When the most massive stars die, they collapse under their own gravity and leave behind black holes; when stars that are a bit less massive die, they explode in a supernova and leave behind dense, dead remnants of stars called neutron stars.

For decades, astronomers have been puzzled by a gap that lies between the mass of neutron stars and black holes: The heaviest known neutron star is no more than 2.5 times the mass of our sun – or 2.5 “solar masses” – and the lightest known black hole is about 5 solar masses.

The question remained: Does anything lie in this so-called “mass gap”?

Now, in a study from the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European Virgo detector, scientists have announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap. The object was found on Aug. 14, 2019, as it merged with a black hole of 23 solar masses, generating a splash of gravitational waves detected back on Earth by LIGO and Virgo.

A paper about the detection appears in The Astrophysical Journal Letters.

“This is going to change how scientists talk about neutron stars and black holes,” says UWM physics professor and co-author Patrick Brady, who also is the LIGO Scientific Collaboration spokesperson. “The mass gap may, in fact, not exist at all but may have been due to limitations in observational capabilities. Time and more observations will tell.”

The cosmic merger described in the study, an event dubbed GW190814, resulted in a final black hole that lies about 800 million light-years away from Earth and has about 25 times the mass of our sun. Before the two objects merged, their masses differed by a factor of 9, making this the most extreme mass ratio known for a gravitational-wave event.

“The mystery object may be a neutron star merging with a black hole, an exciting possibility expected theoretically but not yet confirmed observationally,” says Vicky Kalogera, a co-author and professor at Northwestern University.

LIGO and Virgo detected the gravitational waves created when some of the merged mass was converted to a blast of energy. When they spotted this merger, scientists immediately sent out an alert to the astronomical community. Dozens of ground- and space-based telescopes followed up in search of light waves generated in the event, but no signals were picked up.

So far, such light counterparts to gravitational-wave signals have been seen only once, in an event called GW170817. That event, discovered by the LIGO-Virgo network in 2017, involved a fiery collision between two neutron stars that was subsequently witnessed by dozens of telescopes on Earth and in space. But black hole mergers, in most circumstances, are thought not to produce light because they have such strong gravitational fields that no light or radiation can escape.

One reason that the August 2019 event was not visible could be that, if the object was a neutron star, its 9-fold more massive black-hole partner might have swallowed it whole, says Kalogera. A neutron star consumed whole by a black hole would not give off any light.

How will researchers ever know if the mystery object in the mass gap was a neutron star or black hole? Future observations with LIGO and possibly other telescopes may catch similar events that would help reveal whether additional objects exist in the mass gap.

LIGO is funded by the National Science Foundation and operated by Caltech and MIT, which conceived of LIGO and lead the project. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. About 1,300 scientists from around the world, including five faculty members at UWM, participate in the effort through the LIGO Scientific Collaboration.

The Virgo Collaboration is currently composed of about 520 members from 99 institutes in 11 different countries. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands.