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
(3) Humphreys, R.M., 1978, Astrophysical Journal, Supplement Series