In the Large Magellanic Cloud - a satellite galaxy of the Milky Way - there is a star cluster NGC 1850. There is an object there, NGC 1850 BH1, which since 2022 had been listed as a candidate "dormant" black hole. That is the name given to stellar-mass black holes that emit almost no X-rays and reveal themselves only through their influence on the motion of a neighboring star.
At the time, researchers suggested that around an ordinary star of about five solar masses orbits an invisible companion of roughly 11 solar masses. The logic was simple: if there were an ordinary star of that mass nearby, it would shine brightly and would have been noticed. Since there is no light, it must be a compact object, that is, a black hole.
However, a new work led by Rees Sieberger of the University of Liverpool named after John Moores has shown that this is not the case. The paper was published on the arXiv preprint server on October 27. The authors used the method of spectral decomposition - it makes it possible to separate the mixed light of two stars into individual spectra, that is, to "break down" the overall signal into its components. For this they used new, sharper observations from the FLAMES spectrograph - a high-resolution instrument on the Very Large Telescope - covering shorter wavelengths as well. If a bright star were hiding in the system, it would show up more strongly at such wavelengths.
And it did show up. Instead of one star and an invisible companion, the researchers saw two luminous stars. The first is a comparatively cool, bloated and "stripped" star that has lost its outer layers. The second is more massive, hot and rapidly rotating, with a rotation speed of about 280 kilometers per second. According to the authors, the simplest way to explain such a picture is mass transfer in the past: matter flowed from one star to the other.
"Spectral decomposition showed that the system NGC 1850 BH1 actually does not contain a compact companion, but belongs to the class of "impostor" systems with two bright components," the researchers write. Thus, a black hole in NGC 1850 BH1 is ruled out.
At the same time, the masses have also been refined. Previously the visible star was estimated at five solar masses, and the invisible companion at 11. Now the talk is of a donor star with a mass of about 1.7 solar masses and a bright companion of approximately 5.2 solar masses.
True, questions remain. The mass ratio of the two stars is known imprecisely, so the system cannot yet be fully described. This is important because the object is located in a cluster with a well-known age, composition and distance - and therefore serves as a valuable testing ground for understanding how binary stars interact. There is also a discrepancy with the earlier work: previously it was believed that the companion contributes less than 10% of the light in a certain wavelength range, while the new study says 30-50%. Why the contradiction arose is not yet clear.
The next step is to obtain even sharper and "cleaner" spectra. This is difficult to do: NGC 1850 BH1 is located in a densely populated region, and the stars themselves are relatively faint. At present such observations are possible only from space - for example, with the Hubble telescope's spectroscopic survey - or with ground-based instruments such as X-Shooter and integral-field spectrographs on eight-meter telescopes.