The newly identified star holds a dual record: it is both the fastest star ever spotted and the closest known star to a supermassive black hole. Stefan Gillessen and his team at the Max Planck Institute for Extraterrestrial Physics in Germany tracked the S301 star using the Very Large Telescope in Chile. They have monitored it since 2023 to map its unprecedented orbit around the Milky Way center.
Extreme Gravity at the Milky Way Center
At its closest approach, the S301 star comes within 1.8 billion kilometres of the Sagittarius A* black hole. This distance is just 12 times the gap between Earth and the sun, placing it 10 times closer to the supermassive black hole than the previous record-holder. Traveling at more than 8 percent the speed of light—roughly 24,000 kilometers per second—the star completes its orbital dance in a highly warped region of space.
Sagittarius A* sits at the very heart of our galaxy, boasting a mass roughly 4 million times that of our sun. The gravitational well created by such an immense object dominates the dynamics of the entire Milky Way. For decades, astronomers have watched stars whip around this invisible point in space, providing the strongest evidence for the existence of supermassive black holes. The S301 star, however, ventures into territory previously thought too chaotic for stable stellar orbits.
This proximity plunges the S301 star into one of the most extreme gravitational environments in the universe. The intense gravity near a black hole stretches and warps the fabric of space-time. If the black hole is spinning, it should twist space-time around it in a phenomenon known as the frame dragging effect. This effect, predicted by Einstein’s theory of general relativity, has never been measured around a supermassive black hole.
“If you were living on a planet around this star, the size of the black hole at the closest approach would appear similar to the full moon from Earth – it would be absolutely stunning,” says Gillessen. The sheer scale of the black hole looming in the sky highlights the extreme conditions the S301 star endures.
Unprecedented Orbital Speeds
The velocity of the S301 star is a direct consequence of Kepler’s laws of planetary motion applied to extreme masses. As the star plunges toward the Sagittarius A* black hole, the gravitational pull accelerates it to tremendous speeds. At 8 percent of the speed of light, the star is moving fast enough to cross the distance between Earth and the moon in less than two seconds. This speed makes it an incredibly difficult target to track, requiring the advanced adaptive optics of the Very Large Telescope and its GRAVITY interferometer instrument.
The GRAVITY collaboration has been instrumental in observing stars near the Milky Way center. By combining light from four separate telescopes, the instrument achieves a resolution equivalent to a single mirror hundreds of meters across. This technological leap is what allowed Gillessen and his colleagues to distinguish the S301 star from the crowded stellar field at the galactic core. Prior to this, the star S2 held the record for the closest approach to Sagittarius A*, providing the first successful test of gravitational redshift in a supermassive black hole environment.
Measuring the Black Hole Spin
While measuring the spin of a body like Earth is relatively straightforward using satellite data, determining a black hole spin is notoriously difficult because it lacks a visible surface. The most precise method to measure this property is by observing the frame dragging effect, and the S301 star is the first discovered close enough to feel it directly.
Tuan Do at the University of California, Los Angeles, who was not involved in the research, explains the significance of this discovery. “Earth does this as well, and we can measure with satellites around Earth that there is a very, very slight frame dragging from Earth’s rotation. This is doing the same thing, just with much different objects.”
Gillessen uses a simple analogy to describe how the S301 star will help scientists map the invisible forces at play. “We drop a leaf in the wind and see how the air is moving by measuring that leaf. A star is just the perfect leaf to drop to see the movement of space-time.”
A Decade of Observation Ahead
Actually measuring the black hole spin of Sagittarius A* using the S301 star will probably take around a decade, according to Gillessen. The astronomical team needs to track the star’s orbit over multiple passes to distinguish the subtle shifts caused by the black hole’s rotation from other gravitational perturbations. However, this timeline could accelerate if astronomers find additional stars in similar orbits near the Milky Way center.
“With one star, it would take a while, but it would still be the best constraint on spin that we’ve ever had by far,” says Ziri Younsi at University College London. “If you can find another star that’s even closer, that’s better still. If you can find a population of these stars, then you’re in business.”
Gillessen and his team currently have several candidates for stars orbiting slightly further from the Sagittarius A* black hole than the S301 star, but none that are closer. The search continues for more of these extreme objects.
What Happens Next
Once scientists successfully measure the black hole spin of Sagittarius A*, it will provide a crucial piece of the cosmic jigsaw. The data will not only illuminate how black holes have shaped the evolution of the universe, but it will also offer a rare probe into the behaviour of gravity in extreme environments that have proved extremely difficult to study.
Understanding these properties could fundamentally shift our grasp of astrophysics and cosmology. A spinning black hole carries an immense amount of rotational energy, which can influence the magnetic fields and jets that shoot out from the galactic core. By quantifying this spin, researchers can better model how Sagittarius A* has interacted with the Milky Way over billions of years.
In 2020, scientists Reinhard Genzel and Andrea Ghez received the Nobel Prize in Physics for their decades of work measuring the mass of the Sagittarius A* black hole. “A black hole only has three measurable properties: a mass, a spin and possibly an electric charge. The mass was worth a Nobel prize in 2020, so, if you find the spin, you might expect a call from Stockholm in 20 years,” says Gillessen.
— Priya Nair, science desk, AXO News