Astrophysical Directions by Michael Erlewine

 

 

In Astrophysical Directions
Introduction Coordinate Systems The Solar System The Solar Neighborhood The Galaxy Galactic Objects The Fixed Stars Star Clusters & Nebulae Non-Visual Astronomy External Galaxies Finder-Lists, etc.
The Solar Systems

 

  The Nearest Stars

  The Local System (Gould's Belt)

  The Southern Stream (Sco-Cen Assoc.)

  The Structure of the Local Spiral Arm

 

The Nearest Stars

The 26 nearest stars to our Sun are mapped in the accompanying diagram. The stars are mapped using New Galactic Longitude and Latitude and the numbers within the circles indicate the distance of the star above or below the galactic plane (through the Sun) in light years.

 

Near Stars

 

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The Local System (Gould's Belt)

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.

The Local System (Gould's Belt)

 

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The Southern Stream (Sco-Cen Assoc.)

The largest of the near moving clusters and the chief contributor in the Southern Hemisphere to the Local System is the Scorpid-Centaurus association of stars. Charles Jayne points out that this immense loose cluster (stretching from later Libra through late Sagittarius in the Zodiac) must be the origin of the fabled Via Combusta. The entire sign of Scorpio is influenced by the presence of this vast cloud of stars.

 

The Southern Stream

The Scorpio-Centaurus association is also called The Southern Stream and a stream it is, stretching from New Galactic longitude 243° through 360° and latitude +30° and -30°. The centroid is at 12 = 330°, b2 = +15° and the main body of the cluster stretches from galactic longitudes 314° to 347°.

 

The 1950.0 position for the centroid of Sco-Cen is:

R. A. Dec. Long. Lat Intersect Iii bii
227°16'05" 227°16'05" 236°05'50" -21°36'60" 229°43'03" 330° |+15

 

The shape of the cluster is found to be elongated considerably, with the distance between the extreme points in the direction of elongation being about three times its transverse section. The angle between the direction of elongation and the radius vector through the Sun is 52°. It is estimated that between 0.4xl08 and 1.5xl08 years ago, the cluster (which up to that time may have been spherical) was liable to a disturbing force. This force gave it a velocity between 9 and 19 km/sec relative to an imaginary star moving in a circle around the galactic center and coinciding with the center of the cluster at the time of disturbance. The present shape and motion of the cluster are explained by the evolution since this disturbance. The present mean motion of the cluster is found to indicate an initial velocity of 19 km/sec in the direction forming an angle of 87° with the direction of circular motion, that is approximately in the direction outward from the galactic center. The position of the cluster was at that time at the distance 2200 parsecs from the present position of the Sun in the direction of about 59° New Galactic Longitude (Vulpecula-Sagitta direction).

 

The Southern Stream

Variation with time of the Vertex Direction of Sco-Cen

 

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The Structure of the Local Spiral Arm

The diagram on this page represents the general structure of the local portion of the spiral arm in which our Sun is embedded. Even the very nearest stars (and they are few) are at a great distance. It takes light over four years to reach us from our closest stellar neighbors. It is difficult to get a feel for such vast distance. For instance, the 6000 or so naked-eye stars are all very, very near to us in terms of cosmic distance. In fact, almost all of the objects in this catalog (with the exception of the external galaxies and quasars, etc.) are quite near. They are not far from where we are in terms of the size of our mother galaxy. In other words, we cannot see too far out across our galaxy.

If you will refer to this map once in a while, as you get to know some of the different kinds of celestial objects, it will come home to you that most of the famous objects that astronomers refer to (that illustrate the many astronomical books) are more or less our neighbors. Objects like the Pleiades, the Crab Nebula, and the Orion Nebula are on the one hand so distant that it is difficult to imagine and on the other, so close (in terms of any comprehensive cosmic distance scale), that one gets the idea that we know only about our nearest neighbors, and no one else. Although we have developed a receptive or passive knowledge of time and space, we have just begun to become active and reach out or travel through space. The Hubble telescope is changing all of this.

 

The Structure of the Local Spiral Arm

 

 

 

© Copyright © 1997 Michael Erlewine

 

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