Astronomy & Celestial Mechanics
the sky, computed
Concept

Cold neutral medium — where it appears

The dense, cold phase of interstellar atomic hydrogen, at tens to a hundred kelvin and tens of atoms per cubic centimetre. It is cooled mainly by ionised carbon, and it is the gas from which molecular clouds and stars form.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Two densities in thermal balance at one pressure, 121 times apart. Thermal equilibrium for interstellar gas, drawn as pressure against density with both axes logarithmic. Every point on the curve is a temperature between 40 and 9000 K at which cooling exactly balances the 2·10⁻²⁶ erg per second per hydrogen nucleus that grain photoelectrons deliver. The curve is not monotonic: it rises to 5007 K cm⁻³, falls to 1597, and rises again, so a horizontal line anywhere between those two crosses it three times. At the 3000 K cm⁻³ of the local medium the three crossings are a warm phase at 0.47 cm⁻³ and 6354 K, a cold phase at 57 cm⁻³ and 52 K, and one in between drawn dashed because it cannot survive — a parcel there that is squeezed cools and keeps contracting, and one that expands heats and keeps expanding. The two survivors differ by a factor of 121 in density and by exactly the same factor in temperature — necessarily the same, because their product is the pressure both are held at — and yet they press on one another equally, which is why they can share the same volume of the disc indefinitely rather than mixing.

Two temperatures, and nothing in between

The gas between the stars is not one gas at one temperature. Thermal balance has three solutions at any ordinary interstellar pressure, the middle one cannot survive being nudged, and the two that can differ by a factor of over a hundred in both density and temperature — which is why a map of the interstellar medium looks like clouds rather than like fog.

galaxies · Interstellar medium Molecules take longer to form than a cloud takes to fall. Three timescales for a shielded cloud of 100 hydrogen atoms per cubic centimetre, against its metallicity relative to the Sun's on a logarithmic scale, all in years on a logarithmic scale. Molecular hydrogen forms on dust grains, and the time to turn the cloud's atoms into molecules is inversely proportional to the dust, so to the metallicity: 5.3 million years at solar, 53 million years at a tenth, 530 million years at a hundredth. The cloud's free-fall time, 4.3 million years, does not depend on its metals. The two cross near solar metallicity: above it the molecules form as fast as the cloud can collapse, below it the cloud collapses before most of its hydrogen has become molecular. The dashed line is a galaxy's molecular depletion time, two billion years — the timescale that equilibrium arguments implicitly assume is short compared with everything else.

A molecule that forms too slowly to wait for

The atomic hydrogen in a galaxy's disc turns molecular wherever a cloud is thick enough to shield itself from starlight, and stars form where it does. That is an equilibrium statement, and equilibrium takes time. Molecular hydrogen forms on dust grains, so in gas with a tenth of the Sun's metals it forms ten times more slowly — slowly enough that a cloud can finish collapsing before most of its hydrogen has become molecular, and form its stars from atoms.

galaxies · Interstellar medium

Named alongside it

The objects these essays reach for when they reach for this one.

Interstellar mediumCooling functionDepletion timeField criterionFine-structure coolingFree-fall timeHot ionised mediumInterstellar dustMetallicityMolecular cloudsMolecular hydrogenPhotoelectric heating

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