NGC 1850 BH1 is not a black hole, new telescope data shows

Not everyone bought it. Later analyses suggested the visible star could be the leftover of a heavier star that had handed much of its material to a companion. In that case the invisible partner might simply be an ordinary star that is hard to see.

How was NGC 1850 BH1 unmasked?

A team led by Rhys Seeburger of Liverpool John Moores University used the FLAMES instrument on the European Southern Observatory's Very Large Telescope. They gathered more precise spectra, including shorter wavelengths, and applied spectral disentangling. When two stars are too close to separate, their light arrives as one blend. Each star leaves its own set of spectral lines, a kind of fingerprint, so many observations across the orbit let researchers pull the two signals apart.

The result: two bright components. One is cooler, puffed up and stripped of much of its envelope. The other is hotter, more massive and spins fast, with a rotation component of about 275 km/s. That fits a star that has gained material and angular momentum from its partner.

The revised masses are about 1.7 Suns for the star losing material and about 5.2 Suns for its companion, though the latter carries an uncertainty of 2.6 Suns. The results were posted on arXiv, where papers appear before formal journal publication.

Why it matters for black hole counts

Models suggest the Milky Way could host around 100,000 dormant black holes in binary systems, yet only a handful are confirmed. According to the authors, BH1 is another binary that can convincingly pass as one, so each misclassified system skews the count of black holes out there.