A star named S2 orbits Sagittarius A*, a supermassive black hole at the Milky Way’s center, reaching nearly 3% the speed of light. Tracking its 16-year journey helped astronomers Reinhard Genzel and Andrea Ghez win a Nobel Prize for confirming the black hole’s existence.
At the heart of the Milky Way, 26,000 light-years from Earth, a star named S2 performs a gravitational dance. Moving at roughly 2.55 per cent the speed of light—nearly 7,650 kilometers per second—S2 swings around Sagittarius A*, a supermassive black hole packing 4.3 million times the Sun’s mass. This extreme orbit, measured over three decades, not only confirmed the black hole’s presence but also earned Reinhard Genzel and Andrea Ghez the 2020 Nobel Prize in Physics.
The Star That Defied Expectations
S2’s journey around Sagittarius A* is a cosmic ballet. At its closest approach, called pericentre, the star ventures within 120 times the distance from Earth to the Sun of the black hole. There, it accelerates to 7,650 kilometers per second—enough to cross the Earth-Moon distance in under a minute. This speed, though a mere 2.55 per cent of light speed, is enough to test the effects predicted by Einstein, as the star’s motion reveals the black hole’s immense gravitational pull.
The star’s 16-year orbital period, remarkably short by galactic standards, allowed astronomers to map its path and calculate Sagittarius A*’s mass with precision. Astronomers could watch S2 finish an entire orbit within a working career.
How They Saw the Unseen
Observing S2 required overcoming two major hurdles: the dust blocking visible light and the blurring effect of Earth’s atmosphere. Genzel’s team at the European Southern Observatory and Ghez’s team at the Keck Observatory in Hawaii pioneered techniques to pierce this veil. Early efforts used speckle imaging, combining short exposures to recover detail. Later, adaptive optics corrected atmospheric distortions in real time, while the GRAVITY instrument combined light from four telescopes to increase the achievable image resolution by more than a thousandfold.
The GRAVITY Collaboration’s 2018 analysis placed it roughly 120 astronomical units from the black hole at that moment.
A confused blur of infrared light became individual moving stars whose positions could be measured night after night.
These advancements transformed the galactic center from an obscured mystery into a laboratory for testing general relativity.
Confirming the Black Hole
Sagittarius A* itself emits no light, but its gravitational influence on S2 and other stars left no doubt about its nature. The central gravitational field is dominated by this compact mass inside S2’s closest distance of roughly 120 astronomical units from the centre.
For decades, astronomers inferred the black hole’s existence from stellar orbits. In 1974, Bruce Balick and Robert L. Brown detected an unusually strong radio signal from the region, but it wasn’t until S2’s orbit was mapped that the evidence became irrefutable. Just about everyone was convinced that S2’s orbit could only be due to the tremendous gravitational pull of an SMBH.
A Nobel for the Invisible
Genzel and Ghez’s work culminated in the 2020 Nobel Prize in physics. In an interview with Science Friday, Ghez described the breakthrough: We developed techniques that allowed us to peer through galactic dust clouds with the Keck telescopes and observe the movements of stars orbiting the galactic center.
This confirmation came decades after the initial discovery of Sagittarius A*’s radio signal, underscoring the patience and precision required to unravel the galaxy’s deepest secrets.
What’s Next for the Milky Way’s Core?
In May 2022, the Event Horizon Telescope Collaboration released the first image of Sagittarius A*, which looks eerily like a blurred Eye of Sauron. For now, S2 continues its 16-year journey, a celestial clockwork that keeps proving Einstein’s theories. But for astronomers, the real challenge remains: understanding how supermassive black holes shape galaxies, including our own.
