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LBV 1806-20
LBV 1806-20 is a candidate luminous blue variable (LBV) and likely binary star located nearly 40,000 light-years from the Sun, towards the center of the Milky Way. It has an estimated system mass of around 130 solar masses and an estimated variable luminosity of around two million times that of the Sun.
Despite its high luminosity, it is invisible from the Solar System at visual wavelengths, because less than one billionth of its visible light reaches us. The rest is absorbed by intervening interstellar gas and dust. Although the star is 8th magnitude at the near infrared wavelength of 2 micrometers, it is calculated to be about 35th magnitude at visible wavelengths, which is undetectable with current equipment. On the basis of its luminosity and spectral type it is suspected of being an LBV, but despite the name the characteristic photometric and spectroscopic variations have not yet been observed so it remains just a candidate.
When first discovered, LBV 1806-20 was considered both the most luminous and most massive star known, challenging our understanding of the formation of massive stars. Recent estimates place it somewhat nearer to Earth, which when combined with its binary nature mean that it is now well within the expected range of parameters for the most luminous stars in the galaxy.
==Spectrum==
The spectral type of LBV 1806-20 is uncertain and possibly variable. It has been constrained to between O9 and B2 on the basis of an infrared HeI line equivalent width. The spectrum shows strong emission in the Paschen and Brackett series of hydrogen, but also emission lines of helium, FeII, MgII, and NaI. The lines are broad and have uneven profiles, some showing P Cygni profiles.〔 High resolution spectra show that some HeI absorption lines are doubled.〔
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