LBV 1806−20
A candidate luminous blue variable and likely binary star.
LBV 1806−20 is a candidate luminous blue variable (LBV) and likely binary star system. It lies about 28,000 light-years away, toward the center of the Milky Way. Its mass is estimated at roughly 36 times that of the Sun, and its luminosity varies but averages around two million times the Sun’s output. Despite this extreme brightness, it is invisible at visual wavelengths from our solar system because less than one billionth of its visible light reaches us.
When first identified, LBV 1806−20 was thought to be both the most luminous and most massive star known, which posed a challenge to theories of massive star formation. More recent measurements place it somewhat closer to Earth, and its binary nature means it now falls within the expected range for extremely luminous stars in the galaxy. At two million solar luminosities, it remains one of the brightest stars in the Milky Way.
The star sits at the core of the radio nebula G10.0–0.3, which is believed to be primarily powered by its stellar wind. It belongs to the open cluster 1806−20, which is part of W31, one of the largest H II regions in the galaxy. This cluster contains several unusual stars, including four Wolf–Rayet stars, multiple OB stars, and a magnetar (SGR 1806−20).
The spectral type of LBV 1806−20 is uncertain and may vary, but it has been narrowed to between O9 and B2 based on the equivalent width of an infrared helium line. Its spectrum shows strong emission in the Paschen and Brackett hydrogen series, along with emission lines of helium, FeII, MgII, and NaI. The lines are broad and have uneven profiles, some showing P Cygni features. High-resolution spectra reveal that some HeI absorption lines are doubled.
Intervening dust toward the Galactic Center absorbs an estimated 35 magnitudes at visual wavelengths, so most observations are done with infrared telescopes. Based on its luminosity and spectral type, it is suspected to be an LBV, but the characteristic photometric and spectroscopic variations have not yet been observed, so it remains only a candidate.
To explain the doubled HeI absorption lines and the inconsistent mass, luminosity, and age estimates, LBV 1806−20 is thought to be a binary. The emission lines are single, suggesting that only one star has a dense stellar wind, as might be expected from an LBV.
- distance
- ~28,000 light-years
- mass
- ~36 solar masses
- luminosity
- ~2 million solar luminosities
- spectral_type
- uncertain, between O9 and B2
- constellation
- Sagittarius (toward Galactic Center)
- cluster
- 1806−20 open cluster
Lore & Background
LBV 1806−20 lies at the core of radio nebula G10.0–0.3, which is believed to be primarily powered by its stellar wind. It is a member of the 1806−20 open cluster, itself a component of W31, one of the largest H II regions in the Milky Way. The cluster contains several highly unusual stars, including four Wolf–Rayet stars, several OB stars, and a magnetar (SGR 1806−20).
The spectral type of LBV 1806−20 is uncertain and possibly variable, constrained between O9 and B2 based on an infrared HeI line equivalent width. Its spectrum shows strong emission in the Paschen and Brackett series of hydrogen, as well as emission lines of helium, FeII, MgII, and NaI. The lines are broad with uneven profiles, some showing P Cygni profiles, and high-resolution spectra reveal doubled HeI absorption lines.
Intervening dust toward the Galactic Center absorbs an estimated 35 magnitudes at visual wavelengths, so most observations are conducted using infrared telescopes. Despite its name, the characteristic photometric and spectroscopic variations of an LBV have not yet been observed, so it remains only a candidate. The doubled HeI lines and inconsistent mass, luminosity, and age estimates suggest it is a binary, with only one star appearing to have a dense stellar wind.
Reader's Guide
LBV 1806−20 is significant as a candidate luminous blue variable that once challenged theories of massive star formation due to its initially estimated extreme luminosity and mass. Its revised parameters—a mass of around 36 solar masses and a luminosity of about two million times the Sun—place it within the expected range for extremely luminous stars in the galaxy, yet it remains one of the most luminous known. The star is invisible at visual wavelengths from Earth because less than one billionth of its visible light reaches us, a consequence of heavy dust extinction. Its binary nature, suggested by doubled spectral lines and inconsistent age estimates, adds complexity to its classification. As part of the unusual 1806−20 cluster, which includes Wolf–Rayet stars and a magnetar, LBV 1806−20 provides insight into the environments of massive star formation and the role of binarity in stellar evolution. Its status as a candidate LBV underscores the difficulty of confirming such classifications without observed photometric or spectroscopic variability.
Did You Know?
- LBV 1806−20 is invisible at visual wavelengths because less than one billionth of its visible light reaches Earth.
- It is a member of the 1806−20 open cluster, which also contains a magnetar (SGR 1806−20).
- The star's spectrum shows doubled HeI absorption lines, suggesting it is a binary system.
- Intervening dust absorbs an estimated 35 magnitudes at visual wavelengths toward the Galactic Center.
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