[madbadgalaxyman (Robert)]

In the process of writing an article about the OB association called Scorpius OB1 (nothing to do with Obi Wan Kenobi !!), which is the magnificent and elongated and extended cluster-like object seen between the open cluster NGC 6231 and the nebula IC 4628, I felt the need to start off with an explanation of what an OB association actually is. This turned out to be something of a challenge, as even professional astronomers still debate certain basic issues about these critically important objects, which are some of the most prominent in the sky for binoculars and rich field telescopes.

So here is my explanation of what an OB association is, taken from the beginning of my article.
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OB associations – some facts

OB associations are a very major component of all spiral and irregular galaxies, and they are nearly as important as star clusters. According to Norbert Schulz in his superb 2005 monograph “From Dust to Stars : Studies of the Formation and Early Evolution of Stars”(part is available at Google Books)(new edition out soon), OB associations may contain 50 percent of the massive stars in our Galaxy, yet the published scientific literature about these objects is so remarkably small that one suspects professional astronomers have been too busy chasing extragalactic objects to be bothered with accurately characterizing this vital component of most galaxies. Furthermore, amateur astronomers generally know little about OB associations, despite the fact that they are often superb objects for visual observation with binoculars and Rich Field Telescopes.

C.D.Garmany's review paper on OB associations (1994, PASP, Vol. 106, p.25) defines these associations of O and B stars as being groupings of hot & young & luminous stars that are normally (but not always) much larger than a star cluster, and which are unlike clusters in that they are not gravitationally bound;
therefore, the lack of sufficient self-gravity together with the large internal velocities (of ~4 km/s) will cause the stars in an OB association to quickly disperse. However, an OB association is a real and very-strong overdensity of these (normally very rare) Hot & Massive stars, and the stars in such an association do have a common parent - a giant cloud of molecular hydrogen gas.
Aside from the most important deciding question of “bound vs unbound?”, another major distinction between open star clusters and OB associations is that:

• a star cluster collapses quite rapidly from its natal gas cloud, implying that most of its stars are very close to being the same age (since their formation)
• in contrast, various subgroups of stars that are found within an OB association can have different ages (since formation), because a gas cloud that eventually forms an OB association having subgroups will incorporate (within itself) various dense gaseous clumps that contract into stars at different times. The multi-age nature of the various clumps of young stars has been well studied in the nearby OB star concentration found in the constellation Orion (see: Walter, Sherry & Wolk, (2003), in “Galactic Star Formation Across the Stellar Mass Spectrum”, ASP Conference Series, Vol. 287)

It is important to understand that while the most noticeable thing about an OB association is its striking concentration of short-lived brilliant stars, it must be understood that an OB association also contains many stars of lower mass & luminosity! The distribution of the relative numbers of stars formed at different masses (the mass function) of OB associations is similar to that of star clusters and other star-forming environments, so OB associations are similar to clusters in that they contain enormously more low-luminosity stars than high luminosity stars [see: C.Briceno (2009), RevMexAA(Serie de Conferencias), vol. 35, p.27 ]

Some of the nearest OB associations include:

• Orion OB1, which is an association of young stars that includes the Orion Nebula Star Cluster and NGC 2024 and also many of the brightest stars in the constellation Orion. (it is about 1350 light years away)
• the Scorpius-Lupus-Centaurus complex of bright stars (= the Sco-Cen OB association) located 300-500 light years from the Sun. This association is part of Gould's Belt.