Sometimes massive stars can mimic the behavior of a black hole and trick astronomers into misinterpreting their nature. Such is the case with a strange pair of stars in the Large Magellanic Cloud (LMC) cluster NGC 1850. The system was initially thought to contain a star about five times the mass of the sun orbiting a stellar-mass black hole. Now, a new study led by Rhys Seeburger of Liverpool John Moores University demonstrates that this is a black hole “impostor” system. The research paper was posted to the arXiv preprint server on Oct. 27.
Impostors
It is common for massive stars to be born and live in binary systems. When one star runs out of fuel and explodes, the resulting neutron star or black hole remains in the binary system with its companion. If the dead star’s remnant is a black hole and it barely emits any X-rays, it is called a dormant black hole. Such systems are hard to find because the only way they can be found is through the black hole’s effect on the visible star’s motion.
Models predict around 100,000 such systems in our galaxy. Only a few dormant black holes are confirmed, including VFTS 243 in the LMC and HD 130298 in the Milky Way. But pairs of ordinary stars can also mimic the signal, especially after mass transfer leaves one star stripped and bloated, which can outshine its companion. So, these impostor systems can infiltrate the sample.
In 2022, researchers interpreted NGC-1850 BH1 as a system containing an 11-solar-mass dormant black hole orbiting a 5-solar-mass ordinary star, as one of the objects was “unseen.” Their argument was that a normal star that heavy would outshine its companion. Subsequent studies pointed in a different direction but did not give a definitive result. The companion could either be a compact object like a black hole or a bright star hidden in a thick disk.
Disentanglement
In this study, the team used a technique called spectral disentangling, which separates the mixed light of two stars into each star’s individual spectrum. They applied it to new and sharper observations from the FLAMES instrument (a high-resolution spectrograph of the Very Large Telescope) that extend further into the shorter wavelengths of the spectrum. If a hidden luminous star is present, it would show up more strongly in these observations.
The disentangling revealed two luminous stars. A comparatively cool, puffed-up, stripped primary star orbits a more massive, hotter, faster-spinning secondary with a projected rotational velocity of roughly 280 kilometers per second. This configuration, researchers say, can be most simply explained by earlier mass transfer.
“Spectral disentangling has demonstrated that the system known as NGC 1850 BH1 does in fact not contain a compact companion, but falls into the class of ‘imposter’ systems with two bright components,” the team writes in the paper. The study rules out a black hole in NGC-1850 BH1. Instead, it is an “impostor” with two bright stars that may have interacted in the past.
Not so massive
Building on previous studies, this work provides updated measurements. Previously, the visible star was thought to be a 5-solar-mass star with an invisible 11-solar-mass companion. The current measurements brought down the estimates: the system consists of a 1.7-solar-mass donor star and a luminous companion with an estimated mass of 5.2 solar masses.
“There are, however, some open questions,” the team notes. The ratio between the two stars’ masses is not accurately known, so the system cannot be fully described yet. This matters because the system sits in a cluster whose age, composition and distance are all well known, which makes it a valuable test case for understanding how binary stars interact.
An earlier study found the companion contributes less than 10% of the light in a certain wavelength range, but this paper finds 30%–50% in the same range. Researchers are unsure why this inconsistency arose.
The next step is to obtain sharper, cleaner spectra. The team notes that such observations are difficult because NGC 1850-BH1 is located in a crowded stellar region and the two stars are relatively faint. Currently, they can only be performed from space using spectroscopic imaging with the Hubble Space Telescope or with blue-efficient long-slit spectrographs like the X-Shooter or integral-field spectrographs at 8-meter-class ground-based telescopes.
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Publication details
Rhys Seeburger et al., Revealing the companion of NGC 1850-BH1, arXiv (2026). DOI: 10.48550/arxiv.2610.37262
Journal information:
arXiv
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A suspected black hole turns out to be a rapidly spinning star (2026, November 12)
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