Visit Ara, the Celestial Altar

Just below the tail of mighty Scorpius and solidly wedged between Telescopium and Norma lies the constellation Ara, the Altar. Spanning from declination –45° to –67°, it’s an easy target for Southern Hemisphere observers but not quite so straightforward for those north of the equator — you have to be at fairly low northern latitudes to see its whole extent.

Its declination didn’t stop the ancients from giving it a go, though. Ptolemy, the Alexandrian mathmatician and astronomer, included Ara in his famous Almagest in the 2nd century BC, despite only the northern half of the constellation being visible above the horizon in Egypt, around 30°N.

To my eyes, the overall outline with which Ara is often depicted on star charts reminds me a bit of Orion. Take a look at the chart below and see if you agree. All that’s missing are three stars equivalent to Orion’s belt and something to mimic the stars and nebula that form his sword. I admit that’s pretty much where the similarity ends. At 237 square degrees, Ara is less than half the size of Orion (594 square degrees), placing it at number 63 in the list of constellations by area; Orion comes in at 26th.

Ara has 10 stars brighter than 5th magnitude, which, in addition to its relative compactness, makes it fairly conspicuous to the naked eye. The constellation’s two most prominent stars are similar in brightness, but orange Beta (β) Arae at magnitude 2.85 is slightly brighter than bluish-white Alpha (α) Arae at 2.95. That said, Alpha is actually a variable star that ranges from magnitude 2.7 to about 3.0 in just under 24 hours, although you’d be hard pressed to notice such a small variation.

Alpha is about 5,800 times more luminous than our Sun and 4.5 times bigger; Beta, despite having that same luminosity, is a whopping 140 times the Sun’s size. Gamma (γ) Arae, the fourth-brightest star in the constellation to our eyes, is intrinsically a staggering 120,000 times brighter than the Sun, and 23 times its size.

Having the Milky Way as the backdrop over much of Ara’s area, it’s not surprising that the constellation plays host to fair number of deep-sky objects suitable for modest telescopes. For this tour, we’ll concentrate on just three of them.

Star Clusters in Ara

Starting at Beta, picture a line extending to Theta (θ) Arae. Just over a third of the way between the two  is our first target, the globular cluster NGC 6397, one of Ara’s most outstanding deep-sky object for small apertures. This stellar metropolis is pretty close as globulars go, only about 7,800 light-years away. You’ll find its apparent magnitude listed as being 6.7, but it seems brighter than that — it’s definitely detectable with the naked-eye if you have dark skies and good eyesight. You can easily spot it through 7×50 binoculars.

Through the telescope eyepiece, NGC 6397 looks to me as if someone has spattered dots of paint onto some black card, with smatterings leading off in random directions. Robert Burnham noted its resemblance to Messier 4 in Scorpius in his seminal work Burnham’s Celestial Handbook, describing it as having a “rather loose, scattered structure which permits easy resolution in relatively small telescopes. ” The cluster is about 15-arcminutes across, with a nice, condensed core of easily resolvable 10th-magnitude stars that make it a great target for apertures as small as 3 inches.

Our second target, the open cluster IC 4651, is a bit more challenging. Finding its location is easy — simply start at Alpha and slew just a fraction over 1° to the west. Making out its stars is a little harder, as it’s situated in a busy part of the Milky Way. If you’re having trouble finding it, take a look at the chart and see where the cluster is in relation to Alpha and Kappa (κ ) Arae. Then, compare it to the photo below.

Renowned observer E. J. Hartung described IC 4651 as “an irregular gathering of numerous stars fairly uniform in brightness . . . The stars are in lines, curves and chains, enclosing dark spaces . . .” You’ll probably need at least a 5-inch aperture to resolve this object’s primary 10th- and 11th-magnitude stars, and larger than that to reveal more. I find that low power is best, especially given the busy backdrop.

Much easier to identify is our final target, the globular NGC 6362, as its stellar background is far less complex and confounding. This is because it’s located on its own, down near the very southern border of the constellation and away from the main bulk of the Milky Way. One easy method to locate it is to imagine a line running between the stars α Triangulum Australe and n Pavonis — NGC 6362 is a smidgeon under halfway along.

This cluster’s stars appear as a coarse, gray mass through small apertures, but they’re resolvable through the eyepiece of a 6-inch scope. Overall, the cluster’s brightness rises evenly from its outskirts to its nicely condensed center. There’s a very attractive 7th-magnitude blue star just 14′ to the north-west and a similarly bright orange star 28′ to the northeast.

We’ll return to Ara in a future column to sample some more of its treasures. But for now, enjoy those described above, and be sure to take the opportunity to sweep through the Milky Way in this part of the sky to see what other riches you can find.