See the Celestial Centaur

One of the most interesting of the constellations now dominates the low southern sky at around 10 p.m. local daylight time: the mythical creature that is half-horse, half-man, known as the centaur.

Actually, this particular star pattern, known as Centaurus, is one of two centaurs in our night sky. The other is Sagittarius, the Archer, who traditionally has been depicted as a centaur about to shoot off an arrow in the direction of Scorpius, the Scorpion.

Centaurus is seen completely only from the Florida Keys, southernmost Texas, and the Hawaiian Islands. Yet, in the high latitudes of southern England, an assiduous stargazer must carefully watch that point on the southern horizon for a short look at the 2nd-magnitude star Menkent (about as bright as Polaris, the North Star). In the northern United States, we can use this star (also known as Theta Centauri), to guide us to Iota, Eta and Omega.

The last is not a single star at all but a great swarm of them, indeed, the brightest and most splendid globular star cluster in the entire sky. Shining at a moderately dim magnitude +4, it is easy to glimpse under good sky conditions with the naked eye. It has, in fact, been known since ancient times (albeit as a star), and appeared in the star catalogue of Ptolemy over 18 centuries ago and received the Greek letter designation of Omega from Johannes Bayer.

(On the astronomer's brightness scale, higher magnitudes represent dimmer objects. The brightest stars have negative magnitudes.)

The history

Edmond Halley (of comet fame) called Omega a nebula in 1677, but it was not until 1835 that its true glory as a cluster was revealed by the 18 3/4-inch telescope that Sir John Herschel had taken to South Africa to survey the southern skies. Of Omega he wrote: "It is beyond all comparison the richest and largest object of its kind in the heavens."

Omega Centauri is about 17,000 light-years away and probably contains over one million stars.

Theoretically, Omega Centauri can be seen from places as far north as New York or Philadelphia. But I can offer no encouragement to those residents of the Big Apple or City of Brotherly Love because even if all of their streetlights were somehow to be extinguished and a fresh, clean Canadian air mass were to position itself directly over the Northeastern U.S., the thick haze that is perpetually evident along and near the horizon almost always hides Omega; and even if one were to somehow get it in view through a telescope the cluster would be robbed of its full glory. To see this globular cluster adequately, one should be no farther north than about latitude 35-degrees.

Look around

Of course, Centaurus' greatest claim to fame is that it contains the closest star in the sky, Rigil Kentaurus. Ironically, that is not how this particular star is best known. More often than not it is referred to by its designation Alpha Centauri. This is the third brightest star in the sky and is also a beautiful double star, composed of two yellow stars somewhat like the Sun. It is but a mere 4.3-light years from us and has a faint 11th-magnitude companion about 2-degrees away known as Proxima Centauri.

Proxima's position relative to the main pair actually places it a trifle closer to us at the present time.

To the upper right of Alpha is first magnitude Beta Centauri, which has the name Hadar, and seems to be an apparent neighbor, though it's hardly that. In reality, it's 500 light years away, a bright blue star that outshines our Sun some 10 magnitudes or 10,000 times!

In their way, the ancients had as facile imaginations as ours. While some of us would people the universe with little green men, they placed in the sky a number of exotic beasts of which Centaurus is just one. In the sky itself, we are hard put to imagine the mythological character in the confusing jumble of stars that form Centaurus. This star pattern is often associated with the mythological centaur, Chiron, who was highly skilled in medicine.

This leaves me ending this week's column with a question. If a centaur fell ill, who would he consult: a physician . . . or a veterinarian?

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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.