In a black sky, clear of city lights, your eye can pick out about 6,000 stars. About 300 of the brighter ones have names, such as Polaris, Sirius and Vega. The rest are known to astronomers by numbers.

The names by and large are ancient, given mainly by Arab astronomers, and also Greeks and Romans. The bright star Altair, for example, which you can see on summer evenings, is called after the tag end of the Arabic phrase al-Nasr al-Tair, the flying eagle, which applied to the whole constellation we call Aquila, the Latin for eagle.

The first complete atlas of the sky that systematically identified the bright stars was the “Uranometria” published in 1603, in which Johann Bayer assigned Greek letters to the stars in each constellation. Altair is Alpha Aquila because it’s the brightest star in the constellation. In the 1700s, John Flamsteed used the brighter stars’ positions, rather than their brightness, to assign numbers.

The Bayer and Flamsteed designations are still basic identifiers today. And millions that were too faint for the early modern astronomers to spot, but that can be seen through our much more powerful telescopes, are now cataloged by number in compendiums such as the Yale Bright Star Catalogue, the Smithsonian Astrophysical Observatory Star Catalog, and the Henry Draper Catalogue. Hundreds of billions more in our galaxy are unnamed. Some astronomers pick out individual stars in other galaxies, if you can believe it, as if the 400 billion or so in our cozy little vicinity, just 100,000 light-years across, weren’t enough.

What’s somewhat unsettling about all this star naming is that it can be done at all. Apart from those 6,000 visible stars – which already seem like too many to count – the rest of them, the Milky Way and other patches and blobs of night light, do not look like stars at all, though they are in fact clusters of individuals, like families whose members can be separately counted and focused upon.

Each star has definite, unique characteristics. It has a particular location in relation to us and to other stars. Altair is about 16 light-years from us. Each star has its own color, temperature, size, estimatable age and detectable velocity. Some stars shine with a steady brightness, and others are “variable.” There are two categories of brightness, or magnitude: the “apparent” magnitude we perceive, which depends partly on how close the star is to us, and the “absolute” magnitude a star would have in comparison to others at a fixed distance. The brightest star in the sky (other than the sun) is Sirius, at the head of the dog galloping along at Orion’s heels. It’s among the closest stars to us at 8.65 light-years away, and its apparent magnitude is minus 1.46 (the lower the number, the brighter the star).

Many stars are locked in orbits with one or two or more other stars, as if they’re dancing. Sirius is actually a system of two stars. The star we call Alpha Centauri is actually three stars, and the faintest of the three, a “red dwarf” called Proxima Centauri, is the closest star to us, 4.2 light-years away. Its absolute magnitude is 15.45 and so its apparent magnitude is faint to us, only 11 – findable by telescope, but not by eye.

Each star has its own characteristics, and maybe that’s a way of saying each one has a character. I don’t know. There is more to say about this. Meanwhile, look up.

– DANA WILDE, DWILDE@BANGORDAILYNEWS.NET