Close to the central black hole of our galaxy, stars are subject to enormous gravitational forces. It was therefore long considered unlikely that binary star systems could form and survive in this extreme environment. Now, for the first time, astronomers have discovered such a binary star system in the vicinity of the black hole Sagittarius A* in the center of the Milky Way. The pair of stars is only around 2.7 million years old and orbits each other relatively closely. This protects it from being torn apart by the tidal forces of the black hole, but both stars will merge together in around a million years, as the researchers report. They suspect that other dust-shrouded black hole objects hide similar pairs of stars during or shortly after their merger.
The center of the Milky Way is an extreme place: the enormous gravitational effect of the supermassive black hole Sagittarius A* causes the central stars to orbit around its gravity well at breakneck speed. The orbits and light of these stars are also influenced by the nearby black hole. Under these circumstances, it was long considered almost impossible for new stars to be formed in the center of the Milky Way. The tidal forces of Sagittarius A* would have to tear apart the molecular cloud from which stars form before they can collapse into a protostar – or so the assumption is. But in 2023, astronomers found several young stars in the immediate vicinity of Sagittarius A*. These young stars are only a few tens of thousands of years old and are still partly surrounded by their dust cocoons, as a team led by Florian Peißker from the University of Cologne discovered. How these young stars were able to form despite the enormous gravity at the black hole remains largely unknown.
Young pair of stars at the black hole
Now Peißker’s team has made another discovery in the center of the Milky Way. For their study, they observed the so-called S-cluster – a collection of stars and dusty objects near Sagittarius A* – with two high-resolution spectrographs at the Very Large Telescope (VLT) of the European Southern Observatory in Chile. They came across a star, named D9, that behaved strangely: the spectrographic data showed recurring fluctuations in the star’s speed. “I thought I had made a mistake in my evaluation,” says Peißker. “But the spectroscopic pattern spanned about 15 years.” Further analysis revealed that D9 is not one, but two stars orbiting each other. “It was therefore clear that this discovery was actually the first double star observed in the S cluster,” reports Peißker.
It is the first time a pair of stars has been found near a supermassive black hole. As the astronomers determined, the binary star system consists of a more massive star weighing around 2.8 solar masses and a smaller partner of only around 0.73 solar masses. “The D9 system shows clear signs of the presence of gas and dust around the stars,” explains co-author Michal Zajaček from Masaryk University in the Czech Republic and the University of Cologne. “This suggests that it could be a very young star system.” The researchers estimate the age of the double star to be around 2.7 million years. They suspect that the heavier star is a so-called Herbig Ae/Be star. These protostars are not yet dense or heavy enough to ignite hydrogen fusion, and derive their energy from their contraction after the collapse of large amounts of matter. A dense cocoon of dust and gases is typical for them. They classify the smaller young star as a T Tauri star, a lower-mass precursor to the Herbig Ae/Be stars.
merger in the near future
The discovery of such a young binary star shows that not only new stars, but even pairs of stars can form close to the supermassive black hole. However, their survival time is limited, as Peißker and his colleagues discovered. The two young partner stars are currently orbiting each other at a distance of around 1.43 astronomical units – which corresponds to around one and a half times the distance between the Sun and Earth. This very close distance for two stars protects them from being torn apart by the tidal forces of the nearby black hole. However, Sagittarius A* disrupts their orbital balance strongly enough to bring the two stellar partners ever closer to each other and deform them, as the astronomers explain. In just around a million years, the two young stars could merge with each other. “On cosmic scales, we only have a short time window to observe such a binary star system – and we succeeded,” says co-author Emma Bordier from the University of Cologne.
This future development of the newly discovered star pair D9 could possibly also explain the so-called G objects in the S cluster. They behave like stars, but externally resemble clouds of gas and dust. Based on their findings on D9, Peißker and his colleagues suspect that these G objects could also contain young binary stars that are about to merge or have already merged. “D9 may also soon merge and become a G-Object. This system offers us a first insight into one of the possible development paths of S stars,” write the astronomers. However, it is still unclear exactly how D9 and the other stars in the S cluster formed. The Extremely Large Telescope (ELT), which is currently under construction in Chile, could provide more information. It will allow astronomers to carry out even more detailed observations of the galactic center.
Source: Florian Peißker (University of Cologne) et al., Nature Communications, doi: 10.1038/s41467-024-54748-3