LB-1
Binary system once thought to contain a massive black hole.
LB-1 is a binary star system in the constellation Gemini. It gained attention after a 2019 paper in Nature proposed that the system contained an unusually massive stellar black hole, with a mass about 70 times that of the Sun, which would have fallen outside ordinary single stellar evolution parameters. However, subsequent analyses in 2020 found the original conclusion to be incorrect, and some researchers now believe the system consists of a stripped B-type star and a massive rapidly rotating Be star.
- Constellation
- Gemini
- Distance (gaia parallax)
- about 2,300 pc
- Orbital period
- 78.9 days
- Mass of optically observed star (if main
- 9 M☉
- Mass of stripped helium star (alternativ
- ~1 M☉
- Mass of unseen companion (if black hole)
- ~70 M☉
- Mass of unseen companion (if neutron sta
- ~2–3 M☉
Lore & Background
The optically observed star, LB-1 A (also known as LS V+22 25), is a B-type star nine times the mass of the Sun and located at least 7,000 light-years from Earth. Chinese astronomers using the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and the radial-velocity method discovered that it exhibits radial velocity variations, indicating it orbits an unseen companion every 78.9 days in a 'surprisingly circular' orbit. Follow-up observations using the Gran Telescopio Canarias in Spain and the W. M. Keck Observatory in the United States refined these findings. The parallax to LB-1 from Gaia Data Release 2 implies a distance around 2,300 pc, but the observed spectral properties of the star are inconsistent with those expected for an ordinary main sequence B-type star at that distance.
Reader's Guide
The significance of LB-1 lies in the controversy it sparked regarding the formation of stellar-mass black holes. If the unseen companion were a black hole of about 70 solar masses, it would have been the first black hole discovered in the pair-instability mass gap, challenging models of stellar evolution that predict no black holes in that range. The discovery would also have marked the first time a stellar black hole was found without observing its X-ray emissions. However, subsequent spectroscopic analyses showed that the star is more likely a stripped helium star with only about 1 solar mass, reducing the companion's mass estimate to as little as 2–3 solar masses and raising the possibility of a neutron star. A revised multiepoch spectroscopic study ultimately revealed that LB-1 does not contain a black hole at all; instead, it comprises a rapidly rotating Be star and a slowly rotating stripped helium star, formed through a past mass-transfer event. The system thus serves as a cautionary example of how initial assumptions about stellar type and distance can dramatically affect the interpretation of binary systems.
Did You Know?
- The system was initially proposed to contain a black hole of about 70 solar masses, more than twice the maximum predicted by most current theories of stellar evolution.
- If the companion were a black hole, it would have been the first stellar black hole discovered without observation of its X-ray emissions.
Frequently Asked Questions
What is LB-1 and where can I find it in the sky?
LB-1 is a binary star system located in the constellation Gemini, sitting roughly 2,300 parsecs from Earth. Its two components complete one full orbit every 78.9 days.
Why did LB-1 make headlines in 2019?
A paper published in Nature that year claimed the system harbored a stellar black hole of about 70 solar masses, a value far outside standard single-star evolution predictions. The announcement turned LB-1 into an instant talking point across the astrophysics community.
Is there actually a black hole in LB-1?
No—subsequent analyses in 2020 showed the original black-hole interpretation was incorrect. The current leading view holds that the system is a stripped B-type star paired with a massive, rapidly rotating Be star.
What are the component masses in the revised LB-1 model?
The optically visible component is now thought to be a stripped helium star of roughly one solar mass, while its companion is a Be star estimated at about nine solar masses. The earlier ~70-solar-mass figure belonged to the now-retracted black-hole hypothesis.
Why do variable-star fans still talk about LB-1?
LB-1 serves as a cautionary example of how a sensational dynamical-mass claim can reshape a field overnight and then be walked back within a year. It remains a go-to case study in binary-star classification and the pitfalls of inferring unseen companion masses from wide-orbit systems.
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