Newly Discovered Star Could Finally Reveal the Spin of the Milky Way's Central Black Hole

For decades, our understanding of the universe's most extreme environments has relied on mathematical assumptions rather than direct measurements. If the equations governing gravity are even slightly incomplete, everything from the evolution of galaxies to the fundamental laws of physics requires a rewrite. Now, astronomers have found a natural laboratory to test those equations directly: a newly discovered star whipping around the Milky Way's central supermassive black hole at unprecedented speeds.[1]

The discovery, published this week in the journal Nature, centers on a faint, sun-like star designated S301. Orbiting Sagittarius A* (Sgr A*)—the 4.3-million-solar-mass black hole at the galactic center—S301 is the closest and fastest star ever observed in this extreme environment. Its trajectory is so tight that it penetrates the region where the black hole's rotation physically drags the fabric of spacetime along with it, a phenomenon known as Lense-Thirring precession.[1]

The primary evidence comes from the GRAVITY and GRAVITY+ instruments on the European Southern Observatory's Very Large Telescope Interferometer (VLTI) in Chile. By combining the light from multiple telescopes, the international collaboration achieved the resolution necessary to spot an object roughly two billion times dimmer than the star Betelgeuse, located 27,000 light-years away in the crowded, dust-obscured galactic core.

The data reveals a star pushed to kinetic extremes. S301 completes a highly elliptical orbit in just 8.7 years, shattering previous records for the galactic center. At its closest approach, known as periapsis, it comes within 1.78 billion kilometers of the black hole's event horizon—roughly 12 astronomical units, which is comparable to the distance between our Sun and Saturn.[1]

At that proximity, the gravitational forces accelerate S301 to blistering speeds. The observational data clocks the star at approximately 25,000 kilometers per second during its closest pass. That equates to roughly 55 million miles per hour, or more than 8 percent of the speed of light, making it the fastest known star in the Milky Way.

The mechanism that makes S301 so valuable to physicists is how it interacts with curved spacetime. According to Albert Einstein's general theory of relativity, a massive spinning object does not just sit in space; it twists the surrounding spacetime like a spoon stirring honey. This "frame-dragging" effect alters the orbits of anything passing close by.[2]

Until now, astronomers have relied on other stars, like the well-documented S2, to test relativity. S2's 16-year orbit successfully proved that light loses energy climbing out of a gravity well and that elliptical orbits slowly rotate over time. However, those effects only measure a black hole's mass. Because the frame-dragging effect fades exponentially with distance, S2 never gets close enough to feel the black hole's spin.[1]

S301 crosses that threshold. It is the first star to orbit directly in the region around Sagittarius A* where the frame-dragging effect is extreme. The star acts as a luminous test particle, allowing researchers to measure not just the curvature of spacetime, but its active distortion by rotation.

Despite the robust orbital data, the evidence regarding the black hole's actual spin remains prospective. The current dataset maps the star's position from its initial detection in 2023, combined with archival sightings extracted from 2021 and 2017 observations. This is enough to confirm the orbit, but not yet enough to measure the subtle deviations caused by frame-dragging.[1][2]

The uncertainty lies in the timeline. To isolate the spin effect from other gravitational noise, astronomers need to observe the star through multiple close approaches. The next periapsis will occur in 2031. Researchers explicitly state that it will take at least another decade of continuous tracking to accumulate the precision required to calculate a definitive spin rate.[1]

The origin of S301 also provides secondary evidence for violent galactic dynamics. Based on its faint luminosity, astronomers calculate that S301 is a main-sequence star with only 1.1 to 1.5 times the mass of our Sun. It is far too small to have formed in the turbulent, radiation-soaked environment near the supermassive black hole.[1]

Instead, dynamicists point to the Hills mechanism. The data strongly suggests S301 was originally part of a tight binary star system that wandered too close to Sgr A*. The immense tidal forces ripped the pair apart, flinging one star out of the galaxy entirely as a hypervelocity projectile, while capturing S301 in its current, highly elongated orbit.[1]

Looking ahead, the observational campaign will shift to next-generation hardware. While the VLTI's GRAVITY instrument provided the initial discovery, future tracking will rely on the MICADO instrument slated for the upcoming Extremely Large Telescope (ELT). This will allow astronomers to reconstruct S301's three-dimensional motion with unprecedented fidelity.[2]

If successful, the measurement will test the "no-hair theorem," a foundational concept in astrophysics which posits that isolated black holes can be entirely described by just three properties: mass, charge, and spin. Having already measured the mass of Sgr A*, securing its spin rate would close a major gap in our understanding of the cosmic engine at the center of our galaxy.[2]