Decoding the Milky Way’s Supermassive Black Hole: The Nobel-Winning Star Orbit

According to European and Keck Observatory astronomers, a young blue star designated S2 or S0-2 orbits Sagittarius A*, the supermassive black hole at the center of the Milky Way, reaching speeds of roughly 7,650 kilometers per second—about 2.55 percent of the speed of light—providing a direct test of Einstein’s theory of general relativity.

Tracking the Invisible Monster at the Galactic Core

Near the center of the Milky Way, roughly 26,000 light-years from Earth, S2 repeatedly falls toward an unseen object, accelerates dramatically, and swings away again. At its peak speed during its 2018 pericentre passage, the star could cross the distance from Earth to the Moon in under a minute, according to data from the GRAVITY Collaboration.

Ordinary visible light telescopes cannot view this region because intervening cosmic dust absorbs visible wavelengths. To pierce this barrier, Reinhard Genzel’s team at the European Southern Observatory (ESO) in Chile and Andrea Ghez’s team at the Keck Observatory in Hawaii utilized infrared observations. Over nearly three decades, these independent programs tracked stellar orbits to weigh the central mass, eliminating alternative explanations like dense neutron star clusters in favor of a single supermassive black hole.

Weighing Sagittarius A* Through Stellar Orbits

Astronomers calculate the mass of Sagittarius A* by analyzing the size, shape, and speed of S2’s orbit. Measurements show that roughly 4.3 million times the mass of the Sun is concentrated inside S2’s closest approach distance of approximately 120 astronomical units, according to multi-star orbital analyses by the GRAVITY Collaboration.

While a dense cluster of stellar black holes or neutron stars could theoretically account for heavy invisible mass, such an arrangement would prove unstable, quickly colliding or collapsing. The enduring survival of the observed orbits confirms a single supermassive black hole is present, a finding that earned half of the 2020 Nobel Prize in Physics for Roger Penrose, with the other half divided between Genzel and Ghez.

Testing General Relativity Near the Event Horizon

S2’s extreme velocity near pericentre—reaching roughly 7,650 kilometers per second—allows researchers to test Albert Einstein’s predictions in a gravitational field far stronger than any found in our Solar System. In 2018, astronomers detected a combination of gravitational redshift and the transverse Doppler effect as the star’s light lost energy climbing out of the black hole’s gravitational well.

Furthermore, long-term observation timelines revealed Schwarzschild precession. As reported by the GRAVITY team in 2020, S2 does not return along a perfectly closed ellipse; instead, its point of closest approach advances, rotating the orbit into a slow rosette pattern by about 12 arcminutes per orbit.

Connecting Stellar Orbits to the Event Horizon Image

For decades, stellar orbits served as the premier evidence for the galactic center’s dark heart. That evidence expanded in 2022 when the Event Horizon Telescope collaboration linked global radio observatories to produce the first horizon-scale image of Sagittarius A*.

While S2 traces gravity across tens of billions of kilometers, the Event Horizon Telescope images the immediate plasma glow and shadow surrounding the event horizon itself. Together, these observations bridge macro-scale orbital mechanics with micro-scale imaging of spacetime curvature.

Frequently Asked Questions

How fast does the star S2 travel around Sagittarius A*?

At its closest approach, S2 accelerates to about 7,650 kilometers per second, which is roughly 2.55 percent of the speed of light.

What is Sagittarius A*?

Sagittarius A* is the supermassive black hole located at the exact center of the Milky Way galaxy, possessing a mass equivalent to about 4.3 million Suns.

How do astronomers weigh a black hole that emits no light?

According to astronomical research teams, scientists measure the gravitational pull of an invisible object by tracking the orbits, speeds, and trajectories of surrounding stars like S2.

Did the Event Horizon Telescope replace the need for stellar orbit studies?

No. While the Event Horizon Telescope imaged the immediate shadow of Sagittarius A* in 2022, decades of stellar orbital data from the ESO and Keck Observatory provide complementary verification across vastly different physical scales.