BY:SpaceEyeNews.
The S2 star orbit around Sagittarius A* gave astronomers something extraordinary: a way to measure an object they could not see.
Near the centre of the Milky Way, S2 races around an invisible source of immense gravity. At its closest approach, the star reaches about 7,650 kilometres per second. That equals more than 25 million kilometres per hour, or almost 3% of the speed of light.
For nearly three decades, two independent research teams tracked this remarkable journey. Their measurements revealed that about four million solar masses sit inside a tiny region at the heart of our galaxy. The results also allowed scientists to test Albert Einstein’s theory of gravity in an extreme environment.
S2 therefore became far more than another star near the Galactic Centre. Its orbit helped reveal Sagittarius A* and contributed to research recognised by the 2020 Nobel Prize in Physics.
S2 Star Orbit Around Sagittarius A* Reveals an Invisible Mass
Astronomers do not need to see an object directly to measure its gravity. The path of an orbiting star records the gravitational pull acting upon it.
S2, also known as S0-2, provided an ideal natural measuring tool. Its highly elongated orbit takes just under 16 years to complete. That period is unusually short for a star moving around a galactic centre.
As a result, scientists could follow almost every stage of the orbit within their careers. They measured S2’s position, speed and changing direction. From those observations, they calculated the mass controlling its movement.
The answer pointed to an object containing roughly four million times the Sun’s mass.
Yet mass alone was not enough. Astronomers also needed to know how tightly that material was packed. S2 comes close enough to show that the mass must fit inside an extremely compact region.
The Nobel Prize’s official scientific background describes the stellar orbits as the strongest evidence for a supermassive compact object at the centre of the Milky Way. It also explains that a black hole provides the only known explanation that matches the observations.
Why S2 Reaches Almost 3% of Light Speed
S2 does not travel at a constant speed.
Its orbit is highly elliptical, so its distance from Sagittarius A* changes dramatically. When the star moves inward, the black hole’s gravity accelerates it. The star then reaches its highest speed near the closest point in its orbit, known as pericentre.
During its May 2018 passage, S2 travelled at nearly 3% of light speed. ESO observations placed it less than 20 billion kilometres from the central object. Although that distance sounds enormous, it is remarkably close when compared with the mass involved.
At roughly 7,650 kilometres per second, S2 could cross Earth’s diameter in under two seconds. However, it remains far outside the black hole’s event horizon.
That separation matters. Astronomers can observe the star repeatedly without it disappearing from view. At the same time, S2 travels through a gravitational field strong enough to expose effects that remain tiny within our Solar System.
Decades of Observations Solved the Galactic-Centre Mystery
Tracking the S2 star orbit around Sagittarius A* required more than patience. It demanded major advances in telescope technology.
Dust clouds hide the Galactic Centre from ordinary visible-light observations. Infrared radiation can pass through much of that material, so both major research teams used infrared telescopes.
Reinhard Genzel’s team worked mainly with European Southern Observatory facilities in Chile. Andrea Ghez’s group used the Keck telescopes in Hawaii.
Early observations relied on speckle imaging. This technique combined many short exposures to reduce atmospheric blurring. Later, adaptive optics measured distortions in Earth’s atmosphere and corrected them in real time.
The GRAVITY instrument brought another leap forward. It combined light from four telescopes within ESO’s Very Large Telescope Interferometer. This setup allowed astronomers to measure extremely small changes in S2’s position.
Independent Teams Strengthened the Result
The two observing programmes worked independently. That independence made the conclusion more reliable.
Both groups reconstructed the movements of stars around the same invisible point. Their measurements consistently indicated a compact central mass of about four million Suns.
A short orbital arc could support several possible models. A complete orbit provides far stronger evidence. By observing S2 near its closest and most distant points, researchers greatly reduced uncertainty.
ESO notes that years of tracking S2 produced the strongest empirical evidence for a supermassive black hole at the Galactic Centre.
The S2 Star Orbit Tested Einstein’s Gravity
Finding Sagittarius A* was only part of the achievement. S2 also became a powerful laboratory for general relativity.
Newtonian gravity describes much of the star’s movement. However, small differences appear when S2 reaches its closest approach. Those deviations match effects predicted by Einstein.
Gravitational Redshift Changed S2’s Light
During the 2018 passage, astronomers detected gravitational redshift in S2’s light.
As light moves away from a strong gravitational field, it loses energy. Its wavelength stretches toward the red end of the spectrum. S2’s rapid motion added another relativistic effect linked to the Doppler shift.
Together, these signals differed from the prediction of Newtonian physics. Instead, they matched general relativity. ESO called the result the first successful test of Einstein’s theory near a supermassive black hole.
S2’s Orbit Forms a Slow Rosette
General relativity predicts another effect called Schwarzschild precession.
Under Newtonian gravity, S2 would follow the same closed ellipse during every orbit. In reality, its closest orbital point shifts slightly after each circuit. Over time, the path forms a rosette-like pattern.
In 2020, astronomers announced the first measurement of this effect in a star orbiting a supermassive black hole. The observed motion matched Einstein’s prediction.
Could Sagittarius A* Be Something Else?
Researchers considered alternatives to a supermassive black hole.
For example, a cluster of neutron stars, smaller black holes or other faint remnants could create strong gravity without producing much visible light. However, such a cluster would need to place millions of solar masses inside an extremely small space.
That arrangement would be unstable. Its members would interact, collide, escape or collapse over relatively short astronomical periods.
Precise measurements of S2 and nearby stars also limit the amount of distributed matter that could exist around Sagittarius A*. The central mass behaves like one highly compact object rather than a wide cluster.
Therefore, a supermassive black hole remains the explanation that best fits the evidence.
How S2 Contributed to a Nobel Prize
The 2020 Nobel Prize in Physics recognised a broader scientific achievement, not one star alone.
Roger Penrose received half of the prize for showing that black-hole formation is a robust prediction of general relativity.
Reinhard Genzel and Andrea Ghez shared the other half. The Nobel Committee honoured them for discovering a supermassive compact object at the centre of our galaxy.
S2 provided the clearest individual orbit, but many people contributed to the result. Researchers tracked several stars, developed advanced instruments and analysed decades of measurements.
Their work turned an invisible gravitational source into a precisely measured cosmic object.
S2 Revealed the Black Hole Before Astronomers Imaged It
The S2 star orbit around Sagittarius A* demonstrated the power of patient observation.
Long before scientists produced a horizon-scale image of Sagittarius A*, S2 had already revealed its mass, compactness and gravitational influence. The star’s changing light tested gravitational redshift. Its rotating orbit confirmed Schwarzschild precession.
The Event Horizon Telescope’s 2022 image later examined Sagittarius A* on a much smaller scale. That achievement supported the same picture through a completely different method.
Yet S2 remains one of astronomy’s most valuable natural instruments. Its orbit transformed a dark point at the centre of the Milky Way into measurable evidence—and showed that Einstein’s gravity still works in one of our galaxy’s most extreme environments.
Main Sources:
Spacedaily.com:
https://spacedaily.com/t-s2-star-sagittarius-a-orbit-nobel-prize/
European Southern Observatory — Einstein’s general relativity near Sagittarius A*:
https://www.hq.eso.org/public/news/eso1825/
European Southern Observatory — Schwarzschild precession of S2:
https://www.eso.org/public/news/eso2006/
European Southern Observatory — 2020 Nobel Prize and Galactic Centre research:
https://www.eso.org/public/news/eso2017/
Nobel Prize — 2020 Physics Prize summary:
https://www.nobelprize.org/prizes/physics/2020/summary/
Nobel Prize — Popular scientific background:
https://www.nobelprize.org/prizes/physics/2020/popular-information/
Event Horizon Telescope — First image of Sagittarius A*:
https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy