Centaurus Constellation: Glimpse the Greatest Globular – Sky & Telescope

In last month’s article we looked at some of the many star clusters in Centaurus. This month, while the giant southern constellation is still high in the sky, we’ll search out some more of its marvels, including two of the celestial sphere’s masterpieces — the globular star cluster Omega Centauri and the galaxy Centaurus A (NGC 5128).

Two Clusters and a Star

Let’s start our journey at Alnair (ζ Centauri), a 2.6-magnitude binary star system located near Centaurus’s eastern border with Norma. (Be careful not to confuse this Alnair with the other Alnair, Alpha Gruis, in the constellation Grus.) From here, slew your telescope about 2¼° east-southeast to find the first of our targets, the open cluster NGC 5460. Located around 2,300 light-years away, this cluster has a combined magnitude of 5.6 and is about 23 arcminutes across. Its central portion has more than 90 member stars, while the cluster as a whole might have close to 300.

NGC 5460 was discovered by Scottish astronomer James Dunlop in 1826, during the time he was observing from Sydney. He described it as “. . . a curiously curved line of small stars, of nearly equal magnitudes; two stars of 7th magnitude to the east . . .” John Herschel, noting the central curve of stars as well as its overall scattered nature, opined that it is “a region of large bright stars of 8, 9 . . . etc. magnitude; a very coarse cluster”.

To my eyes, NGC 5460 is a scattered but glorious collection of mainly white stars, featuring a particularly prominent curve of six stars along with other triangular and trapezoid groupings. Expect to make out maybe one or two dozen with apertures of at least 6 inches, including some attractive red stars in the field.

To reach our next target, return to Alnair and head south/southeast in the direction of Eta Centauri. About 4¼ degrees along this line, you’ll come to a lovely side-by-side pair of celestial objects, the star M Centauri and the globular cluster NGC 5286.

M Centauri is a spectroscopic binary about 260 light-years from Earth whose main component is a lovely magnitude 4.6  giant with a yellow (some say orange) hue. Situated seemingly right beside it, but in fact much farther away (by some 35,000 light-years) is the 7.4-magnitude globular.

Visually, NGC 5286 appears quite small and condensed, and it takes a 4-inch or more to start resolving some of its stars. It’s a very old cluster — some studies suggest an age of 12.5 billion years — and is thought to be one of the remnant portions (along with seven other globulars) of a dwarf galaxy that merged with the Milky Way at least 8 billion years ago.

Omega Centauri

Our next target is also a globular that is thought to be part of the remains of a dwarf galaxy. Somewhat closer  than NGC 5286 (at roughly 16,000 light-years), it’s easy to find 4½° southwest of Alnair — you can even spot it with the unaided eye under reasonably dark conditions. This target is the mighty globular star cluster Omega Centauri (NGC 5139).

I say mighty, but in fact most people agree that it is the mightiest of them all, and for good reason — it is the largest, most massive, and brightest of the Milky Way’s globulars. Taking those three attributes in order, Omega Cen is about 150 light-years in diameter, contains around 10 million stars, and is of apparent magnitude 3.9. John Herschel wrote that Omega Cen is “. . . beyond all comparison the richest and largest object of its kind in the heavens. The stars are literally innumerable . . .”

Decades ago, when I still had my keen teenage eyesight and better-than-average suburban skies, it was one of the first deep sky objects I identified without optical aid. If you ever get the chance to see it under  dark skies and with dark-adapted eyes, you’ll find that it spans almost the same apparent diameter as the full Moon. Impressive!

Binoculars reveal Omega Cen to be a dense, unresolved globe reminiscent of a comet, but even a small telescope will begin to resolve its stars. An 8-inch scope will show stars all the way across the cluster’s diameter.

Centaurus A (NGC 5128)

For the last stop on this month’s tour, head 4¼° north-northeast of Omega Centauri until you land upon the galaxy NGC 5128. At magnitude 6.8, NGC 5128 is the fifth-brightest galaxy in the sky and is easily spotted with binoculars as a fuzzy glow. A small telescope reveals its true nature and its most unique feature: a huge, dark, dust lane that bisects the galaxy.

Since its discovery by Dunlop, NGC 5128 has been well studied, with John Herschel describing it as “a most wonderful object; a nebula very bright; very large; little elongated, very gradually much brighter in the middle . . .” Famed southern observer E.J. Hartung wrote that it’s a “bright round luminous haze about 5 arcminutes across, bisected by a clean dark bar about 1-arcsecond wide . . . even a 3-inch shows this object plainly.” Indeed, the dark lane really jumps out when observing visually with a 4-inch scope, while a 6-inch-aperture begins to reveal the lane’s uneven width and edges.

You’ll usually find this galaxy named Centaurus A, the designation for the galaxy’s radio emissions. But should the galaxy’s primary name be NGC 5128 or Centaurus A? There’s one school of thought that Centaurus A should be used to refer only to the region of radio emission while the visible galaxy should be called NGC 5128. But for many years now, it has been widespread practice to refer to the galaxy in all its emissions as Centaurus A.

That’s it for Centaurus for the moment. Next month we’ll move on to another constellation, and continue our tour of the many splendid celestial sights to be found in the southern sky.