Ara, A Southern Delight
24 Jun 2019
Ara may be small and it may not compete well with its magnificent neighbours, Scorpius and Sagittarius, when it comes to objects big and bright and abundant… but it really is an astronomer’s delight, for it offers something special for everyone regardless of your observing preferences. And if you are the kind of observer who enjoys everything the sky has to offer us in binoculars and telescopes… well this most certainly is a constellation in which to spend a night’s observing. Here are some of the highlights I observed last night.
Observations
16″ f/5 Dobs; 70x, 150x, 228x, and 333x
10×50 binoculars
Colourful stars…
Naked eye, Ara’s configuration of eight stars is very striking, obvious even on a brightly moonlit night. And its not hard to see the altar used by Zeus and other Olympic Gods to swear a vow of allegiance before they went to war against Cronus and the Titans and which Zeus placed among the stars to commemorate the gods’ victory, thus forever immortalizing the mighty battle for control of the universe. (The bright glow of the Milky Way represents the smoke rising from the Altar’s burning incense.)
And for those who enjoy beautifully coloured stars in binoculars, the altar’s main stars are lovely, a beautiful assemblage of golden orange and steely white stars:
Alpha Arae (B2) shining at magnitude 2.95 is a lovely cold bluey-white dazzler in the binos.
Beta Arae (K3), shining at 2.84 magnitude is the brightest star of the constellation, and is a beautiful orange star; its colour is quite distinct even to the naked eye, and gloriously so in binoculars.
Gamma Arae (B1), the 3.34 magnitude blue-white supergiant, lies less than one degree south of Beta, and is a steely white diamond in the binos, a gorgeous contrast.
Delta Arae is a double (B8 + G8), and shining at 3.62 magnitude is a lovely white star
Epsilon Arae 1 (K3), shining at 4 magnitude is a gorgeous golden star
Zeta Arae (K3), at 3.13 magnitude is another gloriously golden orange star
Eta Arae (K5) shining at 3.76 magnitude is yet another lovely golden star
Theta Arae (B2) at 3.67 magnitude completes the pattern of the altar, a beautiful bluey-white diamond in the binoculars.
Emission nebulae (and a dark nebula)…
The exquisite open cluster NGC 6193 is surrounded by the beautifully sculpted nebula – it is a wonderful sight. And for those who like their nebulae dark and mysterious, there are superb intriguing dark pools, rivers and wisps of black nothingness to explore…
NGC 6188 – Emission Nebula
RA 16 40 30.0 Dec −48 47 00 Size 20’ x 12’
NGC 6193 – Open Cluster
RA 16 41 20.3 Dec −48 46 00 Mag 5.2 Size 14’
Gum 53 – Dark Nebula
RA 16 40 00.1 Dec -48 51 45
NGC 6188 really was, without a doubt, the highlight of my Ara tour. It is simply stunning. And it is way bigger than the catalogued dimensions of 20’x12′, which seems only to cover the brightest portion of this gorgeous HII/dark nebula complex. At low magnification and with the UHC filter, it is very complex and takes the eye on an incredible journey through a region littered with bright and dark nebulosity. The range of brightnesses of the bright nebulosity are remarkable, made more remarkable by the way they are mixed in with dark wisps and streaks and rifts. It looks like a length of crumpled grey silk in candlelight.
The nebula extends in a north-south orientation; the bright nebulosity being split by a tremendous dark river that meanders through it also in a north-south direction.
Exploring the dark river is intriguing; it comprises a range of darknesses ranging from a dappled leaden grey-darkness to ink-dark region, here and there overlaid with translucent fragments of silky nebulosity.
It is brightest and most prominent around the region containing the beautiful open cluster NGC 6193. The western rim of the nebula is sharply defined with a bright crisp edge delineated by the dark north-south rift that cleaves the nebula in two. The eastern and northern ends fade away in the loveliest way into the rich Milky Way background field.
The open cluster involved in the nebulosity, NGC 6193, really is lovely little cluster; made more beautiful by the gorgeous nebulosity that permeates the field. (Even without the UHC filter and at low power, the nebulosity is apparent, a faint glow of shimmery nebulosity. Gorgeous!) The open cluster is very scattered, and dominated by a brilliant uneven white double with a mag 7 star on the west side. The double is surrounded by a tight grouping of faint stars. The cluster has no central concentration but has a perfect ellipse of moderately bright stars just southwest of the main portion of the cluster, making it a very lovely open cluster to observe.
The stunning nebula may have been the visually highlight of my evening, but the pièce de résistance was undoubtedly the supercluster…
A supercluster…
Westerlund 1 is the most massive young star cluster in our Milky Way galaxy. The cluster contains a large number of rare, evolved, high-mass stars, including: 6 yellow hypergiants, 4 red supergiants including Westerlund 1-26, one of the largest known stars, 24 Wolf-Rayet stars, a luminous blue variable, many OB supergiants, and an unusual supergiant sgB[e] star which has been proposed to be the remnant of a recent stellar merger. In addition, X-ray observations have revealed the presence of the anomalous X-ray pulsar CXO J164710.20-455217, a slow rotating neutron star that must have formed from a high-mass progenitor star. In the future, it will probably evolve into a globular cluster.
Westerlund 1 – Supercluster
RA 16 46 43.0 Dec −45 57 00 Mag – Diam 9’
What a sight to see… but alas, high interstellar absorption in its direction makes Westerlund 1 an extremely challenging object to observe. Indeed, the only reason I got a fleeting glimpse of it was patience and persistence… I tried for it over three nights, spending a long, long time searching the patch of sky wherein it lies (fortuitously, it has a nice arrangement of stars in the vicinity that help guide starhoppers to its location). On the third night, searching with averted vision, the cluster’s feeble glow suddenly and mysteriously materialised out of the sky like some kind of celestial ectoplasm, a tiny ghostly glow with no discernible edge. It hovered on the threshold of visibility, as close to being not-there as a feeble scrap of visible light can get – but it stayed in view long enough for me to feel confident that the faint glow was indeed Westerlund 1; then it vanished back into the starry sky… and stayed resolutely vanished. What a rare treat to have glimpsed this extremely faint and extremely challenging gem among Ara’s glittering treasures.
An overlapping pair of open clusters…
NGC 6204 and Hogg 22 are two sparse, compact open clusters that lie close enough to each other on the sky that they appear to overlap and form one group, often simply (but incorrectly) referred to as NGC 6204. Australian astronomer Arthur Robert Hogg first identified the more southeasterly group of stars as a distinct cluster in 1965. It is a younger and more distant cluster than NGC 6204. (The 22 Hogg clusters are most often [but incorrectly] attributed to Canadian astronomer Helen Sawyer Hogg, who studied and researched open clusters.)
NGC 6204 – Open Cluster
RA 16 46 07.9 Dec −47 00 4″ Mag 8.2 Size 6’
Hogg 22 – Open Cluster
RA 16 46 35.0 Dec −47 04 57 Mag 6.7 Size 1.2’
In the absence of knowing, these two open clusters lying within 6 arcminutes of one another really do look like one group lying in a gloriously rich field of stars. NGC 6204 is a bright cluster with roughly 50 stars concentrated in a 7′ circle and set against a lovely background haze of starlight. Near the centre are two attractive knots of stars lying north-south of each other. The southern knot is the brighter of the two and has ten stars; the northern knot has seven. There is a pretty string of five stars lying at the eastern edge of the cluster.
Hogg 22 is gorgeous… a small grouping of ten dazzling stars elongated in a northwest-southeast direction, with its dazzling white 7.3 mag lucida lying on its southeastern edge.
Open clusters…
Because the northwest corner of Ara is crossed by the galactic plane of the Milky Way, Ara offers some superb open clusters for those who delight in these stellar jewel boxes – from rich large clusters, to some delicate and exquisite clusters. Here are a few of them.
NGC 6200 – Open Cluster
RA 16 44 08.6 Dec −47 28 00 Mag 7.4 Size 15’
This large, rich open cluster lies in a rich Milky Way star field that renders the cluster less prominent than I suspect it otherwise would be. It shows about 35 brighter mag 9.5-11.5 stars over a rich background of mag 12-14 stars. It is dominated by a little knot of stars lying just north-west of the centre, that contains two nice doubles. Very pretty! The catalogued dimension of 12′ appears considerably too small; although the cluster doesn’t have distinct boundaries, it is spread over a roughly 18’ area.
NGC 6208 – Open Cluster
RA 16 49 25.8 Dec −53 42 00 Mag 7.2 Size 18’
This cluster appeared as a slightly enriched cloud of mostly fainter – mag 12-13 – stars against a rich background. The majority of the stars appear to be arrayed in an elongated, curving north-south shape. There is an orange star and a yellow star pretty close to the centre which gives the cluster an attractive appeal.
NGC 6250 – Open Cluster
RA 16 57 55.0 Dec −45 56 00 Mag 5.9 Size 16’
A really interesting cluster… a small tight grouping of about ten bright stars surrounded by a larger, scattered group of about three dozen fainter stars. It looks as if the brighter stars are foreground stars superimposed on a faint cluster. Most of the fainter stars appear to be concentrated around a triangle of three stars – mags 9.8, 8.9 and 7.8. There is a pleasing catenary chain of 5 or 6 stars on the south western side.
IC 4651 – Open Cluster
RA 17 24 42.0 Dec −49 55 00 Mag 6.9 Size 10’
This is a very attractive open cluster – an obvious grouping of stars that stands out well against the starry background. The cluster has looped arcs of stars and small groupings of stars, with narrow tunnels of darkness winding through them. A faint little knot of about 15-20 stars lies at the centre with another little slightly more loose clump lying at its western edge. There are a pair of dark patches separated by faint stars; it looks like a starry letter B.
Galaxies…
NGC 6300 – Galaxy
RA 17 17 00.4 Dec −62 49 12 Mag 10.2 Size 4.3’x2.8’ SB 13.2
This Seyfert 2 galaxy lies in a rich star field and it shows as a relatively large and relatively bright oval halo, slightly elongated northwest-southeast. It has a brightish stellar nucleus at the centre. The halo is smooth and bright and it has four tiny stars superimposed on it, two just south and west of the nucleus and two others at the south south-eastern and northern edges. Averted vision makes its oval-shape more apparent and brightens it up slightly.
NGC 6221 – Galaxy
RA 16 52 46.5 Dec −59 12 57 Mag 9.9 Size 3.5’x2.5’ SB 12.1
Lying in a rich starry field, this galaxy appears as a fairly bright, fairly large, slightly elongated north-south luminous patch that brightens gradually to a brighter stellar nucleus at the centre. Averted vision reveals hints of a spiral arm on the west side of the halo extending to the north, with a tiny dark patch between it and the central brightness. It also revealed a strange and very tiny little knot at the northern end, but whether it is in the spiral arm or not, I couldn’t tell. A lovely string of faint stars wraps itself around the western side of the galaxy.
NGC 6215 – Galaxy
RA 16 51 07.2 Dec −58 59 34 Mag 11.5 Size 2.1’x1.8’ SB 12.8
This galaxy lies in a rich star field with mag 3.8 Eta Arae just 10’ towards the west. It appears as a round soft hazy glow. It has a gradual brightening to the centre, but no nucleus that I could pick up. With averted vision a faint star could be seen superimposed on the northern edge.
ESO 138-29 – Galaxy
RA 17 29 10.8 Dec −62 26 53 Mag 11.7 Size 2.5’x1.2’ SB 12.9
ESO 138-30 – Galaxy
RA 17 29 25.3 Dec −62 28 52 Mag 14.0 Size 1.2’x0.5’ SB 13.3
This is an extraordinary pair of galaxies. ESO 138-29, larger galaxy, is a ring galaxy and in the distant past the smaller galaxy (ESO 138-30) ploughed through it leaving a large dark hole after the encounter and a bridge of matter joining the two galaxies. What a pity they were so very faint and small in the telescope; how astounding would it be to see a galaxy with a hole punched through it? They were both averted vision objects and two pairs of mag 12 stars helped pinpoint them. ESO 138-29 appeared as a very faint oval haze elongated NE-SW. One pair of the mag 12 stars lie off the SW side. ESO 138-30 appears as the tiniest smudge of very faint light between the second pair of mag 12 stars.
Globular clusters…
For those of us who are partial to these glorious old fossils, Ara is adorned with three absolute gems.
NGC 6397 – Globular Cluster
RA 17 40 41.36 Dec −53 40 25.3 Mag 5.3 B * V m 10.0 HB V m 12.9 Size 31.0’
In my 10×50 binoculars NGC 6397 is a big and conspicuous round glow of soft and silky light, with a gradual brightening toward the centre, a sort of silkier glow upon a silky glow.
At low power, it is a breath-taking beauty. Strings of mixed stars curve away from the core, which itself is absolutely exquisite … a dense little circular core shimmering with the brilliant brightness of unresolved starlight. The strings of stars taper off into the surrounding field in a way that no other globular does.
At medium power it is even more gorgeous, a dizzying turmoil of stars exploding outwards, and there are dark speckled dark areas between the strings of stars that spiral out. The core looks like a heap of diamond dust. Three lovely orangey stars lie in the southern region. At high power the core of diamond dust resolves into tiny pinpoints that glitter like crushed diamonds. What a beautiful globular cluster this is!
NGC 6352 – Globular Cluster
RA 17 25 29.16 Dec −48 25 21.7 Mag 7.8 B * V m 13.4 HB V m 15.2 Size 9.0’
A beautiful little cluster. In my 10×50 binoculars it is a beautiful small, faint soft round glow. At low power it is a bright, irregularly round hazy glow, not very dense; a sort of haziness enveloped in a silky glow. Gorgeous against the rich background field. At medium power, the globular cluster takes on the frosted look of sand-blasted glass. A misty opacity of light, with a beautiful glowing brightness in the centre. At high power I can see a bright haziness towards the core but not a single star resolved. I can see a hazy granular busyness of starlight, and a hint of grainy glints scattered around the hazy periphery.
NGC 6362 – Globular Cluster
RA 17 31 54.8 Dec −67 02 53 Mag 8.1 B * V m 12.7 HB V m 15.3 Size 15.0’
Ara really does have a contrasting selection of globular clusters. The only word to describe this beauty is scraggly (although in the binoculars it isn’t scraggly, just a lovely small soft round glow.) At low power it is large and bright, and slowly brightens towards a dense wide core. It has a very bedraggled looking halo, and the whole globular looking as if it is covered in a layer of haziness. Its irregular appearance is very striking.
At medium power the globular is a glorious blaze, with its appealing straggly appearance accentuated. Tiny stars are resolved across the globular, looking like tiny diamonds lying on a bed of diamond dust. High power resolves a host of resolved stars across the core, and out across the scraggly edges and deep into the star-field; some in really lovely arcs and sprays of faint stars. A very unusual string of resolved stars bisects the cluster in a north northwest to south southeast orientation.
Planetary nebulae…
For those who delight in smoldering white dwarfs and their envelopes of beautiful hot glowing gas, Ara contains a couple of beauties, along with the youngest planetary nebula known – the Stingray Nebula.
PN G331.3-12.1 – Stingray Nebula
RA 17 16 21.07 Dec −59 29 53.6 Mag 10.75 Size – Mag cent * –
This was a grand catch… seeing the youngest known planetary nebula! It lies 23′ south west of the small galaxy NGC 6305 in a rich starry field. I found it by blinking with my UHC filter (it took a lot of blinking). It was extremely small, simply a stellar droplet of faint, pale light the colour of moonlight. There is a nice roughly 11 mag star 35″ west which provided an excellent comparison for this stellar PN.
IC 4642 – Planetary Nebula
RA 17 11 45.3 Dec −55 24 02 Mag 13.0 Size 24″ Mag cent * 16.0
This planetary nebula, with the UHC filter, appeared as a faint, pale grey, smooth oval, round disk with sharp edges. No sign of a central star.
NGC 6326 – Planetary Nebula
RA 17 20 46.7 Dec −51 45 18 Mag 11.7 Size 36″ Mag cent * 16.7
Using the UHC filter, this is a very bright, small, slightly off-round compact planetary nebula lying in a rich field of stars. Its periphery is well defined, and the planetary nebula is an even, silvery-grey glow with no sign of a central star.
Double stars…
For the double star aficionado, Ara offers a lot of delightful double stars to observe – Sissy Haas’ tremendous book, Double Stars for Small Telescopes, has twenty three on offer. This is a showpiece double…
h 4949 Double Star
RA 17 26.9 Dec −45 51
AB: PA 253° Sep 2.1″ Mag1 5.6 Mag2 6.5
AC: PA 312° Sep 102.4″ Mag1 5.6 Mag2 7.1
A beautiful tight, bright, white pair, with a very wide third companion, also a lovely white star.
Copyright © Susan Young 2017