I was reading about the energy transportation in stars here, and I found this: "The outer portion of solar mass stars is cool enough that hydrogen is neutral and thus opaque to ultraviolet photons, so convection dominates".

I always thought that almost all hydrogen and helium inside the Sun (and even in the convection zone) are completely ionized, so this doesn't seem to be the correct explanation of why convection dominates there for me.

I know that the solar convection zone is 200,000 kilometers deep from the photosphere, and temperature at this depth is 2 million °K which keeps decreasing until it reaches the 5700 °K of the photosphere.

So my question here is, at what depth from the photosphere is the temperature low enough for hydrogen to exist as atoms and not ions ? And is this explanation of neutral hydrogen correct ?

EDIT: I added a picture of a relation between the temperature and the radius inside the Sun. Hope it helps. enter image description here

  • $\begingroup$ 13.6eV is about 170,000K, so certainly below 17,000K you don't have much ionized hydrogen. But, I'm not an expert on where in the convection zone will it get that low... $\endgroup$
    – Jon Custer
    Oct 7, 2015 at 23:38
  • $\begingroup$ @JonCuster, so is it that simple ? Just convert the ionization energy to temperature and consider atoms in higher temperature to be completely ionized ? $\endgroup$ Oct 7, 2015 at 23:55
  • $\begingroup$ Well, it is a chemical reaction following detailed balance, so, yes. $\endgroup$
    – Jon Custer
    Oct 7, 2015 at 23:57
  • $\begingroup$ Note the numbers of zeros in @JonCuster's numbers: you can be significantly ionized a bit below the ionization energy temperature, since there is a tail to the Maxwell-Boltzmann distribution, and since there is an entropy effect (there are more ways to arrange N ions + N electrons than N neutral atoms). See the Saha equation. Convection is also an involved thing in its own right; having UV absorbers is neither necessary nor sufficient to induce it. All that said, I don't really know the solar model well enough to answer your actual question. $\endgroup$
    – user10851
    Oct 8, 2015 at 3:02
  • $\begingroup$ @ChrisWhite, I added a picture of temperature vs radius of solar model. I don't know if this is enough to answer the question though. $\endgroup$ Oct 8, 2015 at 3:13

2 Answers 2


The boundary between the radiative and convective zones is at a temperature of around $10^6$ K. Hydrogen atoms will not survive temperatures much above 20,000 K (to see why, study the Saha equation). Therefore the explanation that convection begins because of the increased opacity brought about by hydrogen atoms is incorrect and is there is very little un-ionised hydrogen in the convection zone. The same goes for helium, although that obviously survives to slightly higher temperatures - it starts to be ionised at about 12,000 K and is almost completely ionised above 30,000 K. Thus almost none of the convection zone (in terms of a mass fraction, or even as a fraction of the radius) contains neutral hydrogen and helium atoms.

Convective heat transport takes over when the temperature gradient in the interior becomes steeper than the "adiabatic temperature gradient". $$\left|\frac{dT}{dr}\right| > \frac{T}{P}\left|\frac{dP}{dr}\right|\left(1 - \frac{1}{\gamma}\right),$$ where $\gamma$ is the adiabatic index of the gas (e.g. 5/3 for a monatomic ideal gas). If we have hydrostatic equilibrium and a perfect gas of mean atomic weight $\mu$ then $$\left|\frac{dT}{dr}\right| > \frac{\mu}{k_B}\frac{GM(r)}{r^2}\left(1 - \frac{1}{\gamma}\right),$$ where $M(r)$ is the mass interior to $r$.

In the core and deep interior, the energy transport is radiative and the temperature gradient is given by radiative diffusion: $$\left|\frac{dT}{dr}\right| = \frac{3\rho \kappa L}{64\pi \sigma T^3 r^2},$$ where $\rho$ is the density, $\kappa$ is the opacity, $L$ is the luminosity (generated interior to $r$) and $\sigma$ is the Stefan-Boltzmann constant.

Now in the outer parts of the star $M(r)$ changes slowly, so comparing the radiative gradient with the adiabatic gradient, we see that convection can be triggered by a combination of increasing opacity and decreasing temperature, but that the gradient is decreased by decreasing density.

At temperatures of a million K, the opacity is given approximately by Kramer's opacity $\kappa \propto \rho T^{-3.5}$, which is dominated by bound-free absorption by abundant highly ionised elements such as oxygen and iron. So the radiative temperature gradient increases as $\rho^2 T^{-6.5}$ (the decreasing temperature and stronger temperature dependence wins out over the decreasing density).

In the end then it is a combination of decreasing temperature and increasing opacity which triggers convective instability in the Sun.


I always thought that almost all hydrogen and helium inside the Sun (and even in the convection zone) are completely ionized...

This question has been bothering me ever since it was asked. I had a similar, apparently naive, assumption until I bugged a solar physicist down the hall.

It turns out that not only does the sun become partially ionized low in the chromosphere and through much of the photosphere into the convection zone. The interesting part is not only are these regions not totally ionized, the ions that do exist were unexpected. For instance, there are several heavy elements/metals (e.g., silicon, sodium, etc.) that will freely give up electrons even at the low temperatures of the photosphere and there are corresponding quasi-stable H- and H2+ states in addition to the usual H+, He+, and He2+.

...so this doesn't seem to be the correct explanation of why convection dominates there for me.

As you might imagine, the ionization states are altitude-/depth-dependent and not trivial [e.g., Fontenla et al., 1990, 1991, 1993, 2002]. The level of complexity increases even further when including even heavier elements and the nuances of their electron orbitals, which have different frequency-dependent opacities.

Further, H- and H2+ can dominate the opacity in some places that should be too cold to support H+ [e.g., Fontenla et al., 1990, 1991, 1993, 2002].

So my question here is, at what depth from the photosphere is the temperature low enough for hydrogen to exist as atoms and not ions?

There is apparently neutral hydrogen well above the photospheric surface but it does not become a significant fraction of the charged density until until within a few thousand kilometers of the surface [e.g., Avrett et al., 1976]. The fraction of neutral hydrogen depends upon the rates of recombination and ionization.


As Rob (Jeffries) pointed out, I did not discuss the Saha equation. It turns out that the regions of interest are not in local thermodynamic equilibrium or LTE, which is required for the use of the Saha equation. The Fontenla et al. group has been actively creating more and more complex solar atmosphere models for over 20 years and are considered the experts on this topic. If you scan through the references I provided, they do not mention the Saha equation, which was surprising to me the first time I read them.

However, Avrett et al. [1976] does discuss the continuum source function, $S_{\nu}^{c}$, for non-LTE systems. In the limit of LTE, the Saha equation can be used to transform $S_{\nu}^{c}$ to recover Planck's law.

Update 2

Rob (Jeffries) made several more arguments as to why the Saha equation is relevant and convinced me it is important for the convection zone. The models to which I refer only go ~1% into the convection zone and below that it starts to become more and more difficult to argue against using something like the Saha equation (i.e., the fluid goes to LTE rather quickly).


  • Avrett, E.H., et al. "Excitation and ionization of helium in the solar atmosphere," Astrophys. J. 207, pp. L199-L204, 1976.
  • Fontenla, J.M., et al. "Energy balance in the solar transition region. I. Hydrostatic thermal models with ambipolar diffusion," Astrophys. J. 355, pp. 700-718, 1990.
  • Fontenla, J.M., et al. "Energy balance in the solar transition region. II. Effects of pressure and energy input on hydrostatic models," Astrophys. J. 377, pp. 712-725, 1991.
  • Fontenla, J.M., et al. "Energy balance in the solar transition region. III. Helium emission in hydrostatic, constant-abundance models with diffusion," Astrophys. J. 406, pp. 319-345, 1993.
  • Fontenla, J.M., et al. "Energy balance in the solar transition region. IV. Hydrogen and helium mass flows with diffusion," Astrophys. J. 572, pp. 636-662, 2002.
  • $\begingroup$ No answer to this question should avoid discussing the Saha equation. $\endgroup$
    – ProfRob
    Nov 29, 2017 at 7:46
  • $\begingroup$ @RobJeffries - I honestly thought that would be the case as well, but apparently much of these regions are not in LTE so apparently the Saha equation cannot be used (at least not with the usual hand waving). I was informed by my solar whisperer colleague to look up work by Fontenla et al. as their group are the experts on the topic, thus the reference list. They do not discuss the Saha equation precisely because it does not apply in much of the lower solar atmosphere. This was news to me... $\endgroup$ Nov 29, 2017 at 14:01
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    $\begingroup$ All the papers you refer to are about the solar transition region, which is above the photosphere. Saha's equation most certainly applies in the photosphere and below, which is what the question is about. Saha's equation tells you the ionisation fraction as a function of temperature and density. $\endgroup$
    – ProfRob
    Nov 29, 2017 at 19:55
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    $\begingroup$ In the plot in the OP. the photosphere is about 100km thick and is at R=1. The question is about what happens below the photosphere. $\endgroup$
    – ProfRob
    Nov 29, 2017 at 20:35
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    $\begingroup$ By how far!? 2000 km is 1% into the convection zone. And the Saha eqn is certainly valid there. $\endgroup$
    – ProfRob
    Nov 29, 2017 at 23:29

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