Astronomy & Celestial Mechanics
the sky, computed
Series

Opacity — the series

5 essays on one idea, from the one that introduces it to the one that assumes the rest.
  1. Opacity against temperature, and the three things that supply it. The Rosseland mean opacity of a gas of composition X = 0.7, Y = 0.28, Z = 0.02, on logarithmic axes, at 10⁻⁷ g/cm³ and 10⁻⁶ g/cm³. The three faint curves are the separate processes at the first density — electron scattering, the Kramers bound-free and free-free term, and the negative hydrogen ion — and the solid curve is their sum. Every one of them is multiplied by the fraction of hydrogen the Saha equation says is ionised at that temperature and density, or by one minus it for H⁻, which is the only reason the low-temperature end is a picture of a star rather than of a formula outside its range: ungated, Kramers alone gives 10,343 cm²/g at 5,800 K, against the 0.40 drawn here. The peak sits at 15,400 K, where hydrogen is 78% ionised — that bump is not a detail, it is the engine of a Cepheid — and the flat floor at high temperature is electron scattering, which is the one term with no temperature in it at all.

    The surface that is a depth

    A star has no surface. What looks like one is the level at which the optical depth reaches about two-thirds — and the edge is sharp only because the opacity climbs so steeply that the transition takes a ten-thousandth of the radius.

    part 1 · starlight
  2. Two means of one opacity, 75 times apart, and only the smaller one is in the equation. Above: a synthetic opacity across the frequencies that carry a star's flux, drawn against x = hν/kT, with a continuum falling as ν⁻³ and a forest of 6 lines per unit x on top of it. The shaded curve is the Rosseland weighting function, ∂B_ν/∂T, which peaks at x = 3.83 and is what decides which frequencies matter. The two horizontal lines are the two ways of averaging. The Planck mean is an ordinary average and lands high, among the lines, because that is where most of the opacity is. The Rosseland mean is a harmonic average — it averages 1/κ rather than κ, because what carries the flux out of a star is transparency and transparencies add — and it lands 75 times lower, close to the continuum, because a harmonic mean is dominated by the smallest values in it. In other words the opacity that appears in the equation of radiative transport is a measurement of the gaps between the lines. Below: what that means for a table. The Planck mean rises as the first power of the line density, slope 0.76 as drawn — every line added is another contribution to an ordinary average. The Rosseland mean does almost nothing at first, slope 0.13, and then turns up sharply, slope 0.99, once the lines are close enough to blanket the windows. That is why adding several million atomic transitions to an opacity table in the early 1990s changed nothing for decades and then changed stellar structure: the new lines were not the first lines, they were the ones that finally closed the gaps.

    A mean dominated by the gaps

    The opacity in the equation of radiative transport is not an average of the opacity. It is a harmonic average weighted by the temperature derivative of the Planck function, which makes it a measurement of the transparent windows between the lines rather than of the lines — and that single fact decides what adding a million spectral lines to a table does.

    part 2 · starlight
  3. A better measurement that made the model worse: 0.9 per cent in the sound speed. The fractional difference between the Sun's sound speed as its own oscillations measure it and as a structural model predicts it, against fractional radius. Zero would be agreement. The lower curve is the model built on the solar abundances used until the mid-2000s, and it hugs the axis: a part in a thousand across most of the interior, which was for a long time the best-tested piece of stellar physics anybody had. The upper curve is the same model with the abundances re-measured using three-dimensional atmospheres and without assuming local thermodynamic equilibrium — better measurements by every methodological standard, which lowered carbon, nitrogen and oxygen by around thirty per cent. The disagreement grows to 0.9 per cent, and it is not spread through the star: it peaks at 0.683 of the radius, just beneath the base of the convection zone at 0.713. The same substitution moves the model's own convection-zone base from 0.715 to 0.729, against a seismic value known to about a thousandth. What is being tested here is not really the abundances but what converts a composition into a structure, which is the opacity: the metals whose abundances fell are exactly the ones whose bound–free absorption dominates at those temperatures, and an opacity larger by some fifteen per cent near that boundary would restore the agreement. Laboratory measurements of iron at those conditions have since come in high by about that much, which is a satisfying result to have arrived at by way of a discrepancy in the sound speed of the Sun. The curves are published inversions and model differences rather than anything computed here; what the figure adds is where they peak and by how much.

    A better measurement that made the model worse

    The Sun's composition was re-measured with better atmospheres and better physics, and the carbon, nitrogen and oxygen abundances fell by about thirty per cent. The improved model then disagreed with the Sun's own oscillations by ten times as much as the model it replaced, and it still does.

    part 3 · starlight
  4. Hayashi lines: a luminosity that moves and a temperature that does not. The effective temperature a fully convective star must have, against its luminosity, for 0.5, 1, 2 solar masses, drawn as a Hertzsprung–Russell diagram with temperature falling to the right. The temperature follows from matching the star's convective adiabat to a photosphere whose opacity, from the negative hydrogen ion, rises as the ninth power of temperature. Over a factor of thirty thousand in luminosity each line moves by only 363 K, 398 K, 436 K respectively: the lines are nearly vertical, at 3650 K, 4000 K, 4383 K for one solar luminosity. A contracting young star, or a swelling giant, moves up or down its line almost without changing colour. To the right of the lines no star in hydrostatic equilibrium can sit.

    A wall built by a steep opacity

    A star that is convective all the way to its surface cannot choose its surface temperature. The interior fixes a relation between pressure and temperature, the photosphere fixes another through its opacity, and the negative hydrogen ion's opacity rises so steeply with temperature that the two meet at nearly the same temperature whatever the star's size. Young stars contracting and old stars swelling slide up and down a line in the diagram almost without changing colour — and nothing in equilibrium can sit to its right.

    part 4 · starlight
  5. The gradient radiation would need, as a young star shrinks. The radiative temperature gradient at the centre of a contracting, fully convective pre-main-sequence star — the gradient radiation alone would need to carry the luminosity — against the star's radius, which shrinks to the right, for 0.3, 0.5, 1, 2 solar masses, logarithmic. The centre stays convective while the gradient exceeds the adiabatic 0.4 (dashed). As the star shrinks its centre gets hotter and denser, Kramers opacity, ∝ ρT⁻³·⁵, falls, and the gradient falls as the radius to the power 2.5; where it crosses 0.4 a radiative core appears. 0.3 M☉: not before the main sequence; 0.5 M☉: not before the main sequence; 1 M☉: at 1.94 R☉; 2 M☉: at 7.62 R☉. Dots: each mass's main-sequence radius, where contraction stops. A Sun-like star leaves its convective line at 1.9 solar radii; the lightest never do.

    Where a shrinking star stops boiling at its centre

    A young star descends the Hertzsprung–Russell diagram at almost fixed temperature, convective from centre to surface, until something at its centre changes and it turns sharply left toward the main sequence. The something is the opacity. As the star shrinks its centre grows hotter and denser, the Kramers opacity there falls, and radiation becomes able to carry the luminosity without the steep gradient that drives convection. A Sun-like star crosses that line at about twice its present radius, two million years old; a star of a third of the Sun's mass never crosses it at all.

    part 5 · starlight

All series