A star with an Extreme Orbit

S301 feels the rotation of the Milky Way’s central black hole

To the point:

• Discovery S301: Researchers at the Max Planck Institute for Extraterrestrial Physics found the star S301, which orbits the Milky Way's central black hole Sagittarius A* (Sgr A*) closer and faster than any known star, with an 8.7-year orbit and closest approach similar to the Sun-Saturn distance.
• Observation Challenges: S301 is extremely faint—two billion times dimmer than Betelgeuse—making detection difficult; advanced instruments like GRAVITY and new data analysis methods enabled precise tracking of its position near Sgr A*.
• Scientific Importance: S301 probes spacetime closer to the black hole than previous stars, allowing researchers to study the black hole's spin and test fundamental physics theories such as the Kerr metric and the no-hair theorem.
• Technological Advances: The use of GRAVITY at the Very Large Telescope Interferometer and future MICADO instrument at the ESO Extremely Large Telescope provides unprecedented resolution and sensitivity to study stars near the galactic center.

A team from the Infrared/Submillimetre Astronomy Group at the Max Planck Institute for Extraterrestrial Physics (MPE) has discovered a new star that comes closer to the supermassive black hole Sagittarius A* (Sgr A*) at the heart of our Milky Way than any other known star. The newly identified object, named S301, swirls around the black hole in a highly elliptical orbit with a period of approximately 8.7 years. At its closest approach, it comes within just over twelve astronomical units of Sagittarius A*, roughly the distance between the Sun and Saturn. As a result, S301 is now the star with the shortest known orbital period and the tightest orbit ever observed in the galactic centre.

Even more importantly, S301 moves through a region where the black hole’s rotation drags spacetime itself, a phenomenon that only occurs in the immediate vicinity of a massive, spinning object. “S301 probes spacetime in the Galactic Center ten times closer to Sagittarius A* than our previous best star, S2,” says Reinhard Genzel, Director at MPE and Nobel Laureate for Physics in 2020. “Within the next decade, we will measure the black hole’s rotation directly — a milestone for general relativity.”

An almost invisible star: a flicker in a sea of light

S301 is two billion times fainter than Betelgeuse, one of the brightest stars in Orion. Its light is so soft that even in the best images of the Galactic Center it appears as no more than a tiny point, lost among numerous, much brighter stars. 

S301 was first identified in spring 2023 using the GRAVITY instrument at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO) in Chile. Developed through an international consortium led by MPE, GRAVITY combines the light collected by four 8-metre telescopes, achieving an angular resolution 40 times sharper than that of a single telescope. The recently completed upgrade to GRAVITY+ , which includes a new adaptive optics system and laser guide stars, has increased the instrument’s sensitivity by a factor of 10 to 100. This unprecedented combination of precision and sensitivity has made it possible to detect S301, a new and extremely faint star.

GRAVITY’s exceptional resolution makes it possible to observe stars at the center of the Milky Way on scales comparable to the size of our solar system, even when they are extremely faint. The discovery of S301 is the result of four decades of systematic observations by the MPE team. Over this period, the team has mapped the region around Sagittarius A* with steadily increasing precision, measuring the stars’ motions and characterising the mass concentrated at the centre of the Galaxy.

Two relativistic effects: one large, one tiny

The orbit of S301 deviates significantly from an ellipse, the path predicted by Newton’s classical theory of gravitation. This is a clear sign of relativistic effects that occur only in the vicinity of a massive, rotating black hole. Two phenomena are at play:

• Schwarzschild Precession: If space is curved around a central mass – such as, in this case, the supermassive black hole – this affects the orbits of the objects in its vicinity. The gradual shift in the orbit’s closest approach point, known as pericentre advance, was previously observed in the star S2. It arises from the curvature of spacetime caused by the black hole’s mass. The Schwarzschild solution of general relativity remains a useful approximation to explain the orbit of S2, even for a spinning black hole at the center of the Galaxy. 

• Lense-Thirring-Effect (Frame-Dragging-Effect): As the black hole at the centre of our Milky Way is not only extremely massive but also rotates, another relativistic effect occurs alongside Schwarzschild precession: the Lense–Thirring effect. A rotating mass, in a sense, drags the surrounding space-time along with it – much like a rotating spoon setting the water in a cup in motion. The effect is significantly weaker and therefore more difficult to detect. S301 is now the first star in which this effect around a black hole has been measured. 

“S301 is the first star to orbit directly in the region around Sagittarius A* where the frame-dragging effect is extreme,” explains Felix Mang, PhD student at MPE and corresponding author of the study published in Nature. “We’re not just measuring spacetime curvature; we are measuring how it gets distorted by the rotation of the black hole itself. That is unique.”

Why is the measurement so challenging?

Tracking such a star is a technical triumph. To measure S301’s orbit, the Scientists analysed years of observational data, recorded at a resolution of just a few milliarcseconds — equivalent to the size of a car on the Moon. The star’s position was monitored across numerous observations over many years.

Until now, detecting such a faint object had been impossible, largely because of the extreme contrast in brightness between S301 and the stars orbiting the black hole. Using a new data-analysis method, a team at MPE has now succeeded in extracting the faint signal. It is like trying to hear the buzz of a fly while a symphony orchestra is playing.

The star’s advantage: faster, more precise and sensitive to Sagittarius A*’s spin

A key advantage of S301 over observing the diffuse hot gas around black holes (such as the one imaged in M87 by the Event Horizon Telescope) is that we can directly observe motion. Combining the position measurements from GRAVITY+ and radial velocity data from MICADO on the future Extremely Large Telescope of ESO, will allow the team to accurately reconstruct the star’s three-dimensional motion and for the first time, directly determine the spin of Sagittarius A*.

“We’re faster, more precise, and more direct,” says Reinhard Genzel. “With S301, we’re measuring the rotating spacetime itself, not indirectly through an image, but directly through the motion of a single star.”

Dreams of the Future: Do black holes really have no hair?

The observation of S301 is part of a long-term vision: testing the Kerr metric, the mathematical description of spacetime around a rotating black hole: Stefan Gillessen, senior scientist at MPE and corresponding author: “In the long run, the analysis of S301’s orbit could even provide evidence for the no-hair theorem, the statement that a black hole is fully described by just three properties: mass, angular momentum, and electric charge.”

Frank Eisenhauer, director at MPE and Principal Investigator of the GRAVITY and GRAVITY+ projects is looking forward: “The chance to discover the beauty of nature in the form of this star is truly a dream come true,” he says. “And with GRAVITY+ and the upcoming MICADO instrument, currently being developed in an international consortium led by MPE for the ESO Extremely Large Telescope: to explore strange new worlds, seek out new physics, and see what nobody has seen before.”

Background

The Max Planck Institute for Extraterrestrial Physics (MPE) investigates physical processes in the Universe. It is particularly renowned for proving that Sagittarius A* (Sgr A*) is a supermassive black hole — a discovery that earned MPE Director Reinhard Genzel the Nobel Prize in Physics in 2020.
The VLTI instrument GRAVITY and its upgrade to GRAVITY+ was built by a consortium led by MPE in close collaboration with institutions from Belgium, France, Germany, Ireland, Mexico, Portugal, and the UK. GRAVITY+ received significant funding from the Max Planck Foundation.

Beginning of November 2025, four lasers were fired into the sky above ESO’s Paranal Observatory in Chile, each creating an artificial star to help astronomers measure and correct atmospheric blur. This impressive launch, one from each 8-meter telescope, marks a key milestone of the GRAVITY+ project, led by MPE, significantly enhancing the VLTI’s ability to observe fainter objects and cover more of the southern sky.

The European Southern Observatory (ESO) is an intergovernmental organisation supported by 16 member states and two associated countries. It develops, builds and operates leading ground-based observatories – with its headquarters in Germany and three sites in Chile. ESO enables researchers worldwide to investigate ground-breaking astronomical questions and to promote public interest in astronomy.