Larry's
OB Association's!

      When one talks of observing Deep-Sky objects, most amateurs tend
   to think of galaxies, nebulae, open and globular clusters at the top
   of their list. There is, however, a somewhat neglected category of
   Deep-Sky object that is overlooked by many amateurs.
   That is the "O-B Association".

      An O-B Association is a large, very loose form of an open star
   cluster consisting of young spectral type "O" and "B" stars.
   They cover large volumes of space, are loosely held together by
   gravity and have short lifetimes (a few million years) as a distinct
   object. While O-B Associations are considered a separate Deep-Sky
   category from open clusters, both types can be found together,
   with an open cluster forming the core region of a larger
   O-B Association. In time, the outlying O-B Association will drift
   apart, merging with the other field stars in the region,
   leaving the more gravitationally bound open cluster as a still
   observable object.

      Very few star charts actually plot O-B Associations as objects.
   But many can be found in "SKY ATLAS 2000" and "URANOMETRIA" because
   of the number of their individual stars that are plotted.
   A good example of this is "PERSEUS OB3", found around Alpha Persei,
   and "ORION OB1-B", surrounding the "Belt" stars of Orion.
   Because of their large size O-B Associations, are best observed with
   binoculars or small telescopes at low powers.

      The following charts show the location of some of the best
   O-B Associations that are visible in small scopes or binoculars.
   Also included with each chart is a sketch of individual objects to
   give you an idea of what to expect. All sketches were made with an
   80mm refractor spotting scope using low-power eyepieces, with an
   amici prism giving a corrected field of view.

      If you wish to read more about "O-B Associations",
   try the following sources:

   -  "IN PURSUIT OF O-B ASSOCIATIONS"  SKY & TELESCOPE    JAN 1986
   -  BURNHAM'S CELESTIAL HANDBOOK
   -  WEBB SOCIETY DEEP-SKY OBSERVERS HANDBOOK
                        # 3   OPEN AND GLOBULAR CLUSTERS
   -  THE OBSERVER'S GUIDE  MAGAZINE
   -  THE MILKY-WAY   BY BART BOK

• "ORION" OB Finder Chart:
  
  
• "PERSEUS" OB Finder Chart:
  
  
• "AURIGA" OB Finder Chart:
  
  
• "GEMINI" OB Finder Chart:
  
  
• "CASSIOPEIA" OB Finder Chart:
  
  
• "SCUTUM" OB Finder Chart:
  
  
• "VULPECULA" OB Finder Chart:
  
  
• "CYGNUS" OB Finder Chart:
  
  
• "CEPHEUS" OB Finder Chart: