Pistol Star

NICMOS, STScI, NASA
(Figer et al, 1995)

Larger infrared image.

This 4.8 light-year-wide view depicts
the Luminous Blue Variable called
the Pistol Star, the brightest known
star in the Milky Way and the creator
of the Pistol Nebula (more at
Astronomy Picture of the Day and
STScI).

Galactic Region around the Pistol

The Pistol Star lies near the Milky Way's hot and violent, but metal-rich galactic center, deep within its central bulge. It is an extremely luminous and massive star that was found in 1990 at the center of the Pistol Nebula. The star is so massive that it was conjectured then later confirmed to be throwing off so much mass to have created the surrounding Pistol Nebula.

Donald F. Figer, NICMOS, STScI, NASA -- larger infrared image
(The Pistol Star is next to the Quintuplet Cluster and near the Arches Cluster -- more).

Based on proximity and age, the Pistol Star appears to be a member of the four-million-year-old Quintuplet Cluster (AFGL 2004) located only about around 6.5 light-years (ly) -- two parsecs -- away from it. The Quintuplet contains a number of smaller but similarly hot, Wolf-Rayet stars that are also throwing off tremendous quantities of mass and ionizing the surrounding gas (Moneti et al, 1998). Also located near the Pistol Star and Nebula is the two-million-years-old, compact Arches Cluster which contains as many as 150 O-type stars alone (more from Chandra X-Ray Observatory and STScI).

Andrea Moneti, MSX Spirit-III

Larger true-color composite image.

The Pistol Nebula and Star
and the Quintuplet Cluster
are located near the Arches
Cluster and Bubbles region
of the galactic center (more
from Andrea Moneti) and
Moneti et al, 2001).

The star has blown off two expanding shells of gas equal to about 10 Solar-masses. Its largest shell is four light-years (ly) wide and would stretch nearly all the way from Sol to the Alpha Centauri triple-star system. The two gas shells are estimated to be only 4,000 and 6,000 years old, respectively (more at STScI). When Pistol Star finally stops blowing off mass, it may be reduced to less than 10 Solar-masses.

Andrea Moneti, MSX Spirit-III

Larger true-color composite image.

Region in galactic center
containing the Arches Cluster
and Bubbles, the Quintuplet
Cluster, and the Pistol
Nebula and Star (more
from Andrea Moneti) and
Moneti et al, 2001). See a
large, bluer image from
Astronomy Picture of the Day.

The Pistol Star lies about 25,000 light-years away from Sol. From the perspective of Earth, the region containing the star lies at the western edge (17:46:15.3:-28:50:4, ICRS 2000.0) of Constellation Sagittarius (see James Kaler's photo), the Archer. This region is located west of Alnasl (Gamma2 Sagittarii) and W (Gamma1) Sagittarii, and Kaus Media (Delta Sagittarii); northwest of Kaus Australis (Epsilon Sagittarii); southwest of the Lagoon Nebula (M8) and open cluster NGC 6530; southeast of Kaus Borealis (Lamda Sagittarii) and Nunki (Sigma Sagittarii); and south of the Trifid Nebula (M20).

Chandra X-Ray Observatory, NASA, UMass -- larger and jumbo x-ray images

This 400 by 900 light-years (ly) mosaic image of the Milky Way galactic center
includes "hundreds of white dwarf stars, neutron stars, and black holes bathed in
an incandescent fog of multimillion-degree gas" -- plus a central black hole in the
bright patch at center (more at Chandra X-Ray Observatory and Wang et al, 2001).

The Star

Similar to Eta Carinae in brightness, the Pistol Star is one of the most luminous stars in the Local Group of galaxies surrounding the Milky Way (Figer et al, 1995). With a diameter larger than Earth's orbit around the Sun, it radiates as much as more than 10 million times more light than Sol (L= 10^6.3) and appears to have more than 150 Solar-masses, having been resolved as a single star down to a projected separation of 110 AUs (Figer et al, 1998). Indeed, the star may have started with as much as 200 to 250 Solar-masses but has been violently shedding much of its substance. With over 40 Solar-masses, extremely high luminosity, a variable spectra, and surrounding ejecta, it is classified as a Luminous Blue Variable like Eta Carinae, as an extremely large star that are now often regarded as the evolutionary link between main-sequence O stars and the more evolved, smaller but similarly explosive, Wolf-Rayet stars. Despite being 25,000 ly away, the Pistol Star would be visible to the naked eye on Earth as a modest 4th magnitude object, if not for intervening dust being it and Sol. It may only be around 1.7 to 2.1 million years old but will explode in a supernovae within only another one to three million years (Figer et al, 1998; STScI press release; and fact sheet). A useful catalogue number for this star is the variable designation: V4647 Sgr.

Andrea Moneti, MSX Spirit-III

Larger true-color composite image.

Pistol Nebula and Star are
located near the Quintuplet
Cluster (more from Andrea
Moneti) and Moneti et al, 2001).

The Pistol Star may be radiating enough energy to halt the further infall of gas and dust that had been agglomerating onto the star, thus limiting its maximum mass. In fact, some astronomers speculate that the initial mass of the star -- at a possible 200 Solar-masses -- may have exceeded Eddington's upper limit, although alternative theories have been developed. That this extremely massive star has been found near galactic center is no accident, as recent observations suggest that star formation there appears to be more favorable to the creation of stars that are much more massive than than Sol.

Given the apparent youth of the Pistol Star, it is likely that any protoplanetary bodies that may have formed around the star are still agglomerating other planetesimals. In any case, any developing carbon-based life on a developing Earth-type planet would be subject to tremendous heat on a newly formed planet that is under intense asteroidal and cometary bombardment, in addition to the intense and deadly radiation produced by nearby supernovae and other massive young stars.

Luminous Blue Variables

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.

Jon Morse, Kris Davidson,
STScI, NASA

Larger infrared image.

Luminous Blue Variables
like Eta Carinae, 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 they eventually may blow up as spectacular Type-II supernovae. 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 some LBVs and more discussion are available.)

Other Information

Up-to-date technical summaries on this star are available at: NASA's ADS Abstract Service for the Astrophysics Data System; and the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access.

Sagittarius is Latin for "archer," often represented as a centaur wielding a bow and arrow since ancient times. The constellation also contains the Great Sagittarius Star Cloud, where a vast milky swarm of millions of stars mark the way to the center of the galaxy. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Sagittarius. For another illustration, see David Haworth's Sagittarius.

For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.