27 May 2005

Septentrional - Ursa Major

While the focus of this log is mostly political, occasionally I indulge other interests, including unusual connections, where one thing leads to another.

I had occasion today to comment to my father, who was offering me several unusual words for use as my “words for the day,” on the word "Septentrional," a more or less interchangeable term with "boreal." "Septentrional" refers to the Seven Stars of Ursa Major (aka "Septentrion"), which dominates the skies of the North. I have been unable to find a truly comparable term linking the regions of the South with some prominent feature of the Southern Sky. "*Argonavital," "Magellanic," or even "Crucial" come to mind, but none of them has any pedigree, and the last one has obvious drawbacks in that it already has other meanings.

It’s a not-too-well known fact that the Sun and its planets are located in a somewhat enriched region of the Milky Way Galaxy, from the point of view of stellar populations and especially prevalence of bright stars in the vicinity. At least part of the reason for this is that the Sun happens to be in the midst of a slowly dissolving star cluster, the Ursa Major Stream or “Moving Group.”

The seven chief stars of Ursa Major (“The Big Dipper,”) and some of the other stars of the constellation, to which could be added Sirius, are the main members of a real star cluster in space, noticeable from any given direction for a distance of at least a couple of hundred light years. You may notice, from the inclusion of Sirius, which is something like 50 degrees of arc across the sky, that the Sun is actually inside this loose association of stars, but it is not a member of it. These are relatively young stars, having formed together from a protostellar nebula something like 150 million years ago, since which time they have been gradually drifting apart, and the cluster losing coherence as the stars "evaporate" into the general stream of stars in the galactic plane. The Sun, in its normal streaming orbit around the Galaxy, just drifted on in to their region.

Probably most stars in the galaxy are somewhere near something prominent like a group of bright stars, but this feature (usually called nowadays the "Ursa Major Moving Group" or “Ursa Major Stream,” and consisting of about 100 stars, most of them dimmer than the Sun), is the most prominent nearby feature for us. 30 or 40 million years ago, the Sun was nowhere near these stars, as it peregrinated on its 250 million year galactic orbit.

Compared to the bright stars of Orion's Belt, which are much younger, brighter, and farther away, this little cluster isn't much, and wouldn't gain any special notice from even 300 light years away.

Septentrional, incidentally, was one of those obscure words which turned up in James Joyce's Ulysses, to titillate the verbophiles of the Literary World.

For more information about Ursa Major as a real feature in nearby space, see this, from which the following is an exceprt:

Many other stars scattered all over the sky share a common space motion with the UMa cluster. And like the cluster the most massive, hence brightest, stars have evolved to the subgiant and even giant phases. One hundred or so stars have been accepted as members of the UMa stream, as it is called. Spanning a few hundred light years of space, it's much too big to be a cluster proper. As these stars are seen in all directions, we are obviously passing through the stream. The spectral types give stellar ages similar to that of the cluster, and this together with the space motions suggests a common origin about 150 million years ago -- just over half a galactic rotation. This age is about midway, ratio-wise, between the youngest and oldest star clusters (one million to 10 billion years).

It seems that the usual antonym for "septentrional" is "meridional." This word, however, doesn't refer to a celestial feature in the South, but to the noonday sun:
1. Of or relating to meridians or a meridian.
2. Located in the south; southern.
3. Of or characteristic of southern areas or people.

An inhabitant of a southern region, especially the south of France.

[Middle English, pertaining to the sun's position at noon, from Old French meridionel, southern, from Late Latin meridionalis, from Latin meridianus, of midday, southern. ]

Why the noonday sun is associated with the South is a bit unclear to me.

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