A 1.5-Solar-Mass Star in a Tight Orbit Around the 4.3-Million Solar-Mass Black Hole at the Milky-Way Center
The fastest moving star, labeled S301, was discovered recently by Stefan Gillessen’s team at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany (as first reported here, with the full paper accessible here). The star was discovered by near-infrared interferometry on 8-meter telescopes, using the GRAVITY instrument in operation at the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT). Last night, I sat next to Stefan at the reception dinner of the annual conference of Harvard’s Black Hole Initiative, for which I served as the founding director a decade ago.
This 1.5-solar-mass star moves on a highly elliptical orbit with a period of 8.7 years and eccentricity of 0.98 around the supermassive black hole at the Milky-Way center, called Sagittarius A*. This black hole has a long history of swallowing 4.3 million solar masses of gas and stars from its environment. The peak velocity of S301 is 25,000 kilometers per second or 8.3% of the speed of light, as it comes down to a distance of 140 times the Schwarzschild radius of the black hole — which defines the scale of the black hole mouth from where even light cannot escape. If the star were to pass ten times closer to the black hole, it would have been ripped apart by tidal gravity into a stream of gas that shines brightly as it feeds the mouth of this spacetime beast.
The orbit of S301 can be used to test expectations from Albert Einstein’s formulation of gravity as the curvature of spacetime. Einstein’s equations predict that S301’s orbit will precess in response to the spin of the black hole, offering a precise new way to measure how fast Sagittarius A* is rotating within the coming decade.
How did this star get so close to the black hole?
A natural mechanism, proposed by Jack Hills in a 1988 paper published here, is the tidal break-up of a pair of stars by the black hole. About half of solar-mass stars form in binaries. When a binary star system gets close enough to the black hole, the tidal gravity becomes stronger than the gravitational binding of the two stars and breaks the binary apart, sending one star out at a speed of up to thousands of kilometers per second and launching the second star into a tighter orbit around the black hole. Indeed, a population of hypervelocity stars had been discovered on their way out in the Milky-Way halo by Warren Brown and collaborators from the Harvard-Smithsonian Center for Astrophysics (as reported here).
In a 2006 paper published here, I proposed with the student, Idan Ginsburg, that the former companions of the observed hypervelocity stars in the Milky-Way halo might have produced the observed population of close-in S-stars on highly eccentric orbits around Sagittarius A*. The Galactic center star S301 is likely one of them, formed via the Hills mechanism out of an initial binary star system with an orbital period of 1–2 weeks over the past 100 million years.
In a follow-up paper published here, I showed with Idan that planets could survive the break-up of binary star systems by Sagittarius A*. As a result, Galactic travel agencies could sell tickets for thrilling journeys on habitable planets around hypervelocity stars. I wonder whether adventurous Galactic passengers would prefer to travel with a hypervelocity star on its way out of the Milky-Way galaxy at a speed of up to 1% of the speed of light or travel with a star like S301 as it reaches 8.3% of the speed of light and gets within a distance of 140 Schwarzschild radii from the largest black hole in our Galaxy. I would personally favor the latter, since the extreme spacetime structure of a supermassive black hole is far more exhilarating than the rarefied environment of intergalactic space. The trip close to the black hole also offers health benefits, since aging slows down by a third of a percent at the closest approach of S301 to Sagittarius A*. This corresponds to a gain of 5 minutes to the passenger’s lifespan every day relative to distant relatives.
The black hole tour with S301 offers a view of the black hole’s mouth from a distance where it occupies roughly the same angle as the Moon or the Sun in our sky. The gas swirling into the event horizon of Sagittarius A* glows bright but at the center of this glow, there is a silhouette — a shadow cast by the absorption of light emanating behind the black hole. Over the decade between 2006 and 2016, I wrote 30 papers in collaboration with my postdoc, Avery Broderick, forecasting the expected portrait of a black hole (as detailed here and summarized for the general public here). By now, Sagittarius A* was imaged by the Event Horizon Telescope (here), whose headquarters was established at Harvard’s Black Hole Initiative during my directorship.
On a tour with S301, it would be fascinating to observe the silhouette image of Sagittarius A* from a minimum distance that is 140 million times closer than the Earth is from the black hole. I would have loved to serve as the tour guide on such a journey. Here’s hoping that Galactic travel agents will pay attention to this essay.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
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