A Ring of Star-forming Clouds and Bright Stars

A luminous, rotating ring or belt of star-forming regions of molecular clouds and very bright, young, and massive bluish stars are located within 1,600 light-years (500 parsecs or pc) of Sol (Spitzer Gould's Belt Survey). Commonly called Gould's Belt or Ring, the structure of around a million Solar-masses spans some 3,000 light-years (ly) and encompasses most of the most massive stars (conspicuous but short-lived spectral type B stars) in the Solar neighborhood, as well as their much more numerous but dimmer sibling stars and x-ray bright, stellar remnants in a disk within the ring -- which includes a "local bubble" that is actually a tubular, "local chimney" of relative low-density but hot and ionized gas created by supernovae (Kenji Bekki, 2009; Ken Croswell, 2005; and Guillout et al, 1998). According to the JCMT Gould's Belt Legacy Survey. Sol is actually located some 650 light-years (200 pc) off the approximate center of this massive ring-like structure, which is tilted around 18 to 20 degrees to the Galactic Plane and does not appear to have been generated as part of the Milky Way's spiral structure. Since its creation some 30 million years ago, the rotation of the Milky Way has stretched the initial circular structure into an oval (that encompasses loose clusters of young bright, massive stars in constellations Cepheus, Centaurus, Lacerta, Lupus, Orion, Perseus, Scorpius, and Vela, as well as other conspicuously bright, massive stars in Canis Major, Carina, Crux, Ophiuchus, Puppis, Serpens, and Taurus).

Discovery History

Despite the connection with Gould, the southern half of this ring was actually first described in an 1847 publication by Sir John Frederick William Herschel (1792-1871) -- the son of Sir William Friedrich Wilhelm Herschel (1738-1822) -- while observing from the Cape of Good Hope in South Africa. According to Ken Croswell, Herschel described a "zone of large stars which is marked out by the brilliant constellation of Orion, the bright stars of Canis Major, and almost all the more conspicuous stars of Argo [modern Puppis, Vela, and Carina] -- the Cross -- the Centaur, Lupus, and Scorpio," and noted its near 20 degree tilt with respect to the Milky Way. Later in 1874, the entire ring was observed from Argentina by Benjamin Apthorp Gould (1824-96), who wrote that a "great circle or zone of bright stars seems to gird the sky, intersecting with the Milky Way at the Southern Cross, and manifest at all seasons" (Ken Croswell, 2005). However, most of the mass of the structure actually resides in interstellar gas and dust (including the well-inown Orion Nebula and the Rho Ophiuchi cloud complex) and in dimmer stars.

Origins of the Ring and Disk

Gould's Belt does not appear to have formed through the shock waves created by a supernovae, which lacks sufficient punch as evidenced in the lack of similar structures created by past supernovae (Stuart Clark, New Scientist, November 23, 2009; and Ken Croswell, 2005). Such an explosion would have compressed the interstellar gas within a giant star-forming, molecular cloud in all directions from the supernovae. Moreover, astronomers have not been able to explain why the resulting structure would be tilted near 20 degrees from the galactic plane and protrude so much out of the galactic disk. While multiple supernovae have been proposed, the lack of similar structures in nearby spiral arms does not support such a scenario.

Recent Ideas

In the mid-1990s, astronomer Fernando Comerón suggested that a high-velocity cloud of intergalactic gas fell into the Milky Way's disk and collided with a giant molecular cloud at a near 20-degree angle to the galactic disk (Stuart Clark, New Scientist, November 23, 2009; and Comerón and Torra, 1994). The resulting shock waves would have compressed interstellar gas within the tilted disk of the impact to set off a massive burst of star-formation. This structure of star-forming nebulae is still marked by bright B-type dwarf as well as giant and supergiant stars in the larger Solar neighborhood and by the galactic superbubbles and stellar remnants created by the supernovae of the shortest-lived, most massive O-type stars created in the collision. Comerón's scenario, however, was considered to be unlikely for a tenuous gas cloud until astronomers came to realize that such clouds could be held together by a massive clump of dark matter. In 2007, a similar collision with the Milky Way was projected for Smith's Cloud within 20 to 40 million years, which is now believed to be surrounded by a massive halo of dark matter so that the entire incoming object has a "tidal mass" of some 300 million Solar-masses.

In 2009, Kenji Bekki published the results of computer simulations that show how a clump of dark matter crashing a giant molecular cloud at an oblique ngle can generate a structure resembling Gould's belt, with its characteristic tilt. A dark matter clump with around 10 million Solar-masses will pass through a giant molecular cloud of around one million Solar-masses, pull gas in towards the center of the collision, and trigger a wave of star formation similar to Gould's Belt. As the clump moves through to the other side, it tilts the cloud and sets it spinning, as observed in Gould's belt (Kenji Bekki, 2009). Bekki estimates that such collisions may have occurred only once in every 300 million years on average, and Gould's bright stellar landmark may only last another few tens of millions of years or so as its bright but short-lived, B stars expire (although their much more numerous but less massive, dimmer sibling stars remain, along with some bright, massive stars recently born from gas compression around the Local Chimney and superbubbles that were created by the supernovae of the most massive O-type and B-type stars born earliest after the collision). Both Bekki and Commeron, however, are finding huge "holes" and Gould's Belt like structure in neighboring galaxies (such as NGC 6822 and M83), and Bekki estimates that the dark-matter clump that formed Gould's Belt should now be about some 9,000 light years away in the direction of Constellation Scorpius, the Scorpion.

Smith's Cloud is approaching the Milky Way's disk at a 45-degree angle and with a speed exceeding
over 150 miles (240 kilometers) per second