By studying the motions of distant stars around the galactic centers, scientists showed that there’s a supermassive black hole at the heart of the Milky Way whose mass exceeds 4 million Suns. Continuing this line of research, which was awarded the 2020 Nobel Prize in Physics, astronomers affiliated with the European Southern Observatory’s Very Large Telescope Interferometer (VLTI) have computed new images of the closest stars observed circling the supermassive black hole so far. The new images zoom in 20 times closer than what was previously possible.

Scientists have long-suspected the Milky Way harbors a massive black hole, like other similarly-sized galaxies in the universe. But proving it is another thing. After all, the gravitational anomaly could also be explained by tight clusters of neutron stars, for instance.

But all shroud of doubt lifted in the spring of 2002 when astrophysicists led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Germany used optical imaging showing a tiny speck of light — a star now known as SO-2 — that passed within 17 light-hours of the galactic center at an astonishing speed. That’s a minuscule distance at the cosmic scale, only three times the distance between Pluto and the sun. There’s only one object in the known universe that is compact enough and has enough mass to accelerate stars to such a high speed, and that’s a supermassive black hole.

Elsewhere, astronomer Andrea Ghez’s Galactic Center Group at UCLA used the 10-meter Keck Telescope in Hawaii to track the motion of S2, reporting in 2000 that the star’s path is curved, a telltale sign it was orbiting something super massive at the galactic center. The UCLA team later also found S2 orbits the galactic center, known as Sagittarius A*, very closely. Genzel, Ghez, and Roger Penrose shared the 2020 Nobel Prize in Physics “for the discovery of a supermassive compact object at the center of our galaxy.”

Research in studying the galactic center is an endless work in progress, though. Genzel’s latest work with the GRAVITY collaboration continues to expand an almost three-decade-long study of stars orbiting Sagittarius A*. GRAVITY is the second-generation VLTI instrument for precision narrow-angle astrometry and interferometric imaging. It brings the most advanced vision to the VLT: with its fiber-fed integrated optics, wavefront sensors, fringe tracker, beam stabilization, and a novel metrology concept, GRAVITY pushes the sensitivity and accuracy far beyond what is offered today. Using novel analysis techniques on GRAVITY data, the scientists obtained the deepest and sharpest images of the galactic center thus far.

In the process, the researchers made more precise measurements of previously identified stars as they approached the black hole. One such star is S29, which in May 2021 passed the galactic center at a distance of just 13 billion kilometers, equivalent to 90 times the distance from Earth to the Sun, at a speed of 8740 kilometers per second. No other star has been found to travel to this close or this fast around the supermassive black hole.

They also found a new, previously hidden star called S300, which demonstrated the power of the new method that combines the light of all four 8.2-meter telescopes of ESO’s Very Large Telescope located in Chile, using a technique known as interferometry. A machine-learning technique called Information Field Theory simulated how GRAVITY should see the new images of the stars around Sagittarius A*, which was then compared to actual GRAVITY observations.

“We have been building GRAVITY for a decade. How many things can go wrong when building such a complex machine? And, indeed, one of the main challenges really was that one needs to control & monitor the four telescopes of the VLT and the common interferometric laboratory to sufficient precision. Many, many control loops are running to keep things stable. And what perhaps is technically amazing: We use the 8m diameter telescopes, with domes as big as a tennis court, bundle the light, feed it into fibers that guide the light in channels with a diameter smaller than 1mm, and combine the light finally in a glass chip, that is only a few centimeters in size. The recorded wave patterns encode in a complex way the image of the sky – but with clever techniques, this can be recovered,” Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics and co-author of the new study told ZME Science.

The new observations also confirm that the paths of the stars in close proximity to Sagittarius A* are exactly those predicted by Einstein’s Theory of General Relativity. Using the new data, the researchers refined the mass of Sagittarius A*, now computed at 4.3 million times that of the Sun, as well as its distance, finding it is about 27,000 light-years away.

But despite the most precise measurements to date of Sagittarius A*, the galactic center remains largely mysterious. Previous work by scientists from the Event Horizon Telescope led to the first and now iconic image of the shadow of a black hole at the heart of galaxy M87. However, The Milky Way’s black hole is trickier to image because it is much more active. “I would not be surprised if also the new stars behave as chaotically as the ones we know at larger radii already. But we better find that out via observations,” Gillessen said.

Later this decade, GRAVITY will be upgraded in order to improve sensitivity and reveal fainter stars that perhaps lurk even closer to the black hole. ESO’s upcoming Extremely Large Telescope (ELT), currently under construction in the Chilean Atacama Desert, will further supercharge these efforts, allowing scientists to measure the velocity of these stars with the highest precision. In the meantime, even the tools currently at our disposal can deliver stunning results when in the right hands.

“Perhaps what is most stunning is how big a step forward it is. With interferometry we can see a factor 15 sharper than what was possible with single telescopes. That is really a huge improvement. Imagine, someone would increase your salary by that factor – it is simply huge. And we are very thrilled by the new opportunities. The Galactic Center continues to be very rich and exciting!” Gillessen said.