Astronomers discovered a star moving at 8% the speed of light
Deep in the center of the Milky Way, a star designated S4716 hurtles around Sagittarius A*, our galaxy's supermassive black hole. At its closest approach, it comes within 100 astronomical units of the event horizon. Traveling along this tight four-year orbit, S4716 accelerates to an astonishing 24,000 kilometers per second—about 8% of the speed of light—making it one of the fastest stars ever recorded.
An Extreme Neighborhood at the Galactic Core
Approximately 26,000 light-years from Earth lies the dense core of the Milky Way, anchored by the supermassive black hole Sagittarius A*. Weighing roughly four million times the mass of our Sun, Sagittarius A* exerts a colossal gravitational pull over its immediate surroundings. In this tightly packed environment, known as the central S-star cluster, stellar bodies operate under physics seen nowhere else in the galaxy. Stars do not drift in lazy arcs over hundreds of millions of years; instead, they whip through intense gravitational fields at fractions of the speed of light.
For decades, astronomers have scrutinized this cluster to track individual stellar paths. Because the center of the galaxy is obscured by vast blankets of interstellar dust, optical telescopes cannot penetrate the core. Observations rely on near-infrared instrumentation capable of slicing through the veil. By tracking the exact trajectories of these S-stars over years and decades, researchers have been able to map the gravitational field of Sagittarius A* with unprecedented precision, turning the galactic center into a natural laboratory for testing the laws of gravity.
Uncovering S4716 Across Twenty Years of Data
In 2022, an astrophysics team led by Florian Peißker reported the detection of a previously uncharacterized star designated S4716. Identifying the star was not the result of a single serendipitous snapshot, but of painstaking archival analysis spanning nearly two decades of high-resolution observations. The researchers synthesized data gathered between 2000 and 2020 by premier ground-based facilities, primarily the European Southern Observatory's Very Large Telescope in Chile and the W. M. Keck Observatory in Hawaii.
Disentangling S4716 from the blinding glare of its stellar neighbors required pushing modern instruments to their limits. The team analyzed observations from adaptive optics systems and near-infrared instruments, including SINFONI, NACO, and GRAVITY at the VLT, alongside OSIRIS and NIRC2 at Keck. Adaptive optics actively deforms telescope mirrors thousands of times per second to correct for the blurring caused by Earth's turbulent atmosphere. Even with this technology, S4716 frequently overlapped with brighter stars along the line of sight, requiring advanced filtering and orbit-fitting algorithms to isolate its faint signal and chart its path.
The Orbital Mechanics of an 8% Light-Speed Dash
When the orbital parameters of S4716 were fully calculated, they revealed one of the most extreme paths ever documented. S4716 orbits Sagittarius A* in roughly 4.0 years, making its orbital period exceptionally short for a clearly resolved star at the galactic center. By comparison, S2—the famous benchmark star used for decades to study Sagittarius A*—takes roughly 16 years to complete a single revolution.
The orbit of S4716 is also heavily elongated, with an orbital eccentricity of around 0.75. At its farthest point, known as apoastron, the star swings outward to a distance of roughly 700 astronomical units—where one astronomical unit (AU) represents the average distance from Earth to the Sun. But at periastron, its closest approach, S4716 swoops inward to a mere 100 AU from the supermassive black hole, or roughly 15 billion kilometers. While 100 AU is more than double the distance between Pluto and our Sun, on a cosmic scale it is an extraordinarily narrow brush with a four-million-solar-mass black hole.
As S4716 plunges toward periastron, the gravitational gradient of Sagittarius A* accelerates the star to staggering velocities. At its peak, S4716 reaches approximately 24,000 kilometers per second. This translates to roughly 8% of the speed of light—fast enough to travel from Earth to the Moon in less than twenty seconds. This tremendous velocity makes S4716 one of the fastest bound stars ever observed in the universe.
The Paradox of Youth in a Hostile Environment
The physical properties of S4716 pose a profound astrophysical puzzle often referred to as the 'paradox of youth.' Spectroscopic measurements indicate that S4716 is a relatively young, main-sequence B-type star. In quiescent regions of the galaxy, such stars form when cold molecular gas clouds collapse under their own gravity over millions of years. However, in the immediate vicinity of a supermassive black hole, the immense tidal shear forces tear molecular clouds apart long before they can condense into stars.
Because S4716 could not easily have formed within 100 AU of Sagittarius A*, it almost certainly originated farther out in the galaxy and migrated inward. Theorists suggest that gravitational interactions within the crowded cluster played a central role. One leading explanation is the Hills mechanism: a binary star system drifts too close to the black hole, where tidal forces disrupt the pair. The black hole captures one star into a tight, highly eccentric orbit while violently ejecting the other star into deep space as a hypervelocity runaway. Alternatively, repeated gravitational nudges from surrounding stars and compact stellar remnants could have steadily driven S4716 onto its current radical trajectory.
Testing General Relativity Near the Event Horizon
Beyond its record-setting speed, S4716 represents an invaluable tool for testing Albert Einstein's general theory of relativity in strong gravitational regimes. Under Newtonian gravity, an isolated two-body system traces a closed ellipse that repeats indefinitely. General relativity, however, predicts that the immense spacetime curvature around a massive object causes the orbit itself to rotate over time—a phenomenon known as relativistic periastron precession or Schwarzschild precession.
Stars that dive deep into the gravitational well of Sagittarius A* experience measurable deviations from classical orbits, including gravitational redshift, where light climbing out of the gravitational field is stretched to longer, redder wavelengths. While S2 provided the first clear detections of both gravitational redshift and Schwarzschild precession around Sagittarius A*, stars with shorter periods like S4716 offer the opportunity to observe these relativistic effects over much shorter observation baselines. Monitoring how S4716 precesses can also help astronomers place upper limits on any extended mass distribution—such as clouds of dark matter or swarms of stellar-mass black holes—cloaking the central singularity.
Pushing the Boundaries of Observation
The discovery of S4716 illustrates how rapidly observational astronomy is advancing through combined data sets and sophisticated post-processing techniques. When researchers first began tracking S-stars in the 1990s, resolving objects within hundreds of astronomical units of the central black hole seemed nearly impossible due to atmospheric distortion and instrumental sensitivity limits. Today, multi-epoch observations spanning decades allow astronomers to reconstruct the trajectories of stars that are continually obscured or cross paths with brighter cluster members.
The search for even tighter orbits is far from over. As next-generation ground-based observatories such as the Extremely Large Telescope come online, alongside enhanced optical interferometry, astronomers anticipate uncovering stars even closer to Sagittarius A*. These future detections may reveal stellar objects orbiting within dozens of astronomical units of the event horizon, pushing observed stellar velocities past 10% of light speed and offering an ever-closer look into the edge of spacetime.
Key takeaways
•S4716 is a B-type star orbiting Sagittarius A* on a tight, four-year path, reaching speeds of roughly 24,000 kilometers per second—about 8% the speed of light.
•At its closest approach (periastron), S4716 comes within approximately 100 astronomical units of the supermassive black hole, following an orbit with an eccentricity of around 0.75.
•The star was uncovered by Florian Peißker and colleagues by analyzing twenty years of near-infrared data from the Very Large Telescope and Keck Observatory.
•Because tidal shear from Sagittarius A* prevents in-situ star formation so close to the core, S4716 likely formed farther out and migrated inward through dynamical processes like binary disruption.