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The star S301 reaches about 25,000 km/s while orbiting Sagittarius A* and may assist scientists in measuring the rotation of the Milky Way’s central black hole.
A star that circulates through the extreme environment surrounding the supermassive black hole at the center of the Milky Way has been identified as the fastest ever observed in the galaxy. Named S301, it can reach approximately 25,000 kilometers per second, equivalent to about 8.3% the speed of light, as it passes through the regions closest to Sagittarius A*.
The scale of this speed becomes clearer when compared to Earth. If one could travel at the maximum speed reached by S301, a full trip around the Equator would take about 1.6 seconds. Maintaining that pace for one minute would allow you to circle the planet more than 37 times.
Star Accelerates as It Approaches Sagittarius A*
The star’s speed is not constant throughout its trajectory. S301 follows a highly elongated orbit and accelerates as it enters the region where the black hole’s gravity becomes more intense, reaching its maximum speed just during its closest approach.
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The black hole responsible for this effect is Sagittarius A*, a supermassive object located at the center of the Milky Way. The enormous concentration of mass creates a gravitational field capable of keeping several stars in extremely fast and compact orbits around it.
In the case of S301, the combination of proximity and speed has made the object particularly interesting to astronomers. The star not only breaks a movement record but also traverses a region where extreme gravitational effects can be measured with great precision.
Thus, tracking its trajectory over the coming years may provide data that goes beyond the star itself. Its motion could act as a natural marker to study properties that are still challenging to determine directly in Sagittarius A*.
S301 Completes an Orbit in Just 8.7 Years
A complete orbit of S301 around the black hole takes approximately 8.7 years. This interval is extremely short in astronomical terms and allows researchers to track significant changes in trajectory within relatively accessible observation periods.
The point of closest approach is even more impressive. The star gets to a distance equivalent to about 12 times the space between Earth and the Sun, approximately 1.8 billion kilometers, before moving away again.
Although billions of kilometers may seem like a vast distance in human scales, it is a very close passage compared to the dimensions of the system surrounding a supermassive black hole. It is precisely in this region that S301 receives the greatest gravitational acceleration.
Felix Mang, a PhD student at the Max Planck Institute for Extraterrestrial Physics (MPE) and author of the study, emphasized that the combination of an 8.7-year orbit and such a close approach makes this object different from others that have been tracked in this region.
First observation occurred in 2023
S301 came onto the team’s radar in 2023, when astronomers managed to identify it for the first time. Subsequent observations allowed for a more precise determination of its position and the calculation of parameters needed to reconstruct its orbit.
Researchers also utilized previous data to backtrack its trajectory to 2017. This reconstruction showed that the star passed the closest approach to Sagittarius A* in early 2023, precisely during one of its most significant phases for study.
The subsequent identification of past positions is crucial because a single observation does not allow for the complete determination of an object’s orbit. By comparing different moments, scientists can calculate velocity, distance, and the shape of the path around the black hole.
Thus, what initially appeared as a new bright point in an extremely crowded region of the Milky Way has become one of the most important stars for future investigations into the galactic center.
GRAVITY can separate movements at the center of the Milky Way
Observing a star so close to Sagittarius A* requires equipment capable of working in an area where many objects appear concentrated in a relatively small section of the sky. For this, the team utilized the GRAVITY instrument.
The equipment is installed at the Very Large Telescope Interferometer (VLTI), associated with the European Southern Observatory. Its technique combines light captured by different telescopes, creating conditions to determine positions and movements with accuracy far superior to that obtained by a single instrument.
This capability is especially important at the galaxy’s center, where stars pass close to one another when viewed from Earth. Small positional errors could significantly alter the calculations of the trajectory around the black hole.
It was the precise tracking provided by GRAVITY that allowed researchers to consolidate the orbital characteristics of S301 and identify the extreme speed achieved during its closest approach.
Movement may reveal how the black hole spins
The next step could turn S301 into a tool for investigating a particularly challenging property of Sagittarius A*: its rotation. The way a black hole spins can cause small changes in the space around it, and consequently, in the orbits of nearby objects.
Since S301 passes very close to the gravitational center, its trajectory provides an opportunity to search for these alterations. The closer a star comes to the black hole, the greater the chance of detecting subtle relativistic effects in its motion.
Mang states that these characteristics could allow for a very direct measurement of the rotation of a massive black hole. If this outcome can be achieved, the data could be compared with predictions made by general relativity.
Star May Put Einstein’s Predictions to a New Test
The theory of general relativity explains how large masses deform spacetime and alter the motion of nearby objects. Black holes represent some of the most extreme environments to test these predictions because they concentrate an enormous amount of mass in very compact regions.
The orbit of S301 could allow astronomers to search for effects that would be nearly impossible to detect in less extreme systems. Small changes accumulated with each pass could reveal how Sagittarius A* influences the surrounding space.
Since the star completes its orbit in just 8.7 years, researchers do not have to wait decades to observe a full cycle. This makes the object especially valuable compared to stars that take much longer to return to the same region.
