解析南十字

https://sandandstars.co.za/2016/08/24/southern-cross/

Southern Cross

24th Aug 2016

Susan

Stars

For me, and probably for most southern hemisphere observers, the Southern Cross is the most famous, the most visible, the most memorable and the most personally relevant constellation.

Every time I look up at night, the first thing my eyes are drawn to is the familiar pattern created by the Southern Cross’ five stars. As soon as I’ve seen it, it makes me feel… well… comforted. It’s always felt like some sort of celestial gyroscope – there it is: wheeling around the South Pole: All is well with the universe.

The Southern Cross lies at the eastern end of a dazzling sea of radiance – a spectacularly rich region of the Milky Way, choc-a-block full of celestial wonders and jewels.

Never mind containing a jewel box, Crux IS a jewel box. So what better place to begin my first night’s observing in this wonderful dark sky place?

And what better place to start than the Jewel Box itself? (For all the observations I used magnifications of 64x, 90x, 144x, 230x, 288x.)

Image credit ESA/Hubble

NGC 4755 Open Cluster – the Jewel Box

12h53m38s -60°21′00″ mag 4.2 diam 10′\

In John Herschel’s immortal words, “… this cluster, which though neither a large or rich one, is yet an extremely brilliant and beautiful object when viewed through an instrument of sufficient aperture to show distinctly very different colours of its constituent stars, which give it the effect of a superb piece of fancy jewellery.”

I reckon this must be one of the most famous quotes about any southern deep-sky object (although strangely it’s also probably one of the most misquoted quotes, too, popping up in all sorts of off beam variations.).

It’s a superb sight at low magnification, red and blue stars intermingled with yellows and whites in a profusion of sparkling light. Three brilliant jewels form a triangle; a dazzling bluish white diamond at the apex, two gorgeous pale yellowish topazes forming the base, one of which (the Yale Kappa) in the south eastern corner resembles a piece of jewellery itself – the lovely topaz set in a pendant of tiny diamond chips.

The Jewel Box’s three most dazzling jewels lie framed by the triangle … the westward star of the three is a sapphire, a beautiful bluish star embedded in an arc-shaped sparkle of unresolved starlight like diamond dust.

The middle star, and the smallest of the three, is a brilliant white diamond.

And then there is the third star, the one Herschel described thus: “… centred is this glittering nest of stars, like a ruby set in a set of diamonds and sapphires, is a red giant comparable to Betelgeuse.” It forms a startling contrast to all the other stars, and to my eyes it has the most subtle hint of golden; truly a gorgeous gem among gorgeous gems.

This is a `close up’ of the Jewel Box, showing some of the gloriously coloured stars it contains. Image credit ESA/Hubble

When you can drag your eyes away from this glorious trio, there are all the other jewels lying scattered carelessly in this celestial jewel box. Over there, to the southwest of the triangle… the brightest stars form two delicate semicircles of stars, like two little diamond clips facing opposite directions. Immediately south of these two diamond clips is a long graceful chain of fainter stars oriented northwest to southeast, that look like a delicate chain of gleaming seed pearls. And everywhere else, tiny sparkling stars are scattered around the Jewel Box like an unstrung necklace of mixed gems carelessly dropped into the Jewel Box.

Herschel, and other observers since, report seeing greenish tinted stars in the Jewel Box. Not me. To my eyes the Jewel Box is filled with diamonds of every size, sapphires, topazes, pearls, all scattered on a sprinkling of diamond dust.

Just gorgeous! And how serendipitous is it that such a dazzling little cluster of jewels is washed up on the shores of the Coal Sack’s giant pool of inky blackness?

Coal Sack Dark Nebula

12h53m00s -63°00′00″ diam 7°x 5°\

Image credit ESO

Steve O’Meara calls the Coal Sack the ‘Shadow of the Cross’ because in photos the size and shape of the Coal Sack mimics that of the interior of the Cross (and it does, perfectly). The inimitable Admiral William Smyth referred to it as “the Black Magellanic Cloud”. I’ve always wondered about that…

As the Coal Sack and the Large Magellanic Cloud are roughly the same angular size, it has always looked to me as if the Large Magellanic Cloud was brutally torn out of the fabric of the Milky Way and thrown over there, leaving behind a gaping hole of nothing. Was this how Smyth saw it?

I’ve always loved dark nebulae; exploring those beautiful dark chasms of gas and dust is very beguiling. At first blush they seem like pools of nothingness, then you discover how many variations of darkness the eye can discern: wisps of darker darkness, rivers of gentler darkness, pools of utter darkness. And the Coal Sack has to be one of the best to explore when it comes to variations in darkness.

Naked eye, it is a grand sight in these utterly dark skies – a veritable hole in the fabric of the Milky Way. The 15x70s reveal amazing rib-like structure among the blackness. It’s almost as if one is seeing the skeleton of the Coal Sack, the blacker blackness of the ribs holding the vast cloud of dust and gas together.

One thing the Coal Sack does that other dark nebulae don’t seem to do, is change in a ghostly fashion with averted vision. The ribs round out, become more wave-like, and if I run my eye around the Coal Sack I get the impression of a vast undulating nebula, like a photo of a sea at midnight with slow rolling waves heaving across it. And the foreground stars scattered hither and thither across it add to the impression, looking like sparks of phosphorescence against the black water.

And there, embedded in this vast black ocean, close to Acrux… a gorgeous charred orange star (BZ Crucis), set against the blackness of the Coal Sack like the last dying ember in a campfire… and also the faint glow of –

DSS image

NGC 4609 Open Cluster

12h42m20s -62°59′36″ mag 6.9 diam 6′\

This is a wow object. Not only because of its stunning neighbourhood, embedded in the Coal Sack with gorgeous orange variable star BZ Crucis between it and the tiny glow of open cluster Hogg 15. And not only because in the 15x70s it seems to flare off BZ Crucis like the silvery spray tail of a comet. But because it actually lies behind the Coal Sack yet its young stars pack enough punch to be visible through this dense dark interstellar cloud. Cool stuff!! It’s a gorgeous little cluster through the telescope…. it shows about 20 stars loosely grouped in a rectangular sort of shape. (There is an unrelated but bright foreground 5.25 mag star just to the southeast of the cluster.)

It’s a stunning cluster to visit after the Jewel Box. The Jewel Box is a bunch of dazzlers. This is a smaller bunch of dazzlers, but all tarnished and dulled by the Coal Sack smog.

Being in the vicinity, it was but a small hop via BZ Crucis to –

DSS image

Hogg 15 Open Cluster

12h43m36s -63°06′06″ mag 10.3 diam 2′\
Also a lovely cluster owing to its neighbourhood. The cluster shows three stars forming a narrow but eye-catching triangular asterism with a fainter extra star thrown in. The triangle is set against a faint glow of unresolved starlight. This is one of the open clusters that remind you that not every open cluster has to be blazingly beautiful to be beautiful.

DSS image

NGC 4052 Open Cluster

12h02m01s -63°14′00″ mag 8.8 diam 10′

Another wow cluster! Beautiful location, positioned 8’ north west of 4.5 mag Theta-1 Crucis and 16′ west of 4.7 mag Theta-2 Crucis. At low power it is a large rich cluster of 10th to 13th mag stars with others glittering in and out of view in the background haze of starlight. At medium power it appears much looser, and beautiful chains of stars crisscross the cluster forming a lovely almost geometric pattern. I think what I love most about open clusters is how different they all are, and each one has something unique to delight the eye.

DSS image

NGC 4103 Open Cluster

12h06m40s -61°15′00″ mag 7.4 diam 6′

The cluster lies in a beautiful star field. (That’s the prob with this incredibly dark sky… the star fields are incredibly rich; I spend half my time getting side tracked and wandering around them, dumbfounded at the sheer number of stars that fill my eyepiece.) The cluster is bright and spread out, a group of about 20 stars of equal brightness, standing out beautifully against a grainy haze of starlight. A long string of mag 9 stars make an asterism that looks like Zorro′s Z slashed into the night sky.

DSS image

NGC 4349 Open Cluster

12h24m07s -61°52′12″ mag 7.4 diam 4′

Lying just short of midway between Acrux and ε Crucis, NGC 4349 is an exceptional cluster. It displays a swarm of stars that spiral away from a slightly more crowded centre. One of the spirals snakes out in faint stars from the cluster’s western side and curves northwards like a string of tiny diamond lying on black velvet. A pretty mag 8 star is noticeable towards the southeast corner of the cluster.

DSS image

Hogg 23 Open Cluster

12h28m37s -60°54′35″ mag 9.5 diam 7’

Situated 11′ south-east of VdB-H 133 (which tends to dominate the star field with its pretty line of bright stars) – but even so Hogg 23 shows a pretty grouping of 6 stars with a yellowish 7.1 mag star in the centre of the cluster. It looks like a little topaz surrounded by tiny ice chips of light. A particularly lovely star field.

DSS image

Harvard 5 Open Cluster

12h27m16s -60°46′44″ mag 7.1 diam 5′

Harvard 5 lies 12′ east of VdB-H 133 and is the third brightest cluster in Crux at 7.1 mag. It is a pretty little cluster with its stars gathered together in a compact diameter of only 5 arc minutes.

DSS image

Hogg 14 Open Cluster

12h28m39s -59°48′00″ mag 9.5 diam 3.0′
At medium power, the cluster shows only a few stars standing out against the rich background star field. The heart of the cluster is a 10 mag star with some fainter stars gathered around it in a vague arc. NGC 4439, another small cluster lies roughly south of Hogg 14 –

DSS image

NGC 4439 Open Cluster

12h28m27s -60°06′12″ mag 8.4, diam 4.0

A beautiful little cluster of stars in a lovely dainty little arc with three faint stars nestled within the arc. A pretty white mag 7 star lies to the NW. Lovely area, rich star field.

DSS image

NGC 4184 Open Cluster

12h13m38s -62°42′20″ mag – diam 2′

The cluster showed as a faint grainy haze; no stars resolved, just the odd twinkle that twinkled out before it fully registered on the eye. The surrounding field wasn’t that rich so the hazy-grained cluster stood out more than it would have against a rich star field. I love these little hazy open clusters; like with the cores of globular clusters, I almost go cross-eyes trying to resolve stars and patterns in the hazy glow.

DSS image

NGC 4337 Open Cluster

12h24m04s -58°07′24″ mag 8.9 diam 3.5′
A dainty small very faintly grainy haze standing out well against the background field. A delicate chain of mag 11 and 12 stars cut through it in a very pretty fashion.

DSS image

Ruprecht 98 Open Cluster

11h58m48s -64°34′00″ mag 7.0 diam 15′
A pretty cluster! The data give a diameter of 15′ but the main concentration seems to be just over half that with between 40 and 50 stars scattered around loosely. A nice mix of moderately bright and faint stars. A faint string of four bright stars with four fainter companions curves gracefully across the northern side of the cluster.

DSS image

Ruprecht 105 Open Cluster

12h34m00s -61°32′00″ mag –, diam 12′\
The cluster is centred on a 7.1 mag star but it was pretty near impossible to distinguish cluster members from the background field. Nonetheless, a lovely view of pretty stars.

DSS image

Ruprecht 165 Open Cluster

12h28m36s -56°26′00″ mag – diam 20′
Tough to separate the cluster’s stars from the background stars, but could make out a loose smattering of stars around a 6 mag yellowish star, but with no way of telling where the cluster ended.

DSS image

Ruprecht 97 Open Cluster

11h57m23s -62°42′18″ mag 9.1, diam 5′
A grainy haze of stars in a circular 5′ area. It looked as if there was a slight concentration of stars in the northern portion of the haze. A few stars glittered in and out of view… resolved for an instant before disappearing back into the haze.

DSS image

Trumpler 20 Open Cluster

12h39m32s -60°38′00″ mag 10.1, diam 7′
Hmmm. A beautiful star field, like seed pearls stitched onto lace, but impossible to distinguish a cluster among it all. But worth the search, a lovely view while I rummaged around.

DSS image

Loden 694 Open Cluster

12h53m31s -60°50′00 mag – diam 16′
Some faint stars but nothing that stood out against the star field. As it’s 16′ in diameter, was expecting to see more something even), but not. So logged as a not.

DSS image

Loden 682 Open Cluster

12h47m19s -60°38′00″ mag – diam 28′
The data record the cluster as large – 28′ – but even so, it was difficult to tell it from the background stars. I saw a faint little unresolved roundish fuzz at the location, but that was it.

DSS image

Loden 565 Open Cluster

12h08m24s -60°51′00″ mag – diam 11′
Picked up the orange 6.2 mag star that lies to the west of the cluster, but it was impossible to define a cluster at all. Just lots of stars with nothing to distinguish a cluster member from the rest of the crowd. Pretty field, though, so a nice view while searching.

DSS image

ESO 131-13 Open Cluster

12h22m49s -59°38′00″ mag – diam 11′
Another pretty field, with lots of stars and no way of distinguishing the cluster members from the rest of the crowd, apart from a nice roughly 9 mag star which was a soft lemony colour.

Then it was time for the Southern Cross’s anchor stars and some nice doubles:

DSS image

Alpha Crucis (Acrux) Triple Star

12h26.6m -63°06′
The naked eye Acrux is a beauty, a gorgeous distinct cold blue steel colour. And it is a superb triple. The main pair are both brilliant bluish white stars separated by 4″. Alpha-1 is mag 1.3 and Alpha-2 is mag 1.6 and they sparkle beside each other like a pair of diamonds. The 4.8 mag third star lies 90″ away and it appears very slightly yellowish, like a somewhat lower-grade diamond beside the two dazzlers. The sight of this triple always thrills me… even though I must have viewed it hundreds of times.

DSS image

Beta Crucis (Mimosa) and Ruby Crucis

12h47.7m -59*°*41′

At mag. 1.30, Mimosa is the second brightest star in Crux and the 20th brightest star in the night sky. Naked eye, it is similar in colour to Acrux, a gorgeous distinct cold blue steel colour.
Through the telescope, Mimosa is a dazzling diamond on a backdrop of black satin… and there within its glow one of the loveliest sights in the sky – a rich deep red incandescent ruby. It’s carbon star DY Crucis, but goes by the lovely nickname Ruby Crucis. It’s one of the most beautiful carbon stars out there, smouldering away like a tiny red ember beside dazzling Mimosa. Very striking indeed.

I adore carbon stars. Another beautiful pairing with a carbon star was back towards the end of 2014 sometime (I don’t have my 2014 log book with me so not sure when) when R Leporis (Hind’s Crimson Star) and Comet Lovejoy were in the same field. It was a special treat to see Lovejoy with its bright little nucleus and gossamer smudge of a stubby little tail flying past a small blood red carbon star… I ask you, how much better can it get?

DSS image

Gamma Crucis Optical Triple Star

12h31.2m -57°07′
Gorgeous naked eye star… a vivid orangery-red or reddish-orange (never can decide) tint. To my eyes this colour is much paler than Antares, but a lot richer than orange Arcturus. This line of sight triple forms a perfect triangle; Gamma and a white star forming the base of the triangle and a fainter bluish star forming the apex. A lovely sight because of the colour contrasts, but to be honest, line of sight doubles and triples don’t do much for me.

DSS image

Delta Crucis

12h15.1m -58°45′

At 2.7 9 mag. it is almost exactly two magnitudes fainter than Acrux. And like Acrux and Mimosa the star has the same distinct steel blue colour, just paler. Apparently, in Portuguese Delta Crucis is known as Pálida (the pale one). Seems like the perfect name for this star which doesn’t have a proper name.

DSS image

Epsilon Crucis

12h21.3m -60°24′

At 3.8 mag. this is the faintest of the five prominent stars; just over 3 magnitudes fainter than Acrux. Naked eye it is a washed out orange-ish star. At 144x it is beautiful; a slightly muddy orange. As an aside, it is odd that New Zealand chose to leave this beautiful little star out of its depiction of the Southern Cross on their flag; I’m guessing their flag-designer was one of the irritants who get my goat big time when they say, “…that other star, that one there, kind of out of line, and ruining the shape…”

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DSS image

Iota Crucis Double Star

12h45.6m -60°59′
A lovely pair with a bright, yellow primary and a 9.5 mag white star separated by 27″ at PA 22°.

DSS image

Mu Crucis Beautiful Double Star

12h54.6m -57°11′
This is simply one of the most beautiful doubles in the sky. The 4 and 5 mag pair, separated by 35’, are both dazzling blue-white in colour. They lie in a beautiful starry field.

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h 4543 Double Star

12h43.5m -58°54′
A yellow-orange primary with a tiny grey-white companion separated by 36.5″ and at PA 95°. Lovely starry field.

Dun 117 Triple Star

12h04.8m -62°00′
Two almost equal white stars, the B star with a very faint blueish tint, separated by 22.7″, at PA 149°. The C star, a fainter, very slightly orangey colour, separated by 25.1″ and at PA 18°.

h 4548 Double Star

12h46.4m -56°29′
The primary is a bright yellowish-white star with a small dim slightly bluish companion, separated by 51.7″, at PA 166°.

h 4524 Double Star

12h28.2m -60°02′
An unequal pair lying on the edge of open cluster NGC 4439. Both white, separated by 31.1″, the smaller companion at PA 338°. Nice field with the open cluster!

HIP 60557 Double Star

2h24.8m -58°06′
Another double lying beside an open cluster – this one lies close to NGC 4337, a dainty small very faintly grainy haze of an open cluster. Two equal stars, white, separated by 5.3″, PA 335°.

Who needs a Rolex when you’ve got a Southern Cross clock?

And that was pretty much it for the Southern Cross.

Or not quite… I brought out my Southern Cross Clock and checked the time. I mean really, what other clock could one possibly use on a Southern Cross excursion?

What a grand constellation Crux is! It not only points out the South Pole, it also tells the time remarkably well. I made myself this Southern Cross clock some time back for a couple of kids, but tarted it up and brought it along so that I could have an appropriate timepiece for the Kalahari, where time seems to move very differently to time elsewhere.

I tried to take a photo of it telling the time with my red torch illuminating it but buggered things up because the flash went off… it was blinding on the laminated white clock… and there went my dark adapted eyes. (By the way, if anyone wants a Southern Cross Clock, drop me an e-mail and I’ll whizz the PDF over.)

But hey, I’d finished my cruise around Crux, it was time for a mug of hot cocoa for me, a bowl of warm milk for Waldo, so all was not lost. (It is absolutely freezing at night in the Kalahari. Like all deserts, the temperatures plummet once the sun sets.)

After the cocoa was finished, my eyes once more dark adapted, and Waldo tucked back under his mohair blankie, I spent the rest of the night visiting old friends… it was a wondrous journey of beauty and awe, made more so because there in the back of my mind the whole time were the beautiful words of Marcus Aurelius: ″Dwell on the beauty of life. Watch the stars, and see yourself running with them.″

Copyright © Susan Young 2016