"Stephen Paul" wrote in message ...
So what's the deal with the high population of red stars in NGC
884
(one of the clusters in the double cluster).

The reddened M class supergaint stars you speak of can be seen in
online color photographs of the two components of the Perseus Double
cluster - NGC884 (the chi Persei cluster) and NGC869 (the h Persei
cluster), all accessed in 2/2004, e.g. -

Mike Halderman's photos:
http://www.mrh.org/20021115-05.html
http://www.mrh.org/39-10.html

Jerry Xiaojin Zhu's photos:
http://www-cgi.cs.cmu.edu/~zhuxj/ast...ngc884869.html

Bruce Glagola's photos:
http://www.glagola.darkhorizons.org/...my/double.html

A Webda online color-magnitude (C-M) diagram for open cluster's NGC
884 (the chi Persei cluster) and NGC 869 (the h Persei cluster) also
shows the outlier reddened stars.
http://obswww.unige.ch/webda/cgi-bin...gc0884+NGC+884
http://obswww.unige.ch/webda/cgi-bin...gc0884+NGC+869

Some articles that can be read online at the NASA astronomical
database site (ADS) discuss these M class supergaints. Hopefully, I
have correctly interperted their content.

---------------------
Wildey, Robert L. 1964. The Stellar Content of H and Chi
Persei-Cluster and Association. ApJ S8:439 (1964)
NASA ADS link:
http://adsabs.harvard.edu/cgi-bin/bi...%2E8%2E%2E439W

Wildey (1964) provides a more detailed raw C-M diagram for the chi
Persei Cluster (NGC884). Wildey at Fig. 17, page 489. He also shows
a schematic of the key C-M diagram components. Fig. 20 at 492. His
key conclusion, at pages 496-497, is that there were three star
formation events at 7, 17 and 60 million years ago in NGC884. They
are evidenced by three bright star sequences seen in his color
magnitude schematic, Figure 20, above absolute magnitude -4.0. Wildey
concluded that the supergaint helium burning M class stars represent
the aged and early-formed supergaints from a first star formation
event 60 million years ago.

---------------------
Schild, Rudolph. 1967. Ages and Structures of Stars in the H and Chi
Persei Association. ApJ 148:449
NASA ADS link:
http://adsabs.harvard.edu/cgi-bin/np...p;db_key=A ST

Schild (1967) provides a schematic diagram of the 2D structure of h
and chi Persei clusters in parsecs distances. Schild at 456. Schild
concluded that the first star formation event which created the large
O Stars in nucleus of the h Persei cluster (NGC869) occured about 6.5
million years ago. Those stars spread out uniformly out between the h
and c Persei clusters. About 11.5 million years ago, a previous
stellar formation event created the nucleus of the chi Persei cluster
and the older (reddened M) supergaints near the core of chi Persei
cluster. Unlike Wildey, Schild felt that the larger M class stars
dispersed outside the core of chi Persei (NGC884) were not part of the
cluster, but were members of the general Perseus Arm. "At the same
time [11.5 million years ago], star formation may have taken place
over much more extensive reaches of the Perseus arm, and many of the
[reddened M] supergaints seen in the direction of the outer group [the
area between h and chi Persei clusters] cannot be ambigously
identified with the assocation." Schild at 456-457. Schild did not
feel there was a third creation event at 60 million years ago, as
discussed by Wildey.

--------------------
Slesnick, C.L. et al. 2002. The Star Formation History and Mass
Function of the Double Cluster h and chi Persei
ApJ 576(2):880-893
NASA ADS link:
http://adsabs.harvard.edu/cgi-bin/np...p;db_key=A ST

I have not read this newest article that showed up the NASA ADS index.
You would have to go a university library to obtain a copy.

I've seen red stars in other open clusters, like the bright core star of M37. Is there
a relationship between red (carbon?) stars and planetary nebulae?

Assuming this is one question, I believe the relationship between
open clusters and red stars primarily is age. The older the cluster,
the more stars have exhausted burning their hydrogren and have
progressed to the helium burning phase. Most open clusters are
created in the galactic plane. Their stars are gravitationally bound.
They tend to be young objects (less than 60 million years old)
composed of white-blue stars because tidal forces in the galactic
plane tend to "tear" them apart as they grow older. The kinematic
energy of the cluster's members also cause the clusters to "evolve"
into ungravitational bound moving groups - like the Hyades moving
group or the Usra Major moving group. Eventually, the kinematic
motions of the cluster's members "evaporate" all of its stars into the
"average" motion of the surrounding galactic plane. Molecular clouds
create new clusters; the cycle continues.

Occasionally, an open cluster will be created at, or will be thrown
into, the high latitudes above the galactic plane. Such clusters can
escape the ravages of galactic tidal forces and "live" to "age" into
an an older and redder open cluster. Open cluster M44 at galactic
latitude +32° is 790 million years old. M44 is currently overhead in
the winter sky, and is the best optical example, IMHO. It is one of
the reddest and oldest open clusters. It's redden faintnest also
makes M44 harder for beginner's to find. M41 at about galactic
latitude -10° is another example.

M44, Beehive Cluster in Cancer
J0840.4+1940 oc G205.92+32.48 oc
Age: 790M Distance: 160 pc
http://seds.lpl.arizona.edu/messier/m/m044.html

M41 OC in Canis Major
J0646.9-2044 oc G231.09-10.25 oc
Age: 290M Distance: 705 pc
http://seds.lpl.arizona.edu/messier/m/m041.html

Hope this answers your question. Thanks for pointing out these red
gaints around NGC884; I had seen them, but had not given them much
thought. Looking up the answer to your question gave me much more
insight into the prominent summer-autumn Double Perseus cluster.

Regards - Kurt