The Milky Way is our home galaxy, a vast collection of stars, gas, dust and dark matter that spans about 100,000 light-years across. But despite its size and familiarity, there is still much we don’t know about it. One of the most intriguing mysteries is what lies at its center, where a supermassive black hole called Sagittarius A* (Sgr A*) lurks.

MeerKAT data of the orientations of all the filaments, color-coded with position angles. (Farhad Yusef-Zadeh/Northwestern University)

Astronomers have been studying the region around Sgr A* for decades, using various telescopes and instruments to probe its secrets. Recently, they have discovered hundreds of mysterious structures that appear to be orbiting the black hole, some of them moving at incredible speeds. These structures are called S-stars, and they are among the most extreme objects in the universe.

S-stars are not ordinary stars. They are young, massive and hot, with temperatures ranging from 10,000 to 40,000 Kelvin. They are also very close to the black hole, within a few light-hours or less. This means they experience intense gravitational forces that distort their shapes and cause them to emit powerful radiation. Some of them also interact with each other and with the surrounding gas and dust, creating spectacular flares and jets.

But how did these S-stars form and end up in such a hostile environment? This is the question that puzzles astronomers and challenges our understanding of star formation and evolution. There are two main hypotheses that attempt to explain their origin.

All the filaments discovered to date, color-coded with position angles. (Farhad Yusef-Zadeh/Northwestern University)

One hypothesis is that the S-stars were 𝐛𝐨𝐫𝐧 far away from the black hole, in a dense cluster of stars called the Nuclear Star Cluster (NSC). The NSC is located about 26 light-years from Sgr A*, and it contains millions of stars of different ages and types. Some of these stars may have been ejected from the cluster due to gravitational interactions with other stars or binary systems. These ejected stars may have then been captured by the black hole’s gravity and settled into orbits around it.

Another hypothesis is that the S-stars were 𝐛𝐨𝐫𝐧 near the black hole, in a disk of gas and dust that surrounds it. This disk is called the Circumnuclear Disk (CND), and it extends from about 1.5 to 4.5 light-years from Sgr A*. The CND may have formed from material that was accreted by the black hole from its surroundings. Within this disk, gas and dust may have condensed into clumps that eventually collapsed into stars.

Both hypotheses have their strengths and weaknesses, and neither can fully account for all the observed properties of the S-stars. For example, the first hypothesis can explain why some of the S-stars have high eccentricities (meaning they have very elongated orbits), but it cannot explain why some of them have low eccentricities (meaning they have more circular orbits). The second hypothesis can explain why some of the S-stars have low eccentricities, but it cannot explain why some of them have high eccentricities.

To solve this puzzle, astronomers need more data and more sophisticated models. They also need to observe the S-stars over longer periods of time, to track their motions and interactions. This is not an easy task, as the center of the Milky Way is obscured by dust and gas that block most of the visible light. Therefore, astronomers have to use infrared or radio telescopes that can penetrate through this veil.

One of the most powerful instruments for studying the S-stars is the GRAVITY interferometer, which combines the light from four 8-meter telescopes at the Very Large Telescope (VLT) in Chile. GRAVITY can achieve an unprecedented resolution and sensitivity in the infrared range, allowing astronomers to measure the positions and velocities of the S-stars with great accuracy. GRAVITY has already made some remarkable discoveries, such as detecting a flare from Sgr A* that was associated with one of the S-stars passing close to it in 2018.

Another promising instrument for studying the S-stars is the Event Horizon Telescope (EHT), which is a global network of radio telescopes that work together as one giant telescope. The EHT can achieve an even higher resolution than GRAVITY, enough to image the shadow of the black hole itself. The EHT has already produced the first image of a black hole in another galaxy in 2019, and it aims to do the same for Sgr A* in the near future.

The S-stars are fascinating objects that offer us a unique opportunity to study the physics and astrophysics of extreme environments. By observing them, we can learn more about how stars form and evolve near supermassive black holes, how these black holes influence their surroundings, and how gravity behaves in such strong fields. The S-stars are also potential sources of gravitational waves, which are ripples in space-time that are generated by accelerating masses. If detected, these waves could provide us with new information about the nature of gravity and the structure of space-time.

The S-stars are a treasure trove of scientific discoveries waiting to be made. They are also a reminder of how much we still don’t know about our own galaxy, and how much more there is to explore. The S-stars are truly hundreds of mystery structures at the heart of the Milky Way.

Reference(s): Astrophysical Journal Letters