Heart Of The Centaur

On Pelion, on the grassy ground,

Chiron, the aged Centaur lay,

The young Achilles standing by.T

he Centaur taught him to explore

The mountains where the glens are dry

And the tired Centaurs come to rest,

And where the soaking springs abound.

Matthew Arnold (Empedocles on Etna)

North northeast by 2.3° lies magnitude 4.6 M Centauri. Here we have a K-type orange giant, the colour superb in the telescope. Lying at 257 ±18 light years, it shines with the light of 72.1 ±9.9 Suns. There is a faint 11th magnitude companion. In the same low power field lies NGC 5286, a 7.2 magnitude globular cluster 4’.0 to the northwest. With a diameter of 14’.0 it is easily spotted as a very condensed and bright fuzzy ball in fine contrast with orange M.

A faint and very small planetary nebula, NGC 5307 lies 44’.5 east and slightly north of NGC 5286. Discovered by Herschel in 1836, its 12”.5 diameter disk appears starlike and hides amongst the background field stars. Care will show the planetary nebula as slightly fuzzy when compared with the surrounding stars. A nebula filter makes identification certain.

North northeast by 4.4° lies magnitude 2.5 zeta Centauri, a B-type blue-white subgiant lying 385 ±34 light years from our Sun. Continuing our northward journey, around 5° finds the naked eye optical pairing of mu and nu Centauri. Mu and nu are B-type blue-white stars of magnitude 3.5 and 3.4 respectively.

West and slightly north of nu by 10.4° is the star designated simply as n Centauri. A magnitude 4.2 A-type star, it merely marks the northwestern part of the Centaur’s body. However, halfway along and south of a line joining nu and n sits one of the most ostentacious of all the galaxies available to the amateur telescope, NGC 5128 or Centaurus A and nicknamed the ‘Hamburger Galaxy’. This extra-galactic delight is wonderful in the smallest of apertures.

James Dunlop discovered this peculiar galaxy from Paramatta, New South Wales. He described it as:

"a very singular double nebula, about 2.5' long, and 1' broad, a little unequal: there is a pretty bright small star in the south extremity of the southernmost of the two, resembling a bright nucleus: the northern and rather smaller nebula is faint in the middle, and has the appearance of a condensation of the nebulous matter near each extremity. These two nebulae are completely distinct from each other, and no connection of the nebulous matters between them. There is a very minute star in the dark space between the preceding extremities of the nebula: they are extended in the parallel of the equator nearly.

"Sir John Herschel observing it at the Cape of Good Hope with an 18-inch f/13 speculum telescope recorded it as:

"A most wonderful object; a nebula very bright; very large; little elongated, very gradually much brighter in the middle; of an elliptic figure, cut away in the middle by a perfectly definite straight cut 40 arcsec broad; pos = 120.3 ; dimensions of the nebula 5' x 4' The internal edges have a gleaming light like the moonlight touching the outline in a transparency."

NGC 5090 was discovered by Sir John Herschel from the Cape of Good Hope. He noted it as:

“bright, pretty large, round; 60 arcseconds.”

It appears as a faint, round glow amongst a field of fainter galaxies including NGC 5091 to the southeast. NGC 5091 shows as a faint sliver of light, the larger the aperture the better here.

Gamma Centauri marks the southwestern extremity of the Centaur’s body shining at magnitude 2.2. An A-type white subgiant 130.4 ±5.3 light years distant, Sir John Herschel recorded it as a double system. Gamma is separated into an equal magnitude 2.9 pair, both of spectral type A0 III with an orbital period of 84.5 years. Tau Centauri lies 45”.3 to the northwest.

Around 3.8° east and slightly south lie the naked eye pair xi¹ and xi² Centauri. Xi¹ shines at magnitude 4.8, a white Sirian-type star. Xi² is brighter at magnitude 4.3 and is a hotter B-type blue-white star. A telescope will split xi² into a pair of magnitude 4.3 and 9.4 stars, the faint companion an F-type sun.

”Dunlop described it in more detail as:

“a beautiful long nebula, about 10' long, and 2' broad, forming an angle with the meridian, about 30 south preceding and north following; the brightest and broadest part is rather nearer the south preceding extremity than the centre, and it gradually diminishes in breadth and brightness towards the extremeties, but the breadth is much better defined than the length. A small star near the north, and a smaller star near the south extremity, but neither of them is involved in he nebula. I have strong suspicions that the nebula is resolvable into stars, with very slight compression towards the centre. I have no doubt but it is resolvable. I can see the stars, they are merely points. This is north following the first zeta Centauri."

Two other galaxies lie near. NGC 4945A is a rather small and roundish galaxy, faint in medium apertures but easily found close north of a star of the 8th magnitude. NGC 4976 is easier, located near to a bright yellowish star. Among a field of stars, a 20cm aperture shows it as a roundish glow with a brighter star like nucleus. Herschel called it “bright, round, gradually brighter in the middle, 80 arcsec across."

Last but by no way least is our next object, arguably the best globular cluster in the heavens. Around 4° northeast of our xi pair lies NGC 5139, omega Centauri. Appearing to the unaided eye as a fuzzy star, omega was recorded by Ptolemy over 1800 years ago. Edmund Halley was the first to unravel its secret when turning a telescope upon it in 1677. Lacaille recorded it as:

“Naked eye, a 3rd mag star [10 Cen] in a fog. Telescope, [Half-an-inch aperture, 8x magnification] like a big diffuse comet”

James Dunlop used a 9” f/12 telescope from Australia and saw it as:

“a beautiful large bright round nebula, about 10' or 12' diameter, easily resolvable to the very centre; it is a beautiful globe of stars very gradually and moderately compressed to the centre; the stars are rather scattered preceding and following, and the greatest condensation is rather north of the centre: the stars are of slightly mixed mags, of a white colour. This is the largest bright nebula in the southern hemisphere”

Sir John Herschel captured its personality rather well after observing it with his 18” f/13 telescope. His description far outweighs any that I could give it (apart from WOW!!!) so I will leave it to him:

“the noble globular cluster w Centauri, beyond all comparison the richest and largest object of its kind in the heavens. The stars are literally innumerable, and as their total light when received by the naked eye affects it hardly more than a star of the 5th or 5..4th magnitude, the minuteness of each may be imagined: it must however be recollected that as the total area over which the stars are diffused is very considerably (not less than a quarter of a square degree), the resultant impression on the sensorium is doubtless thereby much enfeebled, and that the same quantity of light concentrated on a single point of the retina would very probably exceed in effect a star of the 3rd magnitude. On a consideration of all the sweeping descriptions, as well as from a great many occasional inspections of this superb object, I incline to attribute the appearance of two sizes of stars of which mention is made to little groups and knots of stars of the smaller size liying so nearly in the same visual line as to run together by the aberrations of the eye and telescope; and not to real inequality. This explanation of an appearance often noticed in the descriptions of such clusters, is corroborated in this instance by the distribution of these appearently larger stars in rings or mesh-like patterns, chiefly about the centre where the stars are most crowded. An attempt has been made to imitate this appearance in the drawing, but partly from the difficulty of its execution, partly from defect of engraving, the plate fails to convey a just idea of it. Two such rings on an oval crossed by a kind of bridge is especially conspicuous in the central part…

…Diameter full 20'. It much more than fills the field. When the centre is on the edge of the field, the outer stars extend fully half a radius beyond the middle of it. The stars are singularly equal, and distributed with the most exact equality, the condensation being that of a sphere equally filled. - Looking attentively, I retract what is said about the equal scattering and equal sizes of the stars. There are two sizes 12th mag and 13th mag, without greater or less, and the larger stars form rings like lace-work on it. One of these rings, 1.5' in diameter, is so marked as to give the appearance of a comparative darkness like a hole in the centre. There must be thousands of stars. To the naked eye it appears as a star of 5th mag or 5..4th mag, rather hazy. There is a 9th mag star on the S.p. border of it, about 4' or 5' south of centre, and several 8th mag are scattered far away. My attendant (J.S.) called up, who saw the hole and darkness, and described it as I have done above. On further attention the hole is double, or an oval space crossed by a bridge of stars. Position of axis = 150.”

Clear skies and good hunting.