The Fastest Star in the Milky Way Will Put Einstein's Theory of Relativity to the Ultimate Test
Deep within the turbulent heart of our galaxy lies one of the most extreme environments in the known universe — the region surrounding Sagittarius A* (Sgr A*), the supermassive black hole (SMBH) at the center of the Milky Way. For decades, astronomers have studied a remarkable cluster of stars in this region, known as S-stars, whose extraordinarily rapid orbits provided some of the most compelling early evidence that a massive compact object — now known to weigh approximately 4 million solar masses — lurks at the galactic center. Now, the discovery of a new member of this elite stellar family is set to revolutionize our understanding of black hole physics and push the boundaries of Einstein's General Theory of Relativity.
The newly discovered star, designated S301, is not merely another addition to the S-star catalog. It is, by every measurable metric, the most extreme stellar object ever found in the vicinity of Sgr A*. It is simultaneously the fastest, the closest, and the most scientifically promising S-star ever detected — and it may soon allow astronomers to accomplish something that has never been done before: directly measure the spin of a supermassive black hole.
The findings are detailed in a landmark new paper titled "Discovery of a star sensitive to the spin of Sgr A*," set to be published in the prestigious journal Nature. The lead author is K. Abd El Dayem from the Laboratory for Instrumentation and Research in Astrophysics (LIRA) at the Paris Observatory, and the work represents the culmination of years of painstaking observation by an international team of scientists.
The S-Stars: Nature's Cosmic Laboratories
To appreciate the significance of S301, it is important to understand the broader context of S-star research. The S-stars occupy a region known as the central parsec — a volume of space roughly 3.26 light-years across centered on Sgr A*. Despite the chaotic, radiation-drenched environment of the galactic center, these stars follow well-defined Keplerian orbits around the central black hole, tracing elegant ellipses that encode precise information about the gravitational field in which they move.
The study of S-stars has already yielded extraordinary scientific dividends. Most famously, the star S2 (also known as S0-2 in some naming conventions) completed a close approach to Sgr A* in 2018, allowing astronomers to confirm gravitational redshift — a prediction of General Relativity — with unprecedented precision. This work, conducted by the GRAVITY Collaboration at the European Southern Observatory, contributed to the 2020 Nobel Prize in Physics awarded to Reinhard Genzel and Andrea Ghez for their work on the galactic center.
But S2, remarkable as it is, takes roughly 16 years to complete a single orbit. S301 does so in just 8.7 years, at a closest approach — known as periapsis — of only 12 astronomical units (AU) from Sgr A*. For reference, that is approximately the distance from the Sun to Jupiter, yet this star is swooping past a black hole four million times the mass of our Sun at a staggering 25,000 kilometers per second — roughly 8% of the speed of light.
"What is special about this star is that it's orbiting Sagittarius A* on a very tight orbit, taking just 8.7 years to complete it, and is approaching the black hole at a mere 12 times the distance of Earth to the Sun. That is unprecedented." — Felix Mang, PhD student at the Max Planck Institute for Extraterrestrial Physics (MPE)
A Window Into Extreme Spacetime
The discovery of S301 opens a new and remarkable scientific window. At such proximity to Sgr A*, the star is no longer simply orbiting a massive gravitational source — it is navigating the deeply curved, dynamically twisted fabric of spacetime that surrounds a rotating black hole. This is a regime where the subtle but profound predictions of General Relativity become measurable, observable phenomena.
Nobel laureate Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, and a founding member of the collaboration, placed the discovery in its broader historical context:
"Decades carefully tracking stars orbiting our galaxy's central black hole, Sagittarius A*, have led to this breakthrough discovery of a very promising star. Because it orbits so close to Sagittarius A*, S301 opens a new window to the fundamental properties of spacetime in this extreme black-hole environment." — Reinhard Genzel, Nobel Prize Laureate
The key phenomenon that makes S301 so scientifically valuable is the Lense-Thirring effect — a subtle but measurable consequence of General Relativity that arises when a massive rotating body drags the fabric of spacetime along with it. This effect, sometimes called frame-dragging, was first theorized by Austrian physicists Josef Lense and Hans Thirring in 1918, just a few years after Einstein published his general theory. It predicts that a sufficiently massive rotating object will twist spacetime in its immediate vicinity, influencing the motion of nearby objects in ways that Newtonian gravity cannot explain.
For a rotating black hole as massive as Sgr A*, this frame-dragging effect is not merely theoretical — it is strong enough to cause measurable orbital precession in nearby stars. Specifically, the star's point of closest approach to the black hole — its periapsis — will shift slightly with each completed orbit, tracing out a rosette pattern over time rather than a fixed ellipse. The rate and direction of this shift encodes direct information about the spin magnitude and orientation of the black hole itself. Crucially, the closer the star, the more pronounced and rapidly detectable this effect becomes — making S301's extreme proximity an invaluable asset.
Measuring the Unmeasurable: The Spin of Sgr A*
The spin of a black hole — formally described by a dimensionless parameter ranging from 0 (non-rotating) to 1 (maximally rotating) — is one of only three fundamental properties that characterize a black hole, alongside its mass and electric charge. This is the essence of the "no-hair theorem" in General Relativity, which holds that black holes can be entirely described by just these three quantities, regardless of the complexity of the matter that formed them.
While the mass of Sgr A* has been measured with impressive precision through decades of S-star tracking, its spin has remained elusive. Current estimates, derived indirectly from methods such as modeling the emission from the Event Horizon Telescope's observations, remain highly uncertain. A direct stellar measurement would be unprecedented and transformative.
Stefan Gillessen, a senior researcher at MPE who played a leading role in the new study, articulated the magnitude of this opportunity:
"For the first time, we would actually be able to measure very directly the spin of a massive black hole, which would be a key test of Einstein's theory." — Stefan Gillessen, MPE
Study co-author Felix Mang echoed this excitement: "With this star we hope to measure, within the next 10 years, the spin of the black hole." And crucially, S301 dramatically accelerates this timeline. Without it, years — even decades — of additional observations of other S-stars would have been required to accumulate sufficient data.
"Without this star, we would need to measure the motion of other stars for several more decades to get anywhere close to measuring the spin of the black hole." — Juan Osorno, astronomer at LIRA Observatoire de Paris–PSL, France
The Art of Finding a Needle in a Cosmic Haystack
Discovering S301 was a monumental observational challenge. The star is an almost incomprehensibly faint object — approximately two billion times dimmer than Betelgeuse, the bright red supergiant visible to the naked eye in the constellation Orion. Detecting it required not only the world's most powerful astronomical instrumentation but also years of dedicated, methodical observation.
The key instrument in this discovery was GRAVITY, an interferometric instrument mounted on the Very Large Telescope Interferometer (VLTI) at the European Southern Observatory's Paranal site in Chile. GRAVITY combines the light from four 8.2-meter telescopes to achieve an angular resolution equivalent to that of a single mirror 130 meters in diameter — fine enough to resolve structures at the scale of milli-arcseconds, which at the distance of the galactic center (roughly 26,000 light-years) corresponds to physical scales of just a few light-hours.
The research team conducted a dedicated, multi-year observational campaign, investing approximately 100 hours of telescope time per year since 2017, accumulating a rich dataset of stellar positions and motions. The painstaking work paid off in the spring of 2023, when a faint stellar source was detected approximately 15 milli-arcseconds northwest of Sgr A*.
"In spring 2023, we discovered a faint star 15 mas north-west of Sgr A*, which in the following months moved outward, and which we labeled S301," the authors write in the paper. They identified the star in four distinct positions during 2023's observing season, enabling an initial characterization of its speed and trajectory. Dedicated follow-up observations in 2024 and 2025 added a further thirteen astrometric measurements. Even more remarkably, when the team searched archival GRAVITY data, they identified S301 in earlier observations — strongly in 2021 data and more weakly in images from 2017.
"Overall, we have 19 astrometric positions of S301 that outline an ellipse on the sky and yield a consistent orbit," the authors report. This rich positional dataset, spanning nearly eight years, was sufficient to determine a robust orbital solution and confirm S301's extraordinary properties. The key observational findings can be summarized as follows:
- Orbital period: Just 8.7 years — roughly half that of the previous record-holder among S-stars
- Peak velocity: Approximately 25,000 km/s, or about 8% of the speed of light
- Closest approach (periapsis): Only 12 AU from Sgr A* — the closest any star has been observed to approach this black hole
- Sensitivity to frame-dragging: Sufficient to detect the Lense-Thirring effect within approximately one decade of continued monitoring
- Apparent brightness: Approximately two billion times fainter than Betelgeuse, making it one of the most challenging stellar targets ever successfully tracked
How Did S301 Get There? The Hills Mechanism
One of the most intriguing questions raised by S301's discovery is how it came to occupy such an extreme orbit in the first place. Stars cannot form so close to a supermassive black hole — the enormous tidal forces would disrupt any nascent protostellar cloud before it could collapse under its own gravity. S301 must, therefore, have formed elsewhere and migrated inward over time through a process known as orbital evolution.
The most compelling explanation involves the Hills mechanism, a process first proposed by astronomer Jack Hills in 1988. In this scenario, a binary star system — two stars gravitationally bound to each other — wanders too close to the central black hole. The powerful tidal forces of Sgr A* overcome the mutual gravitational attraction of the binary pair, disrupting the system in what is called a Hills disruption or tidal capture event. One star is captured into a tight, highly eccentric orbit around the black hole, while its former companion is violently ejected at extremely high velocity — becoming what astronomers call a hypervelocity star, potentially traveling fast enough to escape the Milky Way entirely.
The properties of S301 fit this picture with remarkable consistency. As the authors conclude in their paper:
"Taken together, the properties of S301 suggest a simple and self-consistent picture: a compact main-sequence binary was tidally separated by Sgr A*, leaving behind S301 on the most relativistic stellar orbit known and ejecting its companion as a hyper-velocity star." — Abd El Dayem et al.
This interpretation connects S301 not only to the dynamics of the galactic center but also to the broader phenomenon of hypervelocity stars — a population of stars discovered in recent decades that are moving so fast they may eventually leave the Milky Way. The ejected companion of S301, if it exists, is now presumably racing away from the galactic center at hundreds of kilometers per second.
Implications for Fundamental Physics and Black Hole Science
The scientific implications of S301 extend well beyond the measurement of Sgr A*'s spin. The star's extreme orbit makes it a powerful probe of relativistic effects that have never been observed with such clarity in a stellar environment. Among the phenomena that S301's continued monitoring could illuminate are:
- Lense-Thirring precession: A direct, high-precision measurement of frame-dragging by a supermassive black hole, providing a key test of General Relativity in the strong-field regime
- Gravitational redshift: As S301 plunges toward periapsis, its light will be increasingly redshifted by Sgr A*'s gravitational field, providing another precision test of GR
- Schwarzschild precession: The in-plane precession of the orbit due to the curvature of spacetime, already detected in S2 and expected to be even more pronounced in S301
- Constraints on alternative theories of gravity: Precise orbital measurements can rule out or constrain modifications to General Relativity, including scalar-tensor theories and other alternatives
- Black hole thermodynamics: The spin measurement will test the Kerr metric — the exact solution to Einstein's field equations describing a rotating black hole — with unprecedented precision
The discovery of S301 is also a powerful testament to the scientific payoff of long-term, patient observational programs. The Max Planck Society and its international partners have invested decades in building the infrastructure, instrumentation
