The problem with hot gas

I joined Physics Today‘s editorial staff in June 1997. Back then, one of the first adjustments I had to make was to edit and write articles for all kinds of physicists, not just for the astronomical community I used to belong to.

As you can imagine, part of the adjustment entailed watching out for specialist jargon. Astronomers know what BL Lac objects and P Cygni profile are, but most other physicists don’t. Biophysicists, geophysicists, and other groups of physicists have their own jargon. My fellow editors and I strive to explain or eliminate it.

Jargon is usually easy to spot—especially if it’s unfamiliar to the editor. But there’s another verbal barrier to smooth communication among physicists: the specialized usage of seemingly nonjargon words and phrases.

I’m not referring to the particle physicists’ flavor, charm, or strangeness, which are almost never misleading in context. Nor am I referring to words like stress and strain, which have precise and distinct meanings in physics and engineering, even though the two words are synonyms in everyday parlance.

No, the words I find treacherously misleading are more subtle. In accelerator physics, “luminosity” means what you might expect: the number of particles per unit area per unit time—except when the particle physicists implicitly integrate over time and use the term as a measure of accumulated events. In its renewed quest to measure the mass of the Higgs boson, Fermilab’s Tevatron isn’t really becoming more luminous; it’s running for longer.

Another tricky usage is astronomers’ “hot gas.” Stellar coronae, galactic winds, and supernova remnants—to name just three examples—contain hot gas. Fine. But the gas in those objects is nearly always ionized and is more properly called plasma.

Does it matter whether you say the Sun’s corona is made up of hot gas or plasma? Yes. Because plasma particles are charged, the physics is far more complex that it is for a collection of neutral gas particles. Just how the Sun’s corona is heated to temperatures far higher than the chromosphere beneath it remains a mystery—despite decades of detailed observations. To refer to another star’s corona as “hot gas” is to brush that complexity, which all stellar coronae more or less manifest, under the metaphorical carpet of self-deception.

That said, most astrophysical plasmas are neutral overall. Some calculations, such as James Jeans’s 1902 estimate of how massive a cloud must be before it collapses under its own gravity, work just as well for neutral gas as they do for plasma.

Still, I think it’s wise to refer to hot astronomical gas as plasma, if only as a reminder of the complexity that might be lurking inside. And if you’d like more than a flavor of the challenging complexity of astrophysical plasmas, I recommend taking a look at the recently published report of a workshop held in January in Princeton, New Jersey.

Incidentally, the report is what gave me the idea for this post. Thanks, Hantao!