FreeLists / az-observing / [AZ-Observing] Re: Red Clump Stars

[Brian Skiff <Brian.Skiff@xxxxxxxxxx>]

     The red-giant 'clump' stars are relatively metal-rich (i.e. roughly
Sun-like in heavy elements beyond hydrogen and helium) and have evolved
past the red-giant-branch phase, and are sitting at a lower brightness
level.  Core helium-burning is still going on, but not much is happening
in terms of output from the surface, so despite their varying age ranges
(or state of evolution) they are all very similar in brightness, and
thus form a clump in a color-magnitude diagram.  The 2000 German page
(and the Jan 2000 Harvard press release on which it is based is, well,
it's three years old.  The clump giants are often used as a test of
distances, though there is still problem with population effects on
the brightness, which is why the 3+ year old press item is wrong. 
(The LMC is still at 50 kiloparsecs = distance modulus 18.5.)
     The four red giants in the Hyades and the near-identical four
giants in Praesepe are all red clump giants.  In one of his final papers,
Phil Keenan redfined the MK classification system for the cool giants
to allow for the slightly fainter red clump stars so that the regular
ascending-giant-branch stars are luminosity class IIIa, and the red clump
stars are IIIb. (See 1999ApJ...518..859K for details.)  Among the
naked-eye stars he classifies as red clump stars are Hamal (alpha Ari)
and Pollux, given as type K1.75IIIb Ca-1 and K0IIIb, respectively.

\Brian
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