Black holes are astronomical objects that have a gravity so strong that not even light can escape. Neutron stars are incredibly dense dead stars. It is estimated that a teaspoon of material from a neutron star weighs about 4 billion tons.
Both objects are cosmic monsters, but black holes are considerably more massive than neutron stars.
In the first collision, which was detected on January 5, 2020, a black hole that is six and a half times the mass of our Sun collided with a neutron star that was 1.5 times more massive than our star. In the second collision, detected just 10 days later, a black hole ten times the solar mass merged with a neutron star twice the solar mass.
When objects as massive as these collide, they create ripples in the fabric of space called gravitational waves. And it was these ripples that the researchers detected.
The researchers looked at the previous observations with new eyes, and many of them were likely similar collisions.The researchers have detected two colliding black holes as well as two neutron stars, but this is the first time they have detected a neutron star colliding with a black hole.
But why is this last collision important? According to current theories and previous observations, neutron stars tend to be found with (and collide with) other neutron stars. And the same must be true for black holes.
But the two neutron star and black hole collisions, published in the Astrophysical Journal Letters, may pose a challenge to that general idea.
This goes in the direction of another set of theories that assume that black holes and neutron stars are actually found with each other. These alternative theories also imply that stars and galaxies formed in ways different from the picture painted by standard views of how the cosmos formed.
None of the ideas mentioned above perfectly explain our observations about the Universe. But, according to Raymond, many of them can be tweaked to better fit what we know.
Sheila Rowan of the University of Glasgow says observations of the type and frequency of collisions between black holes and neutron stars over the past six years are creating an increasingly detailed picture of dynamics within galaxies.
"All of this is giving us a rich insight into stellar evolution. This last observation is another new development for us in our understanding of what's out there in the Universe and how it came to be the way it is," she says.The collisions were detected by measuring waves caused by sudden changes in gravitational forces that occur when two massive celestial bodies collide. These are ripples in the very structure of space, just like a stone thrown into a tranquil lake.
These so-called gravitational waves travel hundreds of millions of light years through space and were detected by sensors in Washington and Louisiana, in the United States, and by the Virgo detector, in Italy. Together, they form the Advanced Light Interferometer Gravitational Wave Observatory (ALIGO), which brings together more than 1,300 scientists from 18 countries.
When they reach us, the ripples are tiny—smaller than the width of an atom. The detectors themselves are among the most sensitive instruments ever built.
In the future, the team hopes to detect collisions between neutron stars and black holes using telescopes, both in space and on the ground. This will allow scientists to discover more about the superheavy materials that neutron stars are made of.