Chapter 1: The Phenomenon of High-Velocity Stars

An artist's depiction illustrates the moment S5-HVS1 was expelled by Sagittarius A*, the supermassive black hole at the heart of our galaxy. In the image, the black hole and its binary companion are positioned far to the left, while S5-HVS1 hurtles away in the foreground. (James Josephides, Swinburne Astronomy Productions)

Astronomers have recently identified a star moving at an astonishing speed of 6 million kilometers per hour, which was expelled from the Milky Way’s center by a supermassive black hole approximately five million years ago. This runaway star, designated S5-HVS1, was discovered by Assistant Professor Sergey Koposov from Carnegie Mellon University during the Southern Stellar Stream Spectroscopic Survey (S5).

Douglas Boubert, a co-author from the University of Oxford, noted, "The speed of the star is so extreme that it will inevitably escape the galaxy and never return."

Chapter 2: The Unique Characteristics of S5-HVS1

S5-HVS1, located in the constellation Grus, belongs to a category of celestial bodies known as high-velocity stars (HVSs). These stars have intrigued astronomers since the first one was identified in 2005, leading to the discovery of many more over the next fourteen years. However, S5-HVS1 stands out due to its remarkable speed and its relatively close proximity to Earth, at about 2.9 million light-years away.

The knowledge of S5-HVS1's extraordinary velocity, combined with its closeness, has enabled astronomers to trace its path back to the Milky Way’s core and the supermassive black hole, Sagittarius A* (Sgr A*), located there.

Koposov, the lead author of the study and a member of Carnegie Mellon’s McWilliams Center for Cosmology, expressed enthusiasm: "This is thrilling, as we've long suspected that black holes can eject stars at such high speeds. However, we previously lacked a clear connection between a fast star and the galactic center. We believe the black hole expelled this star at thousands of kilometers per second, roughly five million years ago. This event coincided with a time when our ancestors were just beginning to walk on two legs."

Chapter 3: The Hills Mechanism Explained

So, what led S5-HVS1 to achieve such a phenomenal speed? Astronomers theorize that the star was once part of a binary system alongside a companion star. Its expulsion occurred when their orbits drifted too close to Sgr A*. While its companion was captured by the black hole’s immense gravitational pull, S5-HVS1 was torn free and propelled into its rapid journey.

This phenomenon is referred to as the 'Hills mechanism,' which was proposed by astronomer Jack Hills three decades ago and has long been thought to explain the origins of high-velocity stars. Ting Li, from Carnegie Observatories and Princeton University, highlighted, "This is the first clear demonstration of the Hills Mechanism in action. Observing this star is truly remarkable, as we know it must have originated from the galactic center, a place very different from our local surroundings. It represents a visitor from an alien realm."

Chapter 4: The Observational Breakthrough

The discovery of S5-HVS1 was made using the 3.9-meter Anglo-Australian Telescope (AAT) located near Coonabarabran, NSW, Australia. The astronomers could accurately assess the star's true speed and its remarkable journey only by combining these observations with additional data from the European Space Agency's Gaia satellite.

Daniel Zucker, an astronomer at Macquarie University and a member of the S5 executive committee, remarked, "These observations would not have been possible without the distinctive capabilities of the 2dF instrument on the AAT. For over two decades, it has been at the forefront of research and remains the best facility for our project."

The findings from this research were published in the Monthly Notices of the Royal Astronomical Society. Kyler Kuehn, from Lowell Observatory and another member of the S5 executive committee, expressed excitement: "I’m thrilled that S5 discovered this fast-moving star. While the primary goal of S5 is to investigate stellar streams from disrupted dwarf galaxies and globular clusters, we allocated spare resources to find intriguing targets in the Milky Way, and we serendipitously found something extraordinary. With future observations, we hope to uncover even more!"