Ara the Altar

'neath the glowing sting of that huge sign

The Scorpion, near the south, the Altar hangs.'

(Aratos)

Ara is a small group of stars lying directly south of Scorpius, between Telescopium and Norma and the star fields of Pavo, Apus and Triangulum Australe to the south. With a total area of 237 sq°, there are 10 stars in total, equal to or brighter than 5th magnitude. The brightest stars are alpha and beta, both at apparent magnitude 2.8.

To the Germans, it was Altar, the French called it Autel and Encensoir while the Italians named it Altare. It was the Latin Ara Centauri, Ara Thymiamatis and Thymele. Other titles include Apta, Altaria and Altarium. The Alfonsine Tables added Puteus, a Pit, and Sacrarius and Templum, a Sacred Place. The early Arabians imagined it as Al Mijmarah, a Censer. Caesius saw it as one of the altars raised by Moses, however Biblicists disagreed, considering Ara as the Altar of Noah, erected after the Deluge.

The following information has been garnered from the Hipparcos Catalogue, compiled from data collected by the Hipparcos satellite that so splendidly studied the stars, giving stellar positions and magnitudes to an accuracy never before achieved. This data allowed stellar distances to be pinned down, thereby also inferring their luminosities.

In this article, a luminosity of 336 (± 44) means a brightness 336 times that of our Sun within an error bar of plus or minus 44. The same goes for distances. A distance of 242 (± 16) light years means 242 light years from our Sun plus or minus 16 light years.

Alpha Arae is an interesting B2 V star, a main sequence dwarf sun surrounded by a shell of gas. Normally shining around magnitude 2.8, alpha was found by Hipparcos to be variable, brightening to magnitude 2.74 at maximum. The computed distance is 242 (± 16) light years, therefore alpha has a luminosity of 336 (± 44). Sir John Herschel found alpha to be a double star with a companion of magnitude 10.99. Widely separated, this orange K0 subgiant is listed in Guide 6.0 as optical only.

Beta Arae is a magnitude 2.8 K3-type supergiant or bright giant star lying at a distance of 603 (± 85) light years and shining with the brightness of 2080 (± 580) Suns.

Gamma lies just to the south of beta, shining with an apparent magnitude of 3.3. This giant or possibly supergiant B-type bluish star has a luminosity of 4800 (± 2400) and lies 1140 (± 290) light years away. Sir John Herschel, scanning the skies in 1835, noted a faint 10th magnitude companion around 18" away. There has been no noted change in separation or position angle since discovery so they may not form a true physical pair.

West of gamma, around 3.7°, lies the giant orange star zeta Arae. Shining at magnitude 3.1, this K5-type star has a luminosity of 1460 (± 370) and a distance of 574 (± 74) light years.

Around 3.3° to the southwest brings us to magnitude 3.7 eta. This K5-type orange giant lies 313 (± 14) light years away and has a luminosity of 238 (± 21).

To complete the picture, delta lies 5.4° to the east and slightly south of eta. Shining with the luminosity of 100 (± 14) Suns, this B8-type dwarf star lies 187 (± 13) light years away, shining with an apparent magnitude of 3.6. There is an optical companion, an orange K0-type star of magnitude 9.7 in fine contrast with delta's bluish hue.

Ara holds a number of nice deep sky objects, many of them clusters, as could be expected, given Ara's position near one of the Galaxy's spiral arms. Therefore, many of the galaxies in this region are pretty obscured, though a few are bright enough to give pleasure.

If we start our tour of Ara's riches from alpha and move 3.6° to the north and slightly west, we visit NGC 6352, a faint but large loosely structured globular cluster. Use this cluster to move just over 2° to the north and slightly west to find the double star BrSO 13. This beautiful pair with their distinctive yellow and orange colours lie amongst a background of faint stars. Their separation as measured in 1990 amounted to 8.3" in PA 250°. These G8 and K0 dwarf stars, of magnitudes 5.5 and 8.7 respectively, orbit a common centre of mass every 693 years or so. The computed distance for this pair is 28.66 (± 0.35) light years.

A line of open clusters run from southwest Scorpius, cross through the northwestern portion of Ara, then disappear into Telescopium. NGC 6250, 6204, 6200 and 6193 belong to Ara. Of these, NGC 6193 is of greater interest. A very young cluster at one million years old, this group of giant early type stars are still immersed within their pre-natal cloud of dust and gas. The area of bright and dark gas is catalogued as NGC 6188, the brightest part, an irregular triangle in shape, was discovered by Sir John Herschel in 1836. NGC 6193 resides at the apex of this triangle, its hot, energetic family of stars pouring out their radiation into the surrounding cloud, stripping away the hydrogen electrons and thereby ionising the gas. This causes the gas to fluoresce and make it visible to the eye.

The brightest star within NGC 6193 is an O-type giant of 3000 times the brightness of the Sun. Sir John Herschel discovered a magnitude 6.8 companion in 1836 while an observer at Melbourne Observatory added another companion of magnitude 8.9 in 1878. A further 3 stars are also bound to this system. Separation, position angle, year measured and magnitude are as follows:

PAIR SEP. PA° DATE MAG.

AB 1.6" 014° 1938 5.6, 8.9

AC 9.6" 266° 1938 6.8

AD 13.4" 160° 1938 10.4

AE 13.9" 015° 1938 11.3

AF 20.8" 192° 1938 12.4

A telescope of around 30cm in aperture, beneath a dark sky, should have no trouble in seeing this complex group of stars and bright and dark gas.

Around 2° southeast of NGC 6193 lies a striking double star, Cor 201. This pair of late A or perhaps early F-type stars shine equally at magnitude 7.3 and 7.4, their beautiful yellow colours dominating the surrounding scattered starfield. Measurements in 1990 put their separation at 3.1". The computed distance is 246 (± 96) light years and the combined luminosity is 12.4 (± 9.7).

Moving south by 3.7° brings us to a very nice open cluster, NGC 6208. This cluster has an age of around 1 billion years, its 50 or so stars ranging in brightness, the brightest around 8th or 9th magnitude. Lying at a probable distance of 3200 light years, it has an apparent size of 50% the Full Moon.

Our next object lies 2.8° southwest of zeta Arae. R Arae is a close yellow pair of stars, the primary star also a variable eclipsing binary star of the Algol type. Most variable stars vary because of differences in their energy output or pulsations of the star itself. In the case of R Arae, the brighter star is eclipsed by a larger but dimmer companion, causing the magnitude of R to drop from 6.5 to 7.2 over a period of 4.425 days. The computed distance of this system is 262 (± 37) light years and the combined luminosity is 10.9 (± 3.1). By the way, both stars are on the main sequence.

Swing the telescope 2.4° to the southeast and centre magnitude 3.7 eta Arae in the finderscope. Just 10.6' east and slightly north lives the galaxy, NGC 6215. This faintish barred spiral appears slightly elongated and has a gradual brightening toward the middle. To the east by 13.6' lies the fainter and smaller spiral galaxy NGC 6215A.

A better target lies just 25.2' southeast of eta. NGC 6221 is a larger and brighter spiral galaxy, appearing almost face-on. There is gradual brightening toward the centre. A supernova, 1990W was discovered on August 16, 1990, shining at a paltry 15th magnitude, typical of the brightness that these stellar explosions show. However, don't be misled by the poor showing. These explosions rip through stars with incredible force, blowing away a large proportion of the mass, thus seeding the cosmos with heavy elements, forged within the stellar furnace.

It's now time to enter the deep south of Ara's borders. Around 2.7° from delta Arae is a large and faint barred spiral galaxy, NGC 6300. Too faint? O.K. then, let's move on further south.

NGC 6362 is a globular cluster lying 1.2° to the northeast of magnitude 4.7 zeta Apodis in Apus. This stellar metropolis is large and bright and has a gradual brightening toward the core. It is catalogued as class 10, that is, loosely structured, making the cluster easier too resolve than most. Globular clusters are classed from 1, where the inner regions of stars seem packed into a solid mass, making resolution near impossible, through to 12, where the cluster imitates a rich open cluster in form. An example of a 1 is NGC 2808 in Carina, where only the edges are resolved in typical amateur telescopes. NGC 4372 in Musca demonstrates a class 12 globular cluster, difficult to discern as globular in form. For comparison, 47 Tucanae is a 3 while omega Centauri is an 8.

Another globular to visit, the best object in Ara and a standout at any time is NGC 6397. Nicolas-Louis De Lacaille, the French astronomer, first observed it in 1755. It is a large and rich cluster of class 9 so it is easily resolved as the stars are rather loosely concentrated. In fact, it is similar in apparent size, brightness and structure as M4 in Scorpius. NGC 6397's brightest stars are luminous red giants, roughly arranged into a triangular pattern, though there seem to be no short period pulsating variable stars usually associated with globulars.

Tearing ourselves away from this celestial delight, we move 3.5° to the west-northwest and discover a bright, round but very small planetary nebula, NGC 6326. Discovered by Sir William Herschel in 1835, use high power on it to see well. Careful observation may show some brightening at the centre.

Finally, we end up back at alpha Arae. But before we leave this region and wander inside for a cup of tea and a biscuit, wander quickly across to IC 4651, an open cluster of around 70 stars. Lying only 1.1° to the west, this old-timer at 2.4 billion years old, shows a gathering of many faint stars, the brightest at magnitude 10.

Clear skies and good hunting.

~CosMos