"Stephen Paul" wrote in message ...

As has been pointed out elsewhere, open clusters are considered to be
relatively young with stars of blue and white. So what's the deal with the
high population of red stars in NGC 884 (one of the clusters in the double
cluster). I've seen red stars in other open clusters, like the bright core
star of M37.

Well, that varies a lot depending on the cluster. Some clusters, like
the Pleiades, are genuinely young, and still haven't evolved any red
giants worth speaking of. Most open clusters are old enough so that a
few of the stars -- the brightest and most massive, which burn fastest --
have evolved into red giants. Most often, you find those at or near
the center of the cluster, because heavy stars tend to "sink" due to
orbital dynamics, while light stars tend to rise -- and eventually
"evaporate". M37 is an excellent example; it has a single red star
almost dead center which is about a magnitude brighter than any of
the cluster's other stars.

Although open clusters are young compared to globulars, some of them
have ages that are significant fractions of the universe's age, like
M67, which is around 5 billion years old.

In general, you can tell young clusters by the preponderance of blue
stars. Blue stars (O and B) burn out in times ranging from 1 million
years to 100 million years, so any cluster with them has to be young.
Also, young clusters tend to be dominated by a handful of very bright
stars -- the Pleiades or M36 being good examples. Old open clusters
look more like globulars, with dozens or hundreds of stars only one
or two magnitudes fainter than the brightest star. That is because
the most massive stars have already gone through their main-sequence
lives and their red-giant lives, and are now white dwarfs or neutron
stars, emitting very little light.

- Tony Flanders