Introduction

In the field of astronomy, astronomers sometimes stumble upon celestial objects that leave them scratching their heads.
In a recent study published in Science, a discovery was reported that is likely to get scientists talking and asking questions.

Neutron Stars: Exceptionally Dense

Incredibly Dense Objects: Neutron stars are some of the densest things in the universe. They’re as compact as an atomic nucleus but as big as a city, pushing our understanding of super-dense matter to the limit.
A Weighty Matter: The heavier a neutron star is, the more likely it is to eventually collapse and become something even denser, like a black hole.

A Cosmic Mystery: To understand what happens when neutron stars turn into black holes, objects that are in-between need to be found. These objects also need to be studied very carefully over a long time.
A New Discovery: A cosmic system has been found in the NGC 1851 star cluster that doesn’t fit neatly into the categories of neutron stars or black holes.

Seeing Something New: Inside NGC 1851, a pair of stars has been spotted that provides fresh insights into the extreme matter in the universe. This system has a millisecond pulsar, a fast-spinning neutron star that sends out beams of radio light, and a massive, dark companion that can’t be seen at any wavelength of light.
The Pulsar’s Role: Millisecond pulsars are like cosmic clocks. They spin steadily, and any changes in their spin can tell important things about what’s around them.

Very Precise Measurements: The MeerKAT radio telescope in South Africa was used to closely watch the NGC 1851E system.
What Was Found: Observations allowed figuring out exactly how the two objects move around each other and how heavy they are together. The system’s mass is almost four times that of the Sun, and the invisible companion is denser than a regular star but not as heavy as a black hole.
A Strange Mass Gap: The companion’s mass falls in a range that’s puzzling to scientists, between the heaviest neutron stars and the lightest black holes. Understanding objects in this range is a big mystery in astrophysics.

A Fascinating Idea: One intriguing possibility is that a pulsar is circling around what’s left after two neutron stars collided, something made possible because there are many stars packed closely together in NGC 1851.
Starry Dance Floor: In this crowded group of stars, they twirl around each other, changing partners as they go. If two neutron stars get too close, they collide, creating a black hole. This black hole can then disturb the dance of other stars in the cluster.
Still Many Questions: The work isn’t finished. Research is continuing to figure out exactly what the companion is. Is it the lightest black hole, the heaviest neutron star, or something completely different?
Exploring New Frontiers: When at the border between neutron stars and black holes, there’s a chance of discovering completely new types of objects.