Transit Date of principal star:
2 May

Centaurus is one of several constellations that deal with the Labours of Heracles.

In the Fourth Labour, Heracles' assignment was to bring back a rampaging wild boar that was bringing death and destruction to the inhabitants of the northern part of the Peloponnesian peninsula. On his way, he stops to visit a friend of his, a Centaur named Pholus.

Centaurs were half-men, half-horse, who had all descended from Ixion and Nephele (who was in fact a cloud, shaped by Zeus to resemble his wife Hera).

Centaurs were featured in a number of Greek myths, but by and large remained on the periphery of Greek fable.

As Heracles finishes the sumptuous meal provided by Pholus, he then has the effrontery of opening the Centaurs' private wine cask, meant for them alone. The rest of the Centaurs catch the odour of their wine, wafting across hill and dale, and they become enraged.

Gathering up huge boulders, ripping out trees to use as clubs, and arming themselves with axes, the Centaurs advance on the dinner party.

Pholus takes fright, so the battle is left to Heracles. After repulsing a number of Centaurs single-handedly, Heracles then chases the rest of them to the cave of their king, Cheiron.

Heracles shoots an arrow at one fleeing Centaur (Elatus by name), but it passes through his arm and strikes Cheiron on the knee. You may recall that Heracles' arrows were all dipped in poison, so each was fatal, no matter how slight the wound. Cheiron was a great friend of Heracles, and our hero is devastated. He tries to assist Cheiron, but there is nothing to be done.

Cheiron was immortal, so the poison couldn't kill him, only cause great pain that would last through eternity. He descends to the depths of his cave, his screams of agony echoing throughout the cavernous walls.

Eventually Prometheus takes pity on the long-suffering king of the Centaurs, and offers to take over Cheiron's immortality, if Zeus would agree. Zeus does agree, so Cheiron's agony finally comes to an end, and Zeus places the great king of the Centaurs in the heavens.

Back to the previous battle. The Centaur Pholus looks over the dead and dying and wonders how Heracles' arrows could be so fatal. He plucks one arrow out of a body and looks at it, but it slips through his fingers and strikes him on the foot, killing him instantly.

Heracles hears of the tragedy and returns to bury his friend, at the foot of the mountain that bears his name: Mt Pholoe.

This high plateau region in the interior of the peninsula is just up the road from Olympia. Now called Pholois, this is where the Centaur stories of antiquity originated.
It is said that Zeus had held Pholus in very high regard, and therefore also put his likeness in the heavens. Thus the constellation Centaurus represents two Centaurs: Pholus and Cheiron.
The fact that two Centaurs are linked with the constellation is no accident. The earliest extant artifact showing the likeness of a Centaur is a piece of Mycenaean jewellery which shows two centaurs together: half-men, half-horse, facing each other and dancing, similar to satyrs.

These half-men half-horse figures were also transformed at times to half-man half-goat. Many rituals are known to have involved dressing as one of these half-beasts, rituals which may date back to Neolithic times.

Centaurus is one of the largest constellations with a clearly discernible asterism: the huge form faces east, with a sword waving menacingly toward Lupus the Wolf on the west.

The constellation has an almost complete list of Bayer stars except for omega, which isn't a star, but a well known globular cluster, NGC 5139 (see below).

The front hooves (or feet, if you wish) are formed by two bright stars: alpha and beta Centauri, known also by the Arabian names of Wazn and Hadar.

Alpha Centauri is best known by the name "Rigil Kentaurus", or the Centaur's foot. This is a triple system, three stars which are the closest to our own Sun.

Alpha1 and alpha2 Centauri form a noted binary (see below). They are 4.393 light years away, and each is approximately the size of the Sun.

The closest star is actually alphaC, known as Proxima Centauri. This is a red dwarf of visual magnitude 11.01 and distance 4.221 light years.

The star has a diameter of about 65,000 km (40,000 miles), or about five times that of the Earth. It is a great distance from the other two (perhaps as far as a sixth of a light year away) and the orbit's period is estimated to be hundreds of thousands of years; Burnham suggests "perhaps ... half a million years".

Proxima Centauri is a flare star and is therefore also known by its variable designation of V645 Centauri. See below for details.

Beta Centauri (Hadar) is the tenth brightest star in the heavens, at 0.61 visual magnitude (which is actually the combined values of its two components). It's 525 light years distant and is a rather difficult visual binary (see below).

Double stars:

Alpha1 and alpha2 Centauri form a wide double with an orbit of 79.92 years: -0.04, 1.2. The 2000.0 values are PA 222 and separation 14.1".

Beta Centauri is a difficult double because of the primary's brightness compared to the companion: 0.58, 3.95; PA 251, separation 1.3". The orbit has not been calculated, but is thought to be at least several hundred years.

Gamma Centauri is a visual double of two nearly identical stars, with orbit of 84.5 years: 2.9, 2.9. The 2000.0 values are a PA of 347 and separation 1.0".

Eta Centauri is a binary with very faint companion: 2.3, 13; PA 270, separation 5.6".

Kappa Centauri also has a faint companion: 3.1, 11; PA 82, separation 3.9"

Many more binaries are found in this enormous constellation. Check the Binocular link below.

Variable stars:

AlphaC Centauri (V645 Centauri) is a flare star. That is, its visual magnitude may change rapidly, perhaps taking only several seconds to change its magnitude.

The prototype of this kind of variable is UV Ceti, which has been known to change 3.5 magnitudes within seven seconds!

Since these stars are extremely dim, only the closest ones have been investigated. There are twenty or so such stars within twenty light years of our Sun; they all have an M4.5-M6.5 spectra. Only two have visual magnitudes brighter than 10.

R Centauri is a Mira type variable, 5.3 to 11.8, with 546.2 year period. In 1999 the maximum should arrive in mid-February; the same in 2002.

As the chances of seeing this star at its maximum are therefore not very likely, you might find Burnham's finder's chart (p. 559) of some use.

Deep Sky Objects:

NGC 5139, also known as omega Centauri. This globular cluster is usually described as the finest in the heavens. It's so bright and compact, Bayer thought it was a hazy star, and named it omega.

The cluster is estimated to be from 15000 to 25000 light years away, and may be comprised of over a million stars.

The cluster lies between gamma and zeta Centauri, about five degrees west of zeta.

For a closer appreciation of Centaurus, visit the Binocular Section.

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