Our Sun is embedded in the leading edge of a belt of gas and dust which is partially condensed into stars, inclined at 180 to the galactic plane, and apparently linked to the lower edge of the Orion spiral arm. This Local System, as it is called, may be moving with the lower edge of the Orion arm and projects from the later, like a tongue, toward and slightly above the direction of galactic center. It is estimated that the majority of all matter in the vicinity of the Sun must be concentrated in the local system, including all of the common constituents of spiral arms, namely: dust, gas, and young stars. In other words, the Sun is embedded in a local system or structure of very recent origin, and it was even suggested that this system might be a tiny galaxy (itself with spiral structure) set within the main galaxy. The idea of a local system and its acceptance as a fact has gained and lost favor several times among astronomers. It is now considered a well-established fact.
It was first noticed by Herschel in 1887 that many of the brightest stars in the southern sky occur in a band inclined some 18° to the plane of the Milky Way. In other words, the near and bright stars in the sky concentrate along a great circle which is not coincident with the galactic equator, but tilted or inclined to it -- a flattened local structure. Around 1880, this phenomenon was reexamined by American Astronomer B. A. Gould, who guessed that the Sun was located in a small cluster whose structure seemed to be evident in the naked-eye (near) stars. Gould's Belt, as it came to be known, contained the 'B' stars brighter than 5.25 magnitude, whereas stars of 'B' spectral class fainter than 7.25 magnitude were confined to the plane of the galaxy. It was then discovered that diffuse nebulae were also distributed in two distinct belts, one coinciding with the plane of the galaxy, the other matching Gould's Belt. At first, all objects whose plane of symmetry deviated greatly from the galactic equator were considered part of Gould's belt. Today this belt (now called the local system) is considered to be defined as a group of 100 million stars flanked by the Scorpio-Centaurus association on one end and the Pleiades cluster at the other.
This local system is made up of the luminous O-B5 stars within 400 parsecs of the Sun, the 'A' stars in the Henry Draper Catalogue, neutral hydrogen, the O-associations: Scorpio-Centaurus, II Perseus, and I Orion, and the two largest dust complexes within 500 parsecs of the Sun. These two dust complexes, the great concentration of dust in the Taurus-Orion-Auriga region below the galactic equator and the Ophiuchus-Sagittarius-Scorpio dust clouds above the equator, were shown to be connected along the line of the equator of the local system. This connection (made by the astronomer Hubble) helped to make clear the shape of the local system which is now defined as follows:
The Local System is a thin sheet of young objects 500-700 parsecs long and several hundred parsecs wide with a thickness of 70 parsecs. This elongated system is parallel to the direction 160°-340° (New Galactic Longitude), inclined at about 18° to the galactic plane, with the Sun located near the leading edge of the system about 100 parsecs from the centroid of the system. The system, in other words, is an elongated form that points roughly in the direction of the galactic center. The axis aligned toward the center of the galaxy makes sense, because differential galactic rotation (the spinning wheel) would destroy any object whose axis might be along the direction of a spiral arm. However, an axis toward the center might persist for 100 million years before being dispersed.
There has been controversy as to whether the local system is independent of the spiral arm or part of that arm. If the local spiral arm were for some reason twisted out of plane by 18°, the main features of the local system would be accounted for. The local system has also been detected at the 21 cm line of neutral hydrogen.
- Extent of the Local System = 700 parsecs
Thickness = 70 parsecs
North pole of system: lII= 202°, bII= 72°
Sun's distance from center @ 100 parsecs
Sun's distance from local plane @12 parsecs North of plane
Direction of centroid of Local System: lii = 270°0 bii -3°
Expansion life of system = 40xl06 years
Mass of system = 2x105
Mass of the Sun Absolute magnitude of system: Mv = -13
The centroid of the Local System is located in the middle of the constellation Vela. It intersects the ecliptic at 165°24' of Longitude and a latitude of -62°30' For more details see the section on Cosmic Centers.
Copyright (c) 1997-99 Michael Erlewine