Stages of stellar evolution

In article ,
Oh No writes:
I am not quite clear about what happens exactly at the end of the main
sequence. As I understand, the subgiant branch starts from ignition of
the thick Hydrogen shell,

You have to distinguish between empirical and theoretical
classifications. "Subgiant branch" is empirical, based on observed
temperature and surface gravity. Hydrogen shell burning is
theoretical. Obviously the two will often correspond (given that
stellar evolution theory is pretty good), but sometimes the
demarcations between one class and another won't coincide.

Another thing to be careful of is that evolution depends critically
on stellar mass. Not all masses go through all stages, and the time
scales are different for different masses.

interpret a diagram, not the text, as indicating a short period of re-
ignition of hydrogen in the core before ignition of the shell.

Back when I was in school -- admittedly some years ago -- the
theoretical end of main sequence burning was when the core hydrogen
was used up. If that's correct, there can't be any reignition.

When the hydrogen is used up, the core contracts and heats up. It
isn't obvious to me (nor do I remember) which starts first: core
helium burning or shell hydrogen burning. I would expect the time
delay from the start of one to the other to be small, but I could be
wrong. Both sequence and timing may well depend on mass.

I don't see a name for either the contraction following the horizontal
branch,

That's an extremely brief phase, so there won't be an empirical name
for it.

or the subsequent phase when there is a thick helium burning
shell (corresponding to the subgiant branch) and before the asymptotic
giant branch.

Subgiant or giant? (You can see my memory is limited.)

Since the initial contraction
is classified as part of the main sequence,

Presumably that refers to the empirical classification, the outer
layers not yet showing observable changes even while the core is
contracting.

why is this contraction not
considered part of the horizontal branch?

HB is an empirical classification. If I recall correctly (and see
above about limited memory), there are several theoretical stages all
belonging to the empirical horizontal branch.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
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Thus spake Steve Willner
In article ,
Oh No writes:
I am not quite clear about what happens exactly at the end of the main
sequence. As I understand, the subgiant branch starts from ignition of
the thick Hydrogen shell,

You have to distinguish between empirical and theoretical
classifications. "Subgiant branch" is empirical, based on observed
temperature and surface gravity. Hydrogen shell burning is
theoretical. Obviously the two will often correspond (given that
stellar evolution theory is pretty good), but sometimes the
demarcations between one class and another won't coincide.

That makes sense. The (theoretical) evolutionary tracks of Iben
(reproduced in Carroll & Ostlie) actually show ignition marginally
before the point described as the start of the subgiant branch.

Another thing to be careful of is that evolution depends critically
on stellar mass. Not all masses go through all stages, and the time
scales are different for different masses.

Very true, but for my purpose it seems ok to simply think of, e.g. low
mass stars, terminating at the Helium flash and skipping the remaining
stages.

interpret a diagram, not the text, as indicating a short period of re-
ignition of hydrogen in the core before ignition of the shell.

Back when I was in school -- admittedly some years ago -- the
theoretical end of main sequence burning was when the core hydrogen
was used up. If that's correct, there can't be any reignition.

I had thought that, but that is actually the source of my confusion. In
the theoretical description, the end of the main sequence is a couple of
steps after core hydrogen burning stops. Core hydrogen burning is
followed by a period of contraction, in which a star gets hotter and
brighter, then the star gets dimmer and cooler again. If I understand it
right this is due to fuel in the core - I assumed it must be residual H,
but I don't think it says, so perhaps it is a very short period of He
burning. I don't think so though, because He burning in the core of
giants is shown in a much smaller region - or so it appears - it could
be illusion, because they don't actually say what the vertical axis on
the graph means :-\. Only after this short phase of core burning does
hydrogen start to burn in the shell, and the star almost immediately
enters the subgiant branch, in which there is only shell burning.

When the hydrogen is used up, the core contracts and heats up. It
isn't obvious to me (nor do I remember) which starts first: core
helium burning or shell hydrogen burning. I would expect the time
delay from the start of one to the other to be small, but I could be
wrong. Both sequence and timing may well depend on mass.

As far as I can tell, the sequence is the same for all stars (except
perhaps those with very extreme masses), though of course the timing is
very different. Core burning, whether He or H, does not last long at
this point. The main He core burning phase does not start until the
horizontal branch.

I don't see a name for either the contraction following the horizontal
branch,

That's an extremely brief phase, so there won't be an empirical name
for it.


or the subsequent phase when there is a thick helium burning
shell (corresponding to the subgiant branch) and before the asymptotic
giant branch.

Subgiant or giant? (You can see my memory is limited.)

Subgiant. The subgiant branch has stars with thick H burning shells,
this phase has stars with thick He burning shells, and follows a similar
track but much brighter (without checking the tables, I think also more
shortlived). As far as I can guess from studying H-R diagrams etc,
Cepheids and W Virganis stars are probably in this phase, and have
variability caused by a similar process to delta scutids and RR Lyrae
(which are in the Horizontal branch). But I can't find any reference to
confirm or deny this guess. Is it even known?

Since the initial contraction
is classified as part of the main sequence,

Presumably that refers to the empirical classification, the outer
layers not yet showing observable changes even while the core is
contracting.

I guess the only observable change is that the star is getting hotter
and brighter - but I imagine we can't observe that over the timescales
for which we have observation.

why is this contraction not
considered part of the horizontal branch?

HB is an empirical classification. If I recall correctly (and see
above about limited memory), there are several theoretical stages all
belonging to the empirical horizontal branch.

C & O, with diagrams from Iben, describe the horizontal branch as the
core Helium burning phase, though there are distinct stages within that,
not all of which are necessarily described. Both books made me think
that horizontal branch described the direction of the theoretical
tracks, just as assymptotic branch describes the direction of the track
directly upwards, similar to the red giant branch.

Possibly, if I am going to really understand this, I need to get a
specialist book on stellar evolution. Any recommendations for something
which I can rely on as being up to date, complete and accurate?

Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.teleconnection.info/rqg/MainIndex

Charles Francis wrote:

Possibly, if I am going to really understand this, I need to get a
specialist book on stellar evolution. Any recommendations for something
which I can rely on as being up to date, complete and accurate?

I recommend Clayton, Principles of Stellar Evolution and
Nucleosynthesis. Not necessarily up to date but puts a lot more effort
into the fundamentals than other (too descriptive) texts.

--
ciao,
Bruce

drift wave turbulence: http://www.rzg.mpg.de/~bds/