A star near the centre of our galaxy reaches almost 3 per cent of the speed of light as it is whipped around something four million times the mass of the Sun that emits no light of its own — and tracking its orbit for nearly three decades helped two astronomers wi
At the center of our galaxy, 26,000 light-years away, a young blue star known as S2 performs a maneuver that defies common expectations. This star falls toward an invisible point, reaching speeds of 7,650 kilometers per second. At this velocity, the star hits nearly 3 per cent of the speed of light. It orbits an object with a mass four million times that of our Sun, which stays completely dark. Because this object emits no light, it remained hidden from early telescopes until astronomers began tracking the star's path.
For nearly thirty years, independent research teams led by Reinhard Genzel and Andrea Ghez observed S2 and its neighbors. Dust clouds between Earth and the galactic core block visible light, so researchers used infrared observations to cut through the interference. They refined their methods over decades, moving from basic speckle imaging to the precise adaptive optics and interferometry used by the GRAVITY instrument. These technological leaps allowed them to measure the star's position with extreme accuracy night after night.
Tracking the star provided a way to weigh the invisible object. By measuring the orbit, its shape, and the speed of the star, physicists calculated the gravitational pull at the center of the Milky Way. This work demonstrated that a cluster of smaller objects would be unstable and collapse. Only a single supermassive black hole accounts for the data. The orbit of S2 takes about 16 years, making it a valuable tool for these calculations compared to the 200 million years our own Sun takes to circle the galaxy.
This long-term observation eventually served as a rigorous test for Einstein’s general relativity. Near the point of closest approach, the star's light shifts due to gravitational effects. Observations confirmed that the orbit rotates over time, a phenomenon known as Schwarzschild precession. These precise measurements of the star's movement were the foundation for the 2020 Nobel Prize in Physics, which recognized the discovery of a supermassive compact object at the heart of our galaxy. The work remains a primary example of how sustained, patient measurement reveals physical truths that cannot be seen directly.