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m1omg
2012-Apr-29, 11:56 AM
Which stage in stellar evolution are orange giants like Arcturus or Pollux? Are they stars just starting to expand to the red giant stage, or "helium burning stars" once the red giant shrinks and stabilizes before expanding again into the asymptotic giant branch? Or something else entirely?

Romanus
2012-Apr-29, 02:26 PM
Most of them are probably "clump giants", burning helium in their cores after the first ascent of the RGB. This is a fairly long-lived stage in post-MS solar-mass stars compared to the two red giant phases; for our own Sun this will last about a billion years.

m1omg
2012-Apr-29, 03:27 PM
Thanks, so are they stars in the helium burning stage as described here http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html , except that this stage really lasts 1 billion years in Solar mass stars instead of just 110 million years as described on that really old website?

George
2012-Apr-29, 08:32 PM
Thanks, so are they stars in the helium burning stage as described here http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html , except that this stage really lasts 1 billion years in Solar mass stars instead of just 110 million years as described on that really old website? The billion year time span seems high compared to other sources. One 2008 textbook shows several hundred million years from the time a solar mass star leaves the MS until it goes through its helium burning stage, with the longest phase being the time when hydrogen is burning in the shell around the helium core (ie subgiant branch).

Arcturus is ascending up the RGB toward the helium burning phase. It is about 1.5x in solar mass, so it will have a hotter surface temp., no doubt, than the Sun will be during this phase.

The helium burning stage lasts for only a few tens of millions of years for a solar mass star.

Romanus
2012-Apr-30, 09:23 PM
Yeah, looks like I messed up; probably got the sub giant phase confused with the clump phase.

George
2012-May-02, 01:39 AM
Yeah, looks like I messed up; probably got the sub giant phase confused with the clump phase.I wouldn't be surprised, however, if certain orange stars [M class but with <1 solar mass] along the MS do take a billion years to go through the red giant phase. :)