Astronomers announced today that they had discovered something new out in the dark: a stellar corpse too heavy to be a neutron star — the remnant of a supernova explosion — but not heavy enough to be a black hole.

Whatever it once was, it is long gone. About 780 million years ago — and 780 light-years away — it was eaten by a black hole 23 times more massive than the sun. That feast left behind an even heavier black hole — a vast, hungry nothing with the mass of 25 suns.

News of that event only recently reached Earth, in the form of space-time ripples known as gravitational waves. These evanescent vibrations were felt on Aug. 14, 2019, by an array of antennas in Italy and the United States called the International LIGO-Virgo Collaboration, and the results were published on Tuesday in Astrophysical Journal Letters.

According to a theory that has been the backbone of decades of astrophysical excitement, a star can wind up in one of three final states, depending on its mass: a perpetually cooling cinder known as a white dwarf; a dense star, with the mass of a couple of suns compressed into a ball only 12 or so miles wide, known as a neutron star; or a black hole, a beast reluctantly predicted by Albert Einstein to be so dense that nothing, not even light, can escape its gravity.

The victim in this collision weighed in at 2.6 solar masses, according to the LIGO-Virgo calculations. That is heavier than the accepted limit of 2.5 suns for a neutron star. But the lightest black hole ever measured was about five solar masses.

So the mystery object lies squarely in what astrophysicists call the “mass gap.” Astronomers have long wondered what, if anything, could occupy this astronomical no-man’s land.

“We’ve been waiting decades to solve this mystery” Vicky Kalogera of Northwestern University, one of the main authors of the paper, said in an interview. “We don’t know if this object is the heaviest known neutron star or the lightest known black hole, but either way it breaks a record.”

She added: “If it’s a neutron star, it’s an exciting neutron star. If it’s a black hole, it’s an exciting black hole.”

In a statement issued by the Science and Technology Facilities Council in Britain, Charlie Hoy, a graduate student at Cardiff University and Dr. Kalogera’s co-author, said, “I did not believe the alert when I first saw it come through.”