Sagittarius A*: The Supermassive Black Hole at the Centre of Our Galaxy We Are All Orbiting

Sagittarius A* has a mass of approximately 4.15 million times that of our Sun, packed into a region smaller than the distance between the Sun and the planet Mercury. That figure comes from decades of tracking stars orbiting the galactic centre, most famously the star S2, which completes one full orbit around Sagittarius A* every 16 years. Astronomers Andrea Ghez at UCLA and Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics spent over 25 years measuring those stellar paths. In 2020, they shared the Nobel Prize in Physics for confirming that the object at the centre of our galaxy is a supermassive black hole.

The event horizon, the point of no return, spans roughly 12 million kilometres across. By cosmic standards, that is compact. The gravity it generates, however, governs the motion of billions of stars across a structure 100,000 light-years wide.

The Sun sits about 26,000 light-years from Sagittarius A*, in a region of the Milky Way called the Orion Arm. At that distance, the galactic orbit takes roughly 225 million years to complete, a unit sometimes called a cosmic year or galactic year. The solar system is currently travelling at approximately 828,000 kilometres per hour around the galactic centre. That speed sounds extreme. Against the scale of the orbit, it is a slow crawl.

The last time Earth was at this position in its galactic orbit, the Triassic period was beginning and dinosaurs had not yet appeared. One orbit before that, the planet did not yet have complex animal life. The orbit of our solar system around Sagittarius A* is one of the longest clocks in nature, and it has been running without interruption.

On 12 May 2022, the Event Horizon Telescope collaboration released the first direct image of Sagittarius A*. The image showed a bright ring of superheated gas surrounding a dark central region, the shadow of the black hole itself. It was the second black hole ever directly imaged, after M87* in 2019, but it was the first image of the black hole that anchors our own galaxy.

Producing the image required synchronising radio telescopes across four continents into a single virtual dish the size of Earth. One of the participating facilities was the South Pole Telescope. The data took years to process. The difficulty was that Sagittarius A* is far more turbulent than M87*, the gas around it shifts on timescales of minutes, compared to days for the larger black hole. The collaboration had to develop new imaging techniques to stabilise the signal.

India's astronomical contribution to understanding the galactic centre runs through facilities like the Giant Metrewave Radio Telescope near Pune, operated by the National Centre for Radio Astrophysics. The GMRT has been used to study radio emissions from the galactic centre region and to probe the structure of the interstellar medium between us and Sagittarius A*. It remains one of the most sensitive low-frequency radio telescopes in the world.

ISRO's AstroSat, launched in 2015, carries instruments for X-ray and ultraviolet observation. X-ray flares from the direction of Sagittarius A*, bursts of energy that occur when gas falls toward the black hole, are among the phenomena AstroSat-class missions are designed to study. India's space science programme is not peripheral to this field. The galactic centre is one of the most X-ray-bright regions of the sky, and Indian instrumentation has a direct line of sight to it.

The reason Sagittarius A* is easy to forget is that its influence on us is entirely invisible at human scales. The gravitational pull of the black hole on our solar system is real but vanishingly small compared to the Sun's local gravity. We feel the Sun. We orbit Sagittarius A* the way a ship at sea feels the Moon's tidal pull, present in the mathematics, absent from the body's experience.

The stars closest to Sagittarius A*, the S-cluster stars, move at a fraction of the speed of light. S2 reaches about 2.7 percent of the speed of light at its closest approach. Our solar system, at 26,000 light-years out, moves at a tiny fraction of that. Distance from the black hole is, in the most literal sense, distance from its grip.

What the image of Sagittarius A* settled is not just a scientific question about what sits at the galactic centre. Every model of how the Milky Way formed, how its spiral arms maintain their shape, how stellar populations distribute themselves across 100,000 light-years, all of it depends on the mass and position of the object at the centre being known precisely. The 2022 image fixed that anchor point with a clarity no prior measurement had achieved. Our galaxy's shape is, in the end, a consequence of that one object's gravity, working outward across every stellar orbit including ours.