Stephen Paulwrote



Is there a relationship between red (carbon?) stars and planetary nebulae?
Are all carbon stars potentially remnants of planetary nebulae? If so, maybe
the central star of the planetary nebula in M46, will appear red in the
future, having blown off some of its shell. Sort of a premature aging.

It's the other way around, to some degree. Red stars must be
very cool. The only very cool stars we can see at the distance
of any open cluster are red giant stars. These stars have run
out of hydrogen in their core, and are fusing helium their
cores (or about to fuse helium, or just finished fusing helium).
The outer layers of the stars puff out to tens or hundreds of times
their size during the hydrogen-fusing stage, growing much cooler
and redder. In a small fraction of these red giants, carbon
atoms which are formed in the core are transported to the
envelope, making carbon stars.

Because the outer layers of the atmosphere are so far from the
center of the star --- where most of the mass lies --- they are
held only weakly by the gravity of the star. Instabilities
in the atmosphere cause many red giants to develop pulsations;
in some cases, the motions grow until the outer layers are blown
clear off the star and float into space, making planetary
nebulae.

So, actually, some carbon stars may turn into planetary
nebulae.

Are all red stars carbon stars?

No, very few.


Michael Richmond