The astronomer’s dictionary
Misleading terms make learning Astronomy harder.
One of our astronomer’s professors in graduate school once said, “Astronomers are extremely good at misnaming things.” That is, the very words they use are often misleading or at least mystifying. The practice must present something of a barrier to learning the subject. All astronomers will admit it; and continue to use the terms.
Take, for example, “early-type” and “late-type” stars. The former are bluer, the latter redder; the term survives from a long-exploded theory of stellar evolution. Early-type stars actually formed no earlier than late-type stars, nor are they necessarily going to become later types with the passage of time. The terms survive because it is often convenient to refer to stars with this kind of sweeping generalization, rather than being pedantic and saying something like, “Stellar types from A3 to F9, not including supergiants.” There are also early-type and late-type galaxies. Here there isn’t even the excuse of a long-dead theory; at the time the terms were coined there wasn’t an accepted theory of galactic formation and evolution. Confusingly, an “early-type” galaxy will consist mostly or completely of “late-type” stars.
Population I stars did not form before Population II stars; in fact, it’s generally the other way around. And Population III stars are the postulated first generation, forming before either one. None have been found yet, to our knowledge; but there is a term for them anyway.
Astronomers call any element other than hydrogen and helium a metal. It is useful for astronomers to lump all the heavier elements together, because they make up so little of the universe. However, it clashes with the usage of chemists, for whom the difference in behavior between (say) sodium and sulfur is important. Astronomers will also refer to “dry mergers” of galaxies, having nothing to do with absence of water; and to the hydrostatic equilibrium of stars, with no water around at all.
There are three types of minutes in astronomy. Two are angular measures on the sky, but a minute of Right Ascension is not the same size as one of Declination. A similar situation holds for seconds. The third type of each measures time. There are hours measuring angle, and hours measuring time. The Earth turns through about an hour of Right Ascension in an hour of time (but not exactly). Celestial Longitude and Latitude are well-defined terms and used in some calculations; but that system is inclined at an angle of about 23 degrees from that of terrestrial Longitude and Latitude.
Stars can change their positions relative to each other. These motions are slow, but measurable, and apparently approved of by astronomers, the technical term being “proper motion.” However, galaxies that deviate from the general expansion of the universe have “peculiar velocities,” despite the fact that they are not at all unusual. Similarly, a planet (or anything else) in orbit has a mean, eccentric and true “anomaly.” There is nothing out of the ordinary, inexplicable or different from the crowd about any of these, nothing anomalous.
The most confusing thing for anyone actually doing astrophysics is the system of “magnitudes.” The word in general, and in most scientific situations, means size. Something of larger magnitude is bigger (in some way) than another. But a star of magnitude 2 is brighter than one of magnitude 4. Without going into the mathematical details, our astronomer assures us that trying to calculate a galaxy’s surface brightness in magnitudes per square arc second is a real pain.
The technical terms in astronomy were not intentionally constructed to make the subject hard to learn. But sometimes they do work that way.
