LB-1
Binary system initially thought to contain an unusually massive black hole.
LB-1 is a binary star system in the constellation Gemini. It gained attention in 2019 when a 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 challenged existing models of stellar evolution. 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
- at least 7,000 light-years (2,100 pc) from Earth
- Primary star type
- B-type star (LB-1 A, or LS V+22 25)
- Primary star mass
- nine times the mass of the Sun (or possibly ~1 M☉ if a stripped helium star)
- Orbital period
- 78.9 days
- Unseen companion mass (if black hole)
- about 70 solar masses
- Unseen companion mass (if neutron star)
- ~2–3 M☉
Lore & Background
A revised multiepoch spectroscopic study of LB-1 has revealed that LB-1 does not contain a black hole at all. Instead, it comprises a rapidly rotating Be star and a slowly-rotating helium star. The system was proposed to have formed through a past mass-transfer event, where the stripped helium star was originally the more massive star and transferred mass to its companion, which became the massive rapidly rotating Be star seen today.
Reader's Guide
LB-1 is significant primarily as a case study in the challenges of interpreting binary star observations and the importance of multiple lines of evidence. The initial 2019 claim of a ~70 solar mass black hole in the pair-instability gap generated widespread interest, as it would have forced a re-examination of models of stellar-mass black hole formation. The subsequent correction in 2020, based on more detailed spectroscopic analysis and Gaia parallax data, demonstrated how assumptions about stellar type and distance can dramatically affect mass estimates. The system is now understood to be a binary consisting of a rapidly rotating Be star and a stripped helium star, likely the product of past mass transfer. This outcome highlights the value of multi-epoch spectroscopy and disentangling techniques in resolving such systems. LB-1 also serves as a cautionary example of how initial excitement over a potentially paradigm-shifting discovery must be tempered by rigorous follow-up analysis.
Did You Know?
- LB-1 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 black hole hypothesis had been correct, LB-1 would have been the first black hole discovered in the pair-instability mass gap.
- The system was discovered using the radial-velocity method, marking the first time a stellar black hole was proposed without observation of its X-ray emissions.
- A revised study found that LB-1 comprises a rapidly rotating Be star and a slowly rotating stripped helium star, formed through a past mass-transfer event.
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