phenomenon has baffled scientists because its mass places it somewhere between being the heaviest known neutron star, or the lightest black hole.
Dr Kalogera added: “If it’s a neutron star, it’s an exciting mass for a neutron star. If it’s a black hole, it’s an exciting mass for a black hole.
“Either way, it perked up our antennae the moment we saw it.”
Observations suggest the heaviest neutron star is around 2.1 solar masses, but most are close to around 1.4, while the lightest black holes are about five solar masses.
The University of Arizona’s Feryal Ozel, who studies the boundaries on these objects said: “If it ends up being a neutron star – if a neutron star can be as massive as 2.6 solar masses – it is truly paradigm-changing.”
The two scientists both suspect the mystery object is a black hole, but note it will be difficult to prove either way.
Dr Ozel added: “I don’t think we have any chance of knowing what this object is.
“The telltale signs that it could have been a neutron star are simply not there – but their absence doesn’t mean anything, either.”
Even without knowing what the object is, the event is unique because they are so mismatched, shattering what astronomers thought they knew about how the cosmic phenomena behave.
Most collisions involve pairs that are relatively similar in mass, but at 23 solar masses, this black hole is roughly nine times heavier than its lighter partner.