LBV 1806-20 AB?
|Home | X-Objects | Stars | Habitability | Life ||
© University of Florida
(Used with permission from
Steve S. Eikenberry)
Located around 49,000 light-years
away, LBV 1806-20 is one of the
most massive stars in the Milky
Way. Like the Pistol Star and
Eta Carinae, it is a Luminous
Blue Variable that may have
more than 200 Solar-masses,
if it is a single star (more).
Galactic Region around LBV 1806-20
Previously known as a distant, bright blue star in large infrared telescopes, a team of astronomers announced in January 2004 that a Luminous Blue Variable star that they've designated as LBV 1806-20 may be the most massive and bright star then known to be located in the Milky Way (press release). It is estimated to be located around 49,000 light-years (ly) -- 15.1 +1.8/-1.3 kiloparsecs -- away from Sol on the other side of the galaxy, at the edge of a cluster of massive stars designated as 1806-20. This cluster is composed of some unusually high-mass stars within a spherical volume that is less than two parsecs (6.5 ly) wide. The cluster includes at least two carbon-rich, Wolf-Rayet stars (WC9d and WCL), two blue hypergiants, and a neutron star, the soft gamma-ray repeater SGR 1806-20 (Eikenberry et al, 2004; and AAS annual meeting session abstracts for 2004 and 2002).
Stephane Corbel, Corbel and Eikenberry, 2003 - permission from Corbel, not publisher
(From the Solar System, the line of sight to LBV 1806-20 is obscured by several molecular clouds.)
The 1806-20 star cluster is also associated with radio nebula G10.0-0.3, which is powered by the tremendous stellar wind generated by LBV 1806-20 at the nebula's core. Both still are located close to their parent molecular cloud MC13A, and all are embedded in the H-II region G10.02-0.3, which is part of the farther of two components of W31, one of the largest H-II complexes in the Milky Way and home to intense star-forming regions (Corbel and Eikenberry, 2003).
Corbel and Eikenberry, 2003
(Permission from Corbel, not publisher)
Larger collage of radio
image and maps.
LBV 1806-20 lies at the core of
radio nebula G10.0-0.3 and is a
component of stellar cluster
1806-20, itself a component of
W31, one of the largest H-II
complexes in the Milky Way.
Despite LBV 1806-20's high luminosity, it is virtually invisible to Human eyes on Earth because nearly 90 percent of its light (visible and infrared) is absorbed by intervening interstellar gas and dust. From Earth, it has an apparent magnitude of 8.4. Fortunately, LBV 1806-20's intrinsic brightness could be estimated by sharpening infrared images of the star through speckle interferometry. Based on very high-resolution speckle images, the star has been determined not to be a cluster of stars.
University of Florida,
Eikenberry et al, forthcoming
(Used with permission)
Larger three-color, near-infrared image.
LBV 1806-20 ("A") is located in stellar
cluster 1806-20, which includes other
extremely massive stars such as blue
hypergiants ("C" and "D"), Wolf-Rayet
stars (such as "B"), and the neutron
stellar remnant, SGR 1806-20 -- where
blue objects are relatively nearby,
foreground stars (more).
Similar to the Pistol Star in brightness, LBV 1806-20 is one of the most luminous and massive stars in the Local Group of galaxies surrounding the Milky Way (Eikenberry et al, 2004). With a mass exceeding 200 times that of Sol, the star appears to be a hypergiant (luminosity class 0) with a total luminosity that varies between more than five to greater than 40 million times that of Sol's (bolometric magnitude of -14.2 +/- 0.4. With a a surface temperature between 18,000 and 32,000 °Kelvin (30,000 and 60,000 degrees?), it has swelled up to around 200 times the diameter of Sol. Its spectral type currently appears to class between O9 and B2 (Eikenberry et al, 2004).
The star could be a binary system with a projected average separation (semi-major axis) of less than 400 AUs, based on speckle imaging. However, its light undergoes periodic variations indicative of a single star. Moreover, if not a single star, astronomers are currently at a loss as to how such massive stars could have formed so close together.
NASA -- larger map
LBV 1806-20's formation may have been influenced by supernova
remnant SGR 1806-20 (labelled here as SRB 1806-20). (More on
the rapidly spinning neutron star SGR 1806-20 is available from
NASA and ESA).
Current theories of star formation are unable to explain how a star as big and bright LBV 1806-20 can have formed in the Milky Way, more than 13 billion years after the age of the first stars. LBV 1806-20 should have destroyed itself before it ever ignited fusion at its core. Astronomers have long theorized that as protostars grew to 120 Solar-masses, their extremely high energy output would lead to such extremely high heat and pressure within such a forming star that it would shear off any additional material from the star's surface. LBV 1806-20, however, formed near where the supernova that created neutron star SGR 1806-20 (a magnetically powered, x-ray emitting neutron star that also emits bright, repeating flashes of soft gamma-rays, as a "soft gamma repeater") exploded around one to two million years ago, and this explosion may have compressed gas and dust sufficiently to enabling the star to grow far beyond the usual size of most giant stars. Currently, its immediate surrounding region contains several other "freakishly massive" stars, including a very large protostar in the process of formation and an intensely powerful magnetic neutron star (one of only four "magnetars" classified as SGRs in the twelve found in the Milky Way by 2005), all of which may have formed as the result of the earlier supernova. (More information is available from the ESA on neutron stars, pulsars, magnetars, and a giant flare released by an extremely powerful "starquake" on SGR 1806-20 and from NASA on the flare's detection and origins.)
Larger animation still of
a 2004 giant flare from
SGR 1806-20 (more).
The supernova explosion
creating SGR 1806-20 may
have compressed gas and
dust sufficiently to enable
LBV 1806-20 to grow far
beyond the usual size of
most giant stars.
According to Steve S. Eikenberry, professor of astronomy at the University of Florida, massive stars generally only shine for about 2 million years. Hence, LBV 1806-20 is already middle-aged at only around a million years old and can shed huge amounts of its gas in a stellar wind as well as in titanic eruptions. Eventually, it should blow itself as a supernova, or hypernova with an intense burst of gamma rays.
To paraphrase an excellent summary by astronomer Stephen White, Luminous Blue Variables (LBVs) are among the most massive stars that astronomers know of. LBVs are more than 40 to more than 100 times as massive as Sol and are presumed to have started out as early O-type, main-sequence dwarf stars. Since the most massive stars tend to also burn hotter and to consume their core hydrogen the quickest, they live just a short while by astronomical standards -- only a few million years at most.
(Figer et al, 1995)
Larger infrared image.
Luminous Blue Variables
like the Pistol Star, at left,
appear to eject huge
amounts of their outer
gas envelopes in quick
but violent outbursts.
Once a massive star has fused most of its core hydrogen to helium ash, it becomes very unstable and eventually may blow up as a spectacular Type-II supernova. Some of the most massive ones may first pass through an LBV stage when they appear to eject huge amounts of mass from their outer layers of gas in a very short time (even more mass in a shorter period than Wolf-Rayet stars). The ejected gas (mostly hydrogen) moves outwards from the star at speeds of 50 to 500 kilometers per second and may be observed as nebulae that are bright in radio wavelengths. (Radio images of other LBVs and more discussion are available.)
Up-to-date technical summaries on this star may become available at: NASA's ADS Abstract Service for the Astrophysics Data System; the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access; and the NSF-funded, arXiv.org Physics e-Print archive's search interface.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
Note: Special thanks to Steve Esson for updating us on the number of SGRs, magnetars, and an interesting ESA article on the causes of a giant flare from SGR 1806-20.
© 2004-2005 Sol Company. All Rights Reserved.