Question about stellar convection

Erik Max Francis kirjutas:
wrote:

Three questions occur.

1) Around what stellar mass is the cutoff from "convective" to "not"?

Googling around suggests the limit is about 0.5 solar masses.

Numbers like 0,25 can also be found.

_Stellar
Structure and Evolution_ (Kippenhahn and Weigert) puts the effective
temperature threshold as 5000 K.

Temperature on surface, then?

2) Is this a sharp break point, or a gradual transition kind of
thing?

I suspect it's a pretty sharp breaking point, but I haven't seen
anything which directly states that.

3) If it's a sharp break point, then you'd expect a sudden kink in
the star's expected lifespan. Stars just below that point would be
convective and have very long lives; stars just above it would not be,
and so would be much more short-lived. Is this in fact the case?

Yes, if 2) is true that would follow.

The key term to look for regarding fully-convective stars is the Hayashi
line, which represents the locus of stars (of a given mass) that are
fully convictive. As these stars evolve, they'll move up these lines,
which are quite steep, almost vertical. Note that states to the right
of this line are unstable; stars that find themselves there will
convectively adjust back to the Hayashi line or to the left of it.

How can they? Hayashi line basically states that a star of a given
mass cannot have temperature below a certain minimum. What about
brown, white and black dwarves? The surface of a star has a lower
bound, while luminosity does not have a lower bound!

wrote:

Erik Max Francis kirjutas:

_Stellar
Structure and Evolution_ (Kippenhahn and Weigert) puts the effective
temperature threshold as 5000 K.

Temperature on surface, then?

Not quite, but close. It would be the temperature of a blackbody with
the same luminosity and radius as the star. It's used in stellar
structure calculations and modeling.

The key term to look for regarding fully-convective stars is the Hayashi
line, which represents the locus of stars (of a given mass) that are
fully convictive. As these stars evolve, they'll move up these lines,
which are quite steep, almost vertical. Note that states to the right
of this line are unstable; stars that find themselves there will
convectively adjust back to the Hayashi line or to the left of it.

How can they? Hayashi line basically states that a star of a given
mass cannot have temperature below a certain minimum. What about
brown, white and black dwarves? The surface of a star has a lower
bound, while luminosity does not have a lower bound!

I misspoke. I meant as the stars evolve along the main sequence, not as
they evolve off the main sequence. On the main sequence they slowly
move up their Hayashi line as they expend their fuel.

--
Erik Max Francis && && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis
The future doesn't belong to the fainthearted; it belongs to the
brave. -- Ronald Wilson Reagan, 1911-2004

wrote:
Erik Max Francis kirjutas:
wrote:
Three questions occur.

1) Around what stellar mass is the cutoff from "convective" to "not"?

Googling around suggests the limit is about 0.5 solar masses.

Numbers like 0,25 can also be found.

Doesn't it depend on the "metal" content?

Older stars have less of the heavy elements and so tend to radiate
easier from down inside.

-HJC

I personally don't think that there is a lot going on below the
photosphere of the sun at all. Its radiative emission could simply be
explained in terms of its gravitational energy and electronic
processes in the photosphere (it is not at all so that this would have
lasted just a few million years as energy conservation is a concept of
classical mechanics and does not apply to light).
See my webpage regarding the coronal heating http://www.plasmaphysics.org.uk/research/sun.htm
and my discussion page http://www.physicsmyths.org.uk/discussions/
sun.htm for more in this respect.

Thomas

I personally don't think that there is a lot going on below the
photosphere of the sun at all. Its radiative emission could simply be
explained in terms of its gravitational energy and electronic
processes in the photosphere (it is not at all so that this would have
lasted just a few million years as energy conservation is a concept of
classical mechanics and does not apply to light).
See my webpage regarding the coronal heating http://
www.plasmaphysics.org.uk/research/sun.htm
and my discussion page http://www.physicsmyths.org.uk/discussions/sun.htm
for more in this respect.

Thomas