What Black Holes Bring to the Galaxy

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Black holes are among the most mysterious phenomena in the universe. Forged from the cores of dead stars, they are so dense that nothing can escape their gravitational pull, not even light, which renders them invisible. Entire stars, once luminous, can be extinguished if they cross a black hole’s boundary, and pass the point of no return.

Albert Einstein predicted more than a century ago, based on his theories untangling the nature of gravity, that such strange objects could exist, but he thought the idea was too far-fetched. In 1965, after Einstein’s death, Penrose, the Oxford professor, published a paper showing, mathematically, that the forces of the universe could indeed produce black holes, and that inside their impenetrable depths resides something called a singularity, an inscrutable point which no known laws of physics can describe.

Such a thing might still seem too incredible to exist, but without black holes, the movements of faraway stars in our galaxy don’t always make sense. Genzel and Ghez spent many years poking into the cosmic cloud of interstellar gas and dust at the very center of the galaxy, with the world’s largest telescopes. They discovered stars orbiting a seemingly empty spot at startling speeds, a chaotic environment that could make sense only in the presence of a supermassive black hole. This region in our galaxy, known as Sagittarius A* (pronounced ay-star), has a mass 4 million times that of our sun, squeezed into a space smaller than our solar system.

Astronomers have found other black holes, too, by watching for the dizzying orbits of the unlucky stars around them. They have seen black holes in the glow coming from matter as it plunges into


Fast-rotating stars at the centre of the Milky Way could have migrated from the outskirts of the galaxy

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Milky Way
Credit: CC0 Public Domain

In a research paper published by The Astrophysical Journal Letters, an international team of astrophysicists, including scientists from the University of Surrey, detail how they discovered a group of stars with different characteristics than their neighbors found in the Milky Way’s Nuclear Star Cluster (NSC).

The team used state-of-the-art high-resolution computer simulations to explain how this group of metal-poor and fast-rotating stars came to be located at the center of our galaxy.

Their calculations found that it is likely that this group of stars are leftovers from the migration of a massive star cluster that formed a few light-years away from the Milky Way’s center. Alternatively, while not as likely as the cluster scenario, the team also noted that the group of stars could possibly have originated from a dwarf galaxy located up to 320,000 light-years away from the galactic center.

All evidence points towards an accretion event that happened 3-5 billion years ago during which a massive cluster migrated towards the center of the Milky Way and was disrupted by the strong tidal forces of the NSC, a region of high stellar density. Cluster stars were deposited in the region and were discovered based on their peculiar velocities and low metal content.

Dr. Alessia Gualandris, senior lecturer in physics from the University of Surrey, added: “This discovery may be the ‘smoking gun’ evidence that the Milky Way has been accreting star clusters or dwarf galaxies over its lifetime. Its past was much more active than we previously thought.”

Dr. Tuan Do, assistant research scientist at UCLA, said: “It is remarkable how these new observations of the NSC can reveal so much about the history of the whole galaxy.”

Dr. Manuel Arca-Sedda, a Humboldt Fellow at the Astronomisches Rechen-Institut, Heidelberg, concluded: “A close collaboration between