Astronomers have made a groundbreaking discovery: a binary star is orbiting perilously close to the supermassive black hole at the center of the Milky Way. The pair of stars temporarily thrived in an environment of immense gravity. The pair of stars, named D9, gives us a rare glimpse into how the system survived or even formed in such extreme conditions.

This image shows the position of the newly discovered binary star D9, which is orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way. This is the first pair of stars discovered so far near a supermassive black hole. Cutaway shows a binary star system detected by the SINFONI spectrograph on the European Southern Observatory's Very Large Telescope. Although the two stars cannot be resolved in this image, spectra captured by SINFONI over the past few years have revealed the binary nature of D9. The spectra show that light from the hydrogen gas surrounding D9 periodically oscillates toward red and blue wavelengths as the two stars orbit each other. Source: ESO/F.Peißkeretal.

An international team of researchers has discovered a binary star system near Sagittarius A*, the supermassive black hole at the center of the Milky Way. This is the first time a pair of stars has been discovered so close to a supermassive black hole. The discovery, made using data from the European Southern Observatory's Very Large Telescope (ESO's VLT), reveals how stars survive under extreme gravity. It also opens up new possibilities for detecting planets near Sagittarius A*.

Florian Peißker, a researcher at the University of Cologne in Germany and the first author of the study published in Nature Communications on December 17, explained: "Black holes are not as destructive as we thought. Binary stars - a pair of stars orbiting each other - are common throughout the universe. However, until now, binaries have not been observed near supermassive black holes because strong gravity usually destabilizes such systems."

The new discovery shows that some binary stars can thrive briefly even in destructive conditions. The newly discovered binary star, known as D9, was discovered at the perfect time: It is estimated to be only 2.7 million years old, and the powerful gravity of a nearby black hole could cause it to merge into a star in just 1 million years, a very narrow time span for such a young system.

"This provides a brief window on cosmic timescales for observing such a binary system - and we succeeded!" explains Emma Bordier, co-author of the article and a researcher at the University of Cologne who studied at the European Southern Observatory.

D9 is the first pair of stars ever discovered near Sagittarius A*, the supermassive black hole at the center of the Milky Way. This image shows hydrogen emission lines mapped by the SINFONI instrument on the European Southern Observatory's Very Large Telescope. Over the years, emission lines around D9 have been found to swing periodically toward red and blue bands, revealing that D9 is actually two stars orbiting each other. Image source: ESO/F.Peißkeretal.

For years, scientists have thought that the extreme conditions near supermassive black holes hinder the formation of new stars. The discovery of several young stars near Sagittarius A* overturns this hypothesis. Now, the discovery of this young binary star shows that even stellar pairs can form under these harsh conditions. Co-author Michal Zajaček, a researcher at Masaryk University and the University of Cologne in the Czech Republic, explained: "The D9 system shows clear signs of gas and dust surrounding the star, which suggests that it may be a very young star system that must have formed near a supermassive black hole."

This animation shows how two stars in the D9 star system orbit each other surrounded by clouds of gas and dust. The blue line shows the orbit of the binary star system around Sagittarius A*, the supermassive black hole at the center of the Milky Way. D9 is the first binary star ever discovered near a supermassive black hole. Its formation and survival in such extreme environments means black holes are not as destructive as we thought. Image source: ESO/M.Kornmesser

The newly discovered binary was discovered in a dense cluster of stars and other objects orbiting Sagittarius A*, known as the S Cluster. The most mysterious of the cluster are G objects, which behave like stars but look like clouds of gas and dust.

It was during the observation of these mysterious objects that the research team discovered a surprising pattern in the D9 cluster. Data obtained with the VLT's ERIS instrument, combined with archival data from the SINFONI instrument, discovered repeated changes in the stars' speed, indicating that D9 is actually two stars orbiting each other. "I thought my analysis was wrong," Peißker said, "but the spectral pattern covers a period of about 15 years, and it is clear that this detection is indeed the first binary star observed in the S cluster."

This animation shows D9, the first pair of stars ever discovered near Sagittarius A*, the supermassive black hole at the center of the Milky Way. We zoom in and out of the black hole and the single stars orbiting it, and then move closer to D9, the first binary star system discovered near a black hole.

The research results reveal the nature of the mysterious G celestial body. The research team suggested that they may actually be a combination of binary stars that have not yet merged and remnants of binary stars that have merged.

The exact nature of the many objects orbiting Sagittarius A*, and how they formed so close to the supermassive black hole, remains a mystery. But soon, the GRAVITY+ upgraded version of the VLT interferometer and the METIS instrument on the European Southern Observatory (ESO) Extremely Large Telescope (ELT) under construction in Chile will change this situation. The two sets of equipment will allow the team to conduct more detailed observations of the galactic center, revealing the nature of known objects and undoubtedly discovering more binaries and young systems. "Our discovery allows us to speculate on the existence of planets, since these planets usually form around young stars. It seems only a matter of time before a planet is discovered at the center of the Milky Way," Peißker concludes.

The Laser Guide Star (LGS) is launched from the VLT's 8.2-meter Yeppen Telescope and is aimed at the galactic center, the center of the brightest part of the Milky Way. The laser beam is part of the VLT adaptive optics system. It created an artificial star at an altitude of 90 kilometers in the Earth's mesosphere. This star was used as a reference for correcting the image and spectrum to remove the blurring effects of the atmosphere. Image source: G.Hüdepohl(atacamaphoto.com)/ESOThis image shows the location of the field of view of Sagittarius A* - the black hole is circled in red and is located in the constellation Sagittarius (Sagittarius). This image shows most of the stars visible to the naked eye under good conditions. Image source: ESO, IAU and Sky&Telescope

DOI:10.1038/s41467-024-54748-3

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