Visit Ara, the Celestial Altar – Sky & Telescope

Just below the tail of mighty Scorpius and wedged firmly between Telescopium and Norma lies the constellation of Ara, the Altar. With a declination of -45° to -67°, it's an easy target for southern hemisphere observers, but not so easy for those north of the equator: you have to be at fairly low northern latitudes to see its full extent.

However, its declination did not stop the ancients from trying. Ptolemy, the Alexandrian mathematician and astronomer, included Ara in his famous Almagest in the 2nd century BC, even though only the northern half of the constellation is visible above the horizon in Egypt, around 30° N.

The general outline by which Ara is often depicted on star charts seems to me to be a bit reminiscent 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'll 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), which puts it at number 63 on the list of constellations by area; Orion is at number 26.

Ara has 10 stars brighter than the magnitude 5 star, which, in addition to its relative compactness, makes it quite striking to the naked eye. The two most prominent stars in the constellation are similar in brightness, but orange in color. Beta (β)Area At magnitude 2.85 it is slightly brighter than the blue-white. Alpha (alpha)Area a 2.95. That said, Alpha is actually a variable star, varying from magnitude 2.7 to about 3.0 in just under 24 hours, although it would be hard to notice such a small variation.

Alpha is about 5,800 times more luminous than our Sun and 4.5 times larger; Beta, despite having the same luminosity, is no less than 140 times the size of the Sun. Gamma (gamma)AreaThe fourth brightest star in the constellation to our eyes, it is intrinsically 120,000 times brighter than the Sun and 23 times its size.

With the Milky Way as a backdrop to much of the Ara area, it is not surprising that the constellation hosts a good number of deep sky objects suitable for modest telescopes. For this tour, we will focus on just three of them.

Star clusters in Ara

Starting at Beta, imagine a line that extends to Tit (θ)AreaJust over a third of the way between the two lies our first target, the globular cluster. NGC 6397one of Ara's most prominent deep sky objects for small apertures. This stellar metropolis is pretty close as far as globulars go, only about 7,800 light-years away. You'll find its apparent magnitude listed as 6.7, but it appears brighter than that – it's definitely detectable with the naked eye if you have dark skies and good eyesight. You can easily spot it with 7×50 binoculars.

Through the telescope eyepiece, NGC 6397 looks to me as if someone had splashed dots of paint on a black cardboard, with the splatters dispersing in random directions. Robert Burnham pointed out its resemblance to Messier 4 in Scorpius in his seminal work Burnham's Celestial Handbookdescribing it as having a “fairly loose and sparse structure that allows for easy resolution in relatively small telescopes.” The cluster is about 15 arcminutes across, with a nice, condensed core of easily resolvable magnitude 10 stars that make it a great target for apertures as small as 3 inches.

Our second target, the open cluster IC 4651It's a bit more challenging. Finding its location is easy: just start at Alpha and slide just a fraction of 1° west. Making out its stars is a bit 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 with fairly uniform brightness… The stars are in lines, curves, and chains, enclosing dark spaces…” You’ll probably need an aperture of at least 5 inches to resolve this object’s 10th- and 11th-magnitude primary stars, and a larger one to reveal more. I find it best to use low magnification, especially considering the busy background.

Much easier to identify is our final objective, the balloon. NGC 6362since its star background is much less complex and confusing. This is because it is located alone, near the southern limit of the constellation and far from the main mass of the Milky Way. An easy method to locate it is to imagine a line running between the stars α Triangulum Australe and n Pavonis; NGC 6362 is a little below the middle.

The stars in this cluster appear as a thick, gray mass through small openings, but can be observed through the eyepiece of a 6-inch telescope. Overall, the cluster's brightness increases uniformly from its outskirts to its pleasantly condensed center. There is a very attractive blue 7th magnitude star just 14' to the northwest and an equally bright orange star 28' to the northeast.

We will return to Ara in a future column to learn more about its treasures. But for now, enjoy the ones we have described above and be sure to take advantage of the opportunity to explore the Milky Way in this part of the sky to see what other treasures you can find.