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The Discovery of an Anomalous Celestial Object in the Milky Way

About 40,000 light-years from the Sun, there is a large group of stars. Hundreds of thousands of suns form the globular cluster NGC 1851 in the constellation Columba. This cluster is mostly composed of old stars densely packed together. These stars interact gravitationally, and in extreme cases, they even collide with each other.

NGC 1851 cluster — [Photo: NASA Hubble Space Telescope, CC BY 2.0, via Wikimedia Commons]

Perhaps the result of such a collision is the strange object that has just been discovered in the NGC 1851 cluster. An international team of scientists described it in an article published in the scientific journal “Science.” This object is a mystery. It could be a neutron star, a black hole, or an as-yet-unknown exotic type of celestial body.

Neutron stars and black holes

One of the most crucial parameters describing planets and stars is their mass. It determines, for example, what will happen to a star when nuclear fusion ceases in its core. If a star is massive enough, its end may be a supernova — a spectacular explosion in which the star ejects its outer layers, leaving behind an ultra-dense core, a neutron star. The heaviest neutron stars have a mass equivalent to about two solar masses.

In places like the NGC 1851 cluster, it occasionally happens that two neutron stars approach each other. This can lead to their collision and merger. As a result, the mass of the resulting object exceeds the critical value, and it collapses gravitationally. This is one way black holes are born. The lightest black holes have a mass equivalent to about five solar masses.

Mysterious gap

As evident, there is a clear gap between the mass of the heaviest neutron stars and the mass of the lightest black holes. An object that would fit precisely into this gap has never been found. That is, an object with a mass of approximately 3–4 solar masses.

Until now, this had not been achieved. As reported in “Science,” researchers have detected an object in the NGC 1851 cluster with a mass of 3.9 solar masses, fitting exactly into the aforementioned gap.

Scientists did not observe it directly. This object is in a binary system, a companion to a radio pulsar: a rotating neutron star. The pulsar spins at a dizzying speed, rotating 170 times per second. It regularly emits pulses towards Earth, which astronomers recorded with the help of the South African MeerKAT radio telescope.

NGC 1851 cluster — [Photo: NASA/JPL-Caltech/SSC, Public domain, via Wikimedia Commons]

What can be deduced from the pulsar

Pulsars are incredibly regular. The super-precise recording of their pulses allows researchers to learn more about their orbit. In this case, it turned out that the pulsar has a hidden companion. It could be the heaviest known neutron star, the lightest known black hole — or something else that is currently unknown.

– Each of these possibilities is exciting — says Ben Stabbers, an astrophysicist at the University of Manchester, one of the study’s authors. — A pulsar-black hole system would allow testing different theories of gravity, while a heavy neutron star would provide a new perspective on nuclear physics at very high densities.

For this reason, astronomers still intend to study the radio pulsar and its mysterious companion. — We’re not done with this system yet — announces Arunima Dutta, co-author of the study. — Discovering the true nature of the pulsar’s companion will be a turning point in our understanding of neutron stars, black holes, and everything that may lurk in between — she concludes.

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