A Highly Irregular Object in the Milky Way May Be the Tiniest Black Hole We’ve Ever Seen

computer artwork of black hole
This May Be the Tiniest Black Hole We’ve Ever SeenMARK GARLICK/SCIENCE PHOTO LIBRARY - Getty Images
  • A group of scientists just discovered a mystery object that is either the biggest neutron star or the smallest black hole we’ve ever seen.

  • It sits in what is called the “lower mass gap”—a range of masses between the smallest black holes and the biggest neutron stars where we see a strangely small number of objects.

  • The system could provide experts with a wealth of new information on everything from black holes to general relativity—but there are still a lot of questions standing between the scientists and their answers.

So… we’re missing some stuff in the universe.

In general, the universe is full of objects of all different sizes and in all different places, ranging from the biggest black holes to the smallest quarks. So, it’s notable when there’s a region or size category that’s just kind of weirdly empty.

One of those is known as the “lower mass gap.” Simply put, it describes the array of masses between the biggest neutron stars and the smallest black holes where there isn’t a whole lot of anything. It’s a puzzle for scientists, as there’s no real reason that we shouldn’t see more objects in those sizes.

Well, we may have just found one more object in that gap. In a recent study, a team lead by experts from the Max Planck Institute for Radio Astronomy announced the discovery of an object “weighing” in at about 2-2.5 times the mass of our Sun—a mass which plops it firmly in the lower mass gap. It’s most likely either the biggest neutron star we’ve ever see or the smallest black hole we’ve ever seen.

Either way, it’s a big deal to the astronomical community. “Either possibility for the nature of the companion is exciting,” Ben Stappers, one of the leaders of the project, said in a press release. “A pulsar–black hole system will be an important target for testing theories of gravity and a heavy neutron star will provide new insights in nuclear physics at very high densities.”

The object was found orbiting a radio pulsar—a type of spinning neutron star that hits Earth with extremely regular “pulses” of radio waves. And it’s that extreme regularity that actually allowed researchers to find the mass of the orbiting mystery object. Through a series of rather complicated mathematical steps, researchers can use those pulses to tell how far away from us a pulsar is. And once we know how far away it is, we can figure out how big it is. And how big the things orbiting around it are.

Knowing size, however, is one thing. Knowing type-of-object is a whole different ball of wax. The researchers are pretty sure the object is either a small black hole or a large pulsar—both of which form after a star dies—and they believe that this mystery object probably formed when two neutron stars collided and merged.

While the scientists are excited to see what answers present themselves in the future, they’re rooting just a little bit for the companion to be a black hole. This would make it the first observed radio pulsar–black hole binary which, according to the press release, “could allow new tests of Einstein’s general relativity and open doors to the study of black holes.”

But until they get a whole lot more information on this binary, they’ll just have to keep digging with open minds. “We’re not done with this system yet,” Arunima Dutta, one of the leads on the study, said in a press release. “Uncovering the true nature of the companion will a turning point in our understanding of neutron stars, black holes, and whatever else might be lurking in the black hole mass gap.”

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