Sagittarius A* cluster

Lore & Background

The cluster consists of stars in close orbit around Sagittarius A*. Their names are not formalized, and stars can have different numbers in different catalogues. Inferred orbits of stars are according to Gillessen et al. 2017, with the exception of S2 (from GRAVITY 2019) and S4711 through S4715 (from Peißker et al., Aug 2020). Data for S62, reported in Peißker et al. Jan 2020, was later found to be further than expected from Sagittarius A* and is now thought to be due to a mistaken observation of S29. Data for S301 are from Abd El Dayem et al. (2026); because its radial velocity has not yet been measured, two possible orbital orientations remain, and the first solution reported in the study is used here. Binary star system D9 was recently found in the S Cluster.

Reader's Guide

The Sagittarius A* cluster is significant as the stellar population immediately surrounding the supermassive black hole at the center of the Milky Way. Its stars, known as S-stars, provide a direct laboratory for testing general relativity and stellar dynamics in extreme gravitational fields. The most studied star, S2, is relatively bright and passes close by Sgr A*. In 2026, S301 was reported as the closest and fastest securely observed star, approaching to about 12 AU from the black hole and reaching approximately 8.5% of the speed of light at pericentre, surpassing the previous record holder S4714. The cluster's legacy includes ongoing refinements of orbital data, as seen with the correction of S62's observation to S29, and the recent discovery of the binary star system D9 within the S Cluster.

Did You Know?

• The individual stars in the cluster are often listed as 'S-stars', but their names and IDs are not formalized.
• S2 is one of the most studied stars in the cluster and is relatively bright.

From Radio Whispers to a Galactic Anchor

In the spring of 1933, Karl Jansky detected a faint radio signal emanating from the direction of Sagittarius, pointing toward the heart of our galaxy. Though his measurements fell short of the exact galactic center, the finding planted a seed. Years later, Jack Piddington and Harry Minnett, working with CSIRO telescopes in Sydney, isolated a discrete bright source they identified in a letter to Nature as the probable Galactic Center. The true compact core, however, eluded detection until February 1974, when Bruce Balick and Robert L. Brown, using the National Radio Astronomy Observatory's baseline interferometer, pinpointed an intensely bright, extremely compact radio emitter nested within the broader Sagittarius A source. Brown later christened it Sgr A* in a 1982 paper, choosing the asterisk because the source was "exciting" and excited atomic states carry that same symbol. From Earth, this object sits roughly 5.6 degrees south of the ecliptic, nestled near the border of Sagittarius and Scorpius, in the visual neighborhood of the Butterfly Cluster and Lambda Scorpii.

Capturing the Unseen: The 2022 Image

On May 12, 2022, the Event Horizon Telescope Collaboration unveiled the first-ever image of the region surrounding Sagittarius A*, built from radio interferometer data gathered in 2017. Processing that data consumed roughly five years of computation. The observation drew on eight radio observatories spread across six geographical sites, and the team produced the image through aperture synthesis. A major complication was that the radio emission from Sgr A* fluctuates on timescales of mere minutes, making the analysis far more demanding than for stable sources. What the image actually reveals is not the black hole itself—no light can escape its gravitational grip—but the glowing ring of gas and dust heated to millions of degrees as it spirals inward. The measured angular diameter of 51.8 ± 2.3 microarcseconds, at a distance of about 26,000 light-years, translates to roughly 51.8 million kilometres. This was the second confirmed black hole image, following Messier 87 in 2019, and the measurement tested Einstein's general relativity more rigorously than any prior observation, with results in perfect agreement.

Stellar Orbits and the Weight of Evidence

For decades, the case for a black hole at the galactic center grew through the motions of nearby stars. In 1994, a Berkeley team including Charles H. Townes and Reinhard Genzel used infrared and sub-millimetre spectroscopy to show that Sgr A*'s mass was tightly concentrated, on the order of three million solar masses. The definitive breakthrough came on October 16, 2002, when Genzel's international team at the Max Planck Institute for Extraterrestrial Physics published a ten-year tracking of star S2 as it swung close to the central object. The analysis used near-infrared interferometry at 2.1 micrometres, a wavelength chosen because interstellar extinction is lower there, and SiO masers helped align the infrared frames with radio observations. The rapid orbital motion of S2 and its companions stood out clearly against slower background stars, which could be subtracted. Crucially, the data excluded alternatives such as a cluster of dark stellar objects or a lump of degenerate fermions, leaving a supermassive black hole as the sole viable explanation. The current best mass estimate stands at 4.297 ± 0.012 million solar masses.

Nobel Recognition and the Physics of the Ring

The 2020 Nobel Prize in Physics recognized the decades-long effort to establish Sagittarius A* as a supermassive compact object. Reinhard Genzel and Andrea Ghez each received a quarter-share for demonstrating that a black hole was the only explanation for the observed stellar dynamics, while Sir Roger Penrose claimed the remaining half for proving that black hole formation is a robust prediction of general relativity. Beyond the prize, the physics surrounding Sgr A* continues to yield surprises. In 2019, measurements from the HAWC+ instrument aboard the SOFIA aircraft revealed that magnetic fields force the surrounding ring of gas and dust—spanning temperatures from roughly −280 to 17,500 °F—into a coherent orbit around the black hole, a configuration that helps keep its emissions comparatively subdued. Observing the object in visible light remains impossible from Earth because 25 magnitudes of dust and gas extinction absorb and scatter virtually all optical photons between us and the galactic center. The proper motion of Sgr A* is approximately −2.70 milliarcseconds per year in right ascension and −5.6 milliarcseconds per year in declination.