A big part of astronomy has always been butterfly collecting.

I mean that observational astronomers have always been interested in "collecting" and cataloging new things: big stars, little stars, not quite stars, big galaxies, little galaxies, in-between galaxies. Of course, there has always been a physical motivation for doing all of this collecting and cataloging -- knowing what's out there helps us to understand how the universe is put together. But for much of the history of modern astronomy, the science consisted largely of cataloging the unknown. This is fine; like butterfly collecting, the discovery and cataloging of astronomical curiosities is as much a celebration of the beauty of nature as it is science.

One of the prettier species of astronomical butterfly are the OB associations. The O and B refer to spectral types of stars rather than obstetrics, but the allusion to neonatology is an apt one. The O and B stars are hot, luminous objects many times more massive than our own Sun. Because massive stars have much shorter lifetimes than their smaller brethren, they tend not to wander too far from where they were born before they die. Thus the OB associations are usually associated with star-forming regions, places in galaxies with lots of raw material for creating new stars.

O and B stars are rare objects in our universe. For whatever reason, most of the stars born during the current epoch in the universe tend to be smaller, feeble little objects, generated in the millions and billions by galaxies like our own Milky Way. The initial mass function of the modern era is heavily biased toward the lowest-mass stars, with the most massive stars making up only an infinitesimal fraction of the stars produced in the universe at any given moment.

When massive stars do form, they tend to do so in the most spectacular ways. They take only a few tens of thousands of years(1) to go from diffuse cloud to raging thermonuclear furnace, unlike the low mass stars that take many tens of millions of years to reach the main sequence. So when we see O and B stars in a star-forming region, we know they're fresh out of the oven, so to speak.

When they reach the main sequence, they also tend to overwhelm the space around them. In star-forming regions, the OB stars are responsible for generating HII regions -- the regions of red, nebulous patches generated by ionization and recombination of hydrogen atoms by light from these stars. Because they're so hot and luminous, they emit lots of ultraviolet light that in turn creates the ionized hydrogen and the associated nebular emission. The Trapezium in the Orion nebula is an example of such an OB association(2), and several others are known in our Milky Way. Because these stars emit so much light and have large winds, they can even disrupt their natal star-forming regions entirely.

Because OB associations are young star-forming regions, they are located in regions of galaxies with lots of material from which stars can form. This means they are found in the spiral arms of gas-rich spiral galaxies. In our Milky Way, they're found in constellations the band of the Milky Way passes through, like Cygnus, Perseus, Carina, Scorpius and Centaurus, and so on(3). In other galaxies, we see them either in the spiral arms, or in disturbed, gas-rich regions if there are no coherent spiral arms. In our Milky Way at least, the stars in OB associations are among the most luminous stars in the Galaxy(3). They're also the most likely places to look for type II supernovae within our Galaxy as well, for only the most massive stars can end their lives this way. (Don't hold your breath though -- though these stars evolve quickly, it may be many millenia before we see another Galactic type II supernova.)

At this time of year, the Milky Way is easily visible to observers in both hemispheres. In the north, there are several OB associations in Cygnus alone, while you lucky folk in the southern hemisphere can view the many rich star fields in Sagittarius and Scorpius. However, don't expect the OB associations to jump out at you; many aren't even visible without sizable telescopes. None of the stars in OB associations (other than the Trapezium in Orion) have bright apparent magnitudes, either because they're many hundreds or thousands of parsecs away, or because there's a lot of dust between us and them -- they do live in spiral arms after all. They're often embedded within or associated with giant molecular clouds, and can sometimes be targets for infrared and radio astronomers as well.

(1) Iben, I. Jr., 1965, Astrophysical Journal 141, 993.
(2) Ostlie, D.A. & Carroll, B.W., 1996, An Introduction to Modern Stellar Astrophysics (Reading, Massachusetts: Addison-Wesley Publishing)
(3) Humphreys, R.M., 1978, Astrophysical Journal, Supplement Series 38, 309

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