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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11A 2024 study estimates that Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, spins at about 60% of the theoretical maximum angular velocity. NASA’s summary of the study also puts its angular momentum at about 90% of the maximum. These are different quantities, and both figures come from an indirect estimate—not a direct observation of the event horizon turning.
What the 2024 result says—and what it does not
Sgr A* lies about 26,000 light-years from Earth. The 2024 estimate suggests it is rotating rapidly, but it does not establish an exact, uncontested spin value. Earlier measurement methods have produced estimates ranging from little or no spin to nearly maximal spin, so the new result adds evidence to a longstanding debate rather than ending it.
“About 60%” refers to angular velocity in the study’s reported comparison with the theoretical maximum. NASA’s companion summary separately reports angular momentum at about 90% of its maximum. Angular velocity describes how quickly something rotates; angular momentum also depends on the object’s mass and how that mass is distributed. The two percentages should not be treated as interchangeable measures.
How astronomers estimated Sgr A*’s spin
The outflow method
The study, led by Ruth Daly of Penn State, used an empirically based approach known as the outflow method. X-ray observations trace hot gas in the disk around Sgr A*, while radio observations trace a collimated outflow. Researchers combined those emissions with an independent estimate of the black hole’s mass to constrain its spin.
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This is an inference from the environment around the black hole, not a measurement made by watching the event horizon rotate. The distinction matters because the result depends on interpreting the gas and outflow signals through the method used. Other approaches have yielded a broad range of spin estimates.
Why a rapidly spinning black hole matters
Spin changes the space around a black hole
Under general relativity, a spinning black hole drags nearby spacetime along with it, an effect called frame dragging. Greater spin also makes the surrounding spacetime more flattened—roughly football-shaped when viewed from the side—than it would be around a non-spinning black hole.
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Spin can help power outflows, given the right conditions
A black hole’s rotational energy can help power narrow outflows or jets when matter and magnetic fields are present in suitable conditions. Sgr A* is comparatively quiet now because its nearby supply of fuel is limited. If more matter becomes available in the future, outflows could become stronger; a high spin alone does not guarantee a bright or powerful jet.
Co-author Biny Sebastian of the University of Manitoba compared a spinning black hole to a rocket waiting on a launch pad: material getting close enough can act like fuel and trigger an outflow. The analogy describes a possible energy source, not a prediction that Sgr A* is about to become active.
What S301 could reveal about the spin
An ESO announcement in 2026 described S301, the fastest known star in the Milky Way, on a close orbit around Sgr A*. ESO reports that Sgr A* has a mass of about 4 million Suns. S301 completes an orbit in 8.7 years, comes within about 1.78 billion kilometres—roughly 12 times the Earth–Sun distance—and reaches about 25,000 kilometres per second, more than 8% of the speed of light.
At that close approach, S301’s orbit is close enough to be affected by frame dragging. Tracking those effects could offer a more direct way to constrain the black hole’s spin than the 2024 outflow-based estimate.
| Approach | What astronomers observe | What it can establish | Status |
|---|---|---|---|
| 2024 outflow method | X-rays from hot gas around Sgr A* and radio emission from its collimated outflow, combined with an independent mass estimate | An indirect spin estimate: about 60% of maximum angular velocity; NASA’s summary gives about 90% of maximum angular momentum | Published estimate; earlier methods have disagreed |
| S301 orbital tracking | Precision measurements of the star’s orbit and frame-dragging effects | Could constrain the spin more directly if observations capture enough of the orbit | Prospective: continued GRAVITY+ observations and future ELT/MICADO observations are expected to advance the effort |
The direct measurement is still ahead
The S301 opportunity is a future test, not a new direct spin measurement. Continued GRAVITY+ observations and future observations with the Extremely Large Telescope’s MICADO instrument could track two full orbits. S301’s next close passage in 2031 is part of that longer effort. Max Planck Institute for Extraterrestrial Physics researcher Stefan Gillessen has described the prospect as a way to measure a massive black hole’s spin very directly and test Einstein’s theory.
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