About 900 million years ago, a black hole probably swallowed up a dead star. Last week, the resulting ripples in space and time were finally detected on Earth.

If these scientific observations are confirmed, the cosmic collision would be the first example of a black hole colliding with a neutron star, possibly offering new insights into the expansion of the universe.

In April, gravitational wave observatories in the U.S. and Europe reignited a search for extreme cosmic events. Astronomers have since detected 23 potential events, but the latest one would be the first of its kind.

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The event, referred to as S190814bv, was detected Wednesday by two LIGO detectors in the U.S. (Laser Interferometer Gravitational-Wave Observatory) and the Virgo detector in Italy. Scientists observed ripples in space-time, suggesting they could have occurred due to a black hole swallowing a neutron star.

Latest @LIGO @ego_virgo update on #S190814bv labels it "NSBH". This means that, if confirmed, our candidate event *could* be an exciting new milestone: the first ever detection of a #NeutronStar #BlackHole merger. But, as always in science, we must be cautious... 1/5 pic.twitter.com/eWMjquMvUg — LIGO (@LIGO) August 17, 2019

A neutron star is created following a supernova explosion and is extremely small and dense, consisting mostly of tightly-packed neutrons. An average neutron star has about 1.5 times the mass of the sun, but a radius of only between 6 and 12 miles.

In the past few months, three other similar events have been detected, but it's possible they were "noise" rather than real events. In the case of S190814bv, scientists said the chance the signal is a false alarm is one-in-trillions-of-years.

While this is the most confident scientists have been, they are still urging caution until the event is confirmed. For example, the collision could have been two merging black holes — still an exciting discovery of a black hole lighter than any seen before.

"There is the slight but intriguing possibility that the swallowed object was a very light black hole — much lighter than any other black hole we know about in the Universe," said Professor Susan Scott, leader of the General Relativity Theory and Data Analysis Group at Australian National University and chief investigator with the ARC Centre of Excellence for Gravitational Wave Discovery. "That would be a truly awesome consolation prize."

An artist's depiction of a black hole swallowing a neutron star. Carl Knox/OzGrav ARC Centre of Excellence

Astronomers are now working to confirm the size of the two objects that crashed together to form the cosmic ripples. They are also scanning the area with telescopes where they believe the event occurred, searching for light that may have radiated by the merger.

LIGO first detected gravitational waves in 2015, a century after Albert Einstein first predicted their existence with his general theory of relativity.

LIGO and Virgo have since detected black holes merging together and neutron stars merging together, but never an interaction between the two. Confirmation of the collision would complete a trifecta of cosmic detections.

"We will either see a neutron star being ripped apart by a black hole, or getting swallowed whole like Pac-Man swallowing a ghost," said Simon Stevenson, an astronomer with Swinburne University of Technology in Australia. "Either way, we are in for a show!"