Stephen Paul wrote:

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.

There can be a few red giant stars in open clusters. This is a key to
actually getting the age of these groups, as the more red giants they have,
the older they tend to be. Most of the stars in NGC 884 are younger lower to
moderate-mass stars, but the cluster is just old enough that the most massive
stars which were formed early in the cluster's history will have evolved off
the main sequence to become red giants (or in some cases, red supergiants).
The more massive the star, the faster its evolution goes, so with a relatively
high-mass star, it can become a red giant while its less massive neighbors are
still puttering along nicely on the main sequence. Some open clusters are
very old (like M67 or NGC 188 for example), so they can contain a lot of red
giants and very few (if any) blue stars.

Is there a relationship between red (carbon?) stars and planetary nebulae?

Well, not immediately. Eventually, red giants may give rise to planetary
nebulae as their outer atmospheres become unstable and their cores collapse,
but you will find very few red stars at the cores of most planetary nebulae.
Most of these central stars are hot pre-white dwarf stars. Carbon stars are
usually just cool giants with a lot of Carbon compounds showing up in their
spectra (which tends to make them redder due to the Carbon compound absorption
bands).

Are all red stars carbon stars?

No, red dwarfs and some red giants are not Carbon stars, but the very reddest
stars you will see in a telescope will often tend to be Carbon stars. I like
V Aquilae, TX Piscium, and R Leporis as probably the reddest stars I have ever
seen. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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