NGC 1850-BH1 Contains Two Stars, Not a Black Hole

How did the team separate the stars?

Spectral disentangling separates light blended from two stars into a separate spectrum for each star. A spectrum shows how light is spread across wavelengths, or colors.

The team applied the method to new, sharper observations from FLAMES, a high-resolution spectrograph on the Very Large Telescope. These data reach farther into shorter wavelengths, where a hidden luminous star should show itself more strongly.

The analysis revealed two luminous components: a comparatively cool, puffed-up, stripped primary star and a more massive, hotter secondary. The primary orbits the secondary, which has a projected rotational velocity—its apparent spin speed across our line of sight—of roughly 280 kilometers per second.

The team says this configuration is most simply explained by earlier mass transfer. The stars may have interacted in the past, leaving one stripped and enlarged.

Seeburger’s team wrote in the paper:

“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.”

On the team’s reading, the observations rule out a black hole in NGC 1850-BH1.

Takeaway: The shorter-wavelength spectra reveal the second star that earlier observations could not separate from a dark companion.

How much do the two stars weigh?

The earlier model paired a visible 5-solar-mass star with an invisible 11-solar-mass companion. The new measurements instead identify a 1.7-solar-mass donor star and a luminous companion with an estimated mass of 5.2 solar masses.

One solar mass means the mass of the Sun. The exact ratio between the two stars’ masses remains uncertain, so researchers cannot yet describe the system fully.

That gap matters because NGC 1850 sits in a cluster whose age, composition and distance are all well known. The cluster therefore offers a useful test of how stars interact in pairs.

Takeaway: The lighter star has about 1.7 Suns of mass, but the exact mass ratio remains unknown.

What questions remain?

An earlier study found that the companion supplied less than 10% of the light in a particular wavelength range. This paper places that contribution at 30%–50% in the same range. The team cannot yet explain the discrepancy.

“There are, however, some open questions,” the team noted about the system’s mass ratio.

Cleaner, sharper spectra could test the two-star interpretation. Observing them is difficult because NGC 1850-BH1 sits in a crowded stellar region and both stars are relatively faint.

The team says such observations are currently possible only from space through spectroscopic imaging with the Hubble Space Telescope. On the ground, the options include blue-efficient long-slit spectrographs such as X-Shooter, which spread starlight into spectra along a narrow slit.

The team also lists integral-field spectrographs on 8-meter-class ground-based telescopes. These instruments record spectra across many positions in an image rather than at a single point.

Takeaway: Cleaner spectra are the next test, because current observations still disagree about how much light the companion contributes.

For black-hole surveys, the practical rule is simple: check the spectrum before counting an unseen companion.