The Galactic Center: Scientific Research Evolution

Early Observations & Identification (Early 20th Century - 1960s)

Early 20th Century: Harlow Shapley's work on globular clusters revises the understanding of the Milky Way's size and places the Sun far from the center, locating the center roughly in the direction of the constellation Sagittarius. Direct optical observation is hampered by immense amounts of interstellar dust and gas.
1933: Karl Jansky, while investigating radio static for Bell Labs, discovers radio waves emanating from the direction of Sagittarius, identifying the first astronomical radio source outside the Solar System, later recognized as originating from the Galactic Center region.
1950s: Radio astronomy develops rapidly. Mapping reveals complex structures and intense radio emission from the Sagittarius region, confirming it as the galactic core. The source Sagittarius A (Sgr A) is identified as a bright, complex radio feature near the dynamical center.
1960s: Infrared astronomy begins to penetrate the dust clouds, revealing a dense cluster of stars at the center.

1971: Downes and Martin identify intense infrared emission from the core.
1974: Bruce Balick and Robert L. Brown use interferometry at the National Radio Astronomy Observatory (NRAO) to discover an extremely bright, compact, and unresolved radio source within Sgr A. They designate this specific point source Sagittarius A* (Sgr A*). Its nature is initially unknown but highly energetic.
Late 1970s - 1980s: Speculation grows that Sgr A* could be a Supermassive Black Hole (SMBH), based on its compactness and energy output. Theoretical models suggest SMBHs reside at the centers of most large galaxies. Infrared observations reveal high stellar velocities near the center, hinting at a massive, unseen object. X-ray observations (e.g., Einstein Observatory) detect high-energy emission from the region.

1990s: Advancements in high-resolution infrared imaging (speckle interferometry, adaptive optics) on large ground-based telescopes (like Keck, VLT) allow astronomers to pierce the dust and track the proper motions of individual stars orbiting very close to Sgr A*.
Late 1990s - Early 2000s: Two independent teams, one led by Reinhard Genzel (using ESO's VLT) and another by Andrea Ghez (using Keck Observatory), meticulously track stellar orbits over years. The star S2 (or S0-2) proves crucial, showing a complete, fast elliptical orbit around an unseen point.
2002 onward: Analysis of these stellar orbits using Kepler's Laws provides definitive evidence for a central dark mass confined within a very small volume. The calculated mass is enormous – around 4 million times the mass of the Sun. This effectively rules out alternatives like dense star clusters and confirms Sgr A* is an SMBH.
2000s: Observations across the spectrum (Chandra X-ray Observatory, VLA, infrared telescopes) study flares and variability from Sgr A*, probing the accretion environment immediately surrounding the black hole. Discovery of complex structures like gas streamers and young, massive star clusters near the center.

2010s: Continued monitoring of stellar orbits allows for increasingly precise mass measurements of Sgr A*. The Event Horizon Telescope (EHT) collaboration, linking radio telescopes worldwide to create a virtual Earth-sized telescope, begins observing Sgr A* with the goal of resolving its event horizon shadow.
2018: The GRAVITY instrument on the VLT detects the gravitational redshift (light stretched to longer wavelengths by strong gravity) in the light of star S2 as it makes its closest approach (periastron) to Sgr A*, providing a direct test of Einstein's General Relativity in the extreme gravitational field near an SMBH. Schwarzschild precession (a shift in the orbit's orientation predicted by GR) is also later detected for S2.
May 12, 2022: The Event Horizon Telescope collaboration releases the first direct image of Sagittarius A*. The image shows a dark central region (the black hole's shadow) surrounded by a bright ring of glowing gas and plasma, shaped by the black hole's intense gravity. This visually confirms the presence of the SMBH and provides further tests of General Relativity.
Ongoing: Research focuses on refining the EHT image, studying the dynamics of gas clouds (like G2) interacting with Sgr A*, understanding accretion physics in this low-luminosity black hole, exploring the properties of the surrounding dense nuclear star cluster, magnetic fields, and the relationship between Sgr A* and the evolution of the Milky Way.

Improving the resolution and detail of the Sgr A* image with EHT.
Testing General Relativity and alternative theories of gravity in the strong-field regime using stellar orbits and EHT data.
Understanding the accretion flow onto Sgr A* and the origin of its variability (flares).
Investigating star formation processes under the extreme conditions near the SMBH.
Searching for intermediate-mass black holes within the central star cluster.
Mapping the magnetic fields near the event horizon.