Ara the AlterConstellation of Ara Star Map

 

A fairly faint constellation south of the tail of Scorpius, located in a spectacular region of the Milky Way, competing with the magnificent constellations of Scorpius and Sagittarius, Ara is most often passed by.

Ara in ancient times was the seen as the Altar of the gods of Olympus; it has also been known as the Altar built by Noah after the flood.

 

The constellation pattern is easily spotted after locating the bright 2.8 magnitude orange supergiant Beta ( β )Arae.  The colour is quite distinct even to the naked eye.  Then less than one degree south of Beta, the 3.3 magnitude blue-white supergiant gamma Arae forms a wide pair with it. (Gamma  has a companion star which requires 10.5 cm to split.) I use β and γ to locate Ara and then the pattern of the altar is easily seen, even on a bright moonlight night.

 

Alpha (α) Arae shining at the same magnitude 2.8 as Beta is a blue-white dwarf.

 

Delta (δ) a blue white 3.6 mag star, Zeta (ζ) 3.1 mag., Eta (η) 3.8 mag , Epsilon ( ε ) 4 mag. and Theta  (θ) 3.7mag complete the pattern of the altar.

 

For small telescopes the Globular cluster NGC 6397 (Caldwell 86) is the object to observe. It is magnitude 5.7 with the brightest star being 10 mag.  Located between Beta (β) and Theta (θ) it is easily located with 7 x 50 binoculars and appears as a small condensed glow.  In very dark skies it may be visible with the naked eye.

The stars are easily resolved with my 78mm refractor. At 63 x magnification using a 10mm Plossl eyepiece the centre is more condensed with outlying stars resolved.  With an 8mm Radian at 79x the condensed centre sits in a sprinkling of scattered bright stars. At 105x using a 6mm Plossl the cluster was quite spectacular for its size (the outliers are scattered over 15’ with the condensed centre 3’across).

NGC 6397 is the second closest globular cluster to us after M4 near Antares in Scorpius, although this has been debated.  I compared the two and found M4 more nebulous in appearance without the nice scattering of bright stars.

When discovered by the French astronomer Nicolas LaCaille in 1752 he described NGC 6397 as a small star within a nebula.

NGC 6352 is a less condensed magnitude 8.2 globular cluster northwest of alpha which requires 15cm to resolve.

NGC 6193 is a 5th magnitude open cluster of 30 stars which is surrounded by the emission nebula 6188 (which can only be seen in photographs). A double star in the centre of the cluster mag 5.6/6.9 requires 15cm to split.

Galaxies NGC 6221 and NGC 6215, magnitude 11 and 12, located just southeast of Eta should be visible if you have a 15mm or larger scope. NGC 6221 is 3.20’, NGC 6215 2.00’.

Finally see if you can locate the Eclipsing Binary star R Arae, the close yellow pair goes from magnitude 6 down to magnitude 6.9 in 4.43 days. The orbital planes of the two stars in an eclipsing binary are viewed almost edge on. This results in mutual eclipses and variations in the two stars’ combined light output as the unseen companion goes in front of the primary star.

 

If you are observing Ara for the first time you will never pass the Scorpion’s tail again without spotting it.

Sources:  Hartung’s Astronomical Objects for Southern Telescopes, Malin & Frew1995; Collins Stars & Planets 2000; Philip’s Astronomy Dictionary1999.