Sand and Stars

Star Clouds

Star cluster NGC 6520 and dark nebula Barnard 86 within the titanic Large Sagittarius Star Cloud. Image credit Johannes Schedler

3 Mar 2017

I don’t think any term in astronomy captures my imagination more than “star clouds”. It conjures up images of exactly what they are… countless stars spanning vast stretches of the night sky; the glow of immeasurable starlight resembling enormous silvery night clouds. No photo can do a star cloud justice; even the most stunning image flattens the glut of bright, sparkling stars into an even flatness of shiny dots, whereas in the telescope, star clouds appear to shimmer and take on a 3-dimensional quality with the multitudes of stars sparkling against the backdrop of nebulous starlight like sterling silver scattered with diamonds.

There is something at every magnification in star clouds – at low magnification, beautiful hazy night-clouds of stars, and they certainly give up their secrets at increasing magnifications: bright stars, coloured stars, oceans of unresolved starlight, mysterious dark nebulae, chains, strings and loops of stars, glutted pockets of stars, asterisms, star trails with speckled dark sections in-between… but above all, beauty overlaid with awe.

Until last night, my star cloud observing was all about the two Sagittarius titans, and I had never before dedicated an observing session solely to star clouds… and thus last night was a sublime observing session coasting at leisure from one star cloud to the next, all so different, some beautiful, some barely discernible against their rich backgrounds, some quite mysterious when I compared what I was seeing with John Herschel’s descriptions. What tranquility one finds up there lost in all that mystery and beauty.

Observing these lovely star clouds

16″ f/5 Dobs; 40x, 70x, 150x and 228x

Scorpius has long made his arrival back in our morning skies and its five star clouds were simply stunning. But that wasn’t where I began. I started off my star cloud expedition with that little beauty in Centaurus…

NGC 5155 Star Cloud – Centaurus

RA 13 29 36.0   Dec -63 24 44   Size 60’

NGC 5155. DSS2 image

Many objects, while beautiful in their own right often have their beauty greatly enhanced by their location – and this star cloud is one of them. Washed up on the eastern shores of the Coalsack lies the very beautiful emission nebula, Ced 122. It is large and bright and an absolute beauty – even in binoculars its glow is distinct and widespread. And there nestled into the north-eastern portion of the nebulous glow is the star cloud.

John Herschel described NGC 5155 thus, “A portion of the milky way broken up into clustering masses of astonishing richness. There must here be at least 200 or 300 stars in the field, none greater than 10th mag.”

And indeed the star cloud does have a frosty appearance that denotes hundreds of faint stars glowing in the background. A beautiful curve of eight stars flank the south side of the star cloud. The very small, very faint, very barely-not-there open cluster – NGC 5120 – lies on the south-western edge. The star cloud’s loveliness is enriched by several gorgeous dark nebulae that flank the cloud in the south, north and west.

NGC 5045 Star Cloud – Centaurus

RA 13 17 02.6   Dec -63 24 54   Size 60’

DSS image
NGC 5045. DSS image

John Herschel described NGC 5045 thus, “A great cluster or a surprisingly rich portion of the milky way. It contains 34 stars 11m, and perhaps 150 or 200 of less magnitudes in the field.”

What a lovely star cloud – it appears as a rich collection of stars among a rich field of stars. I would estimated that up to 200 stars stood out boldly along with a host of smaller stars. Bright, loosely spaced stars form a large north-south zigzag asterism. Pretty yellowy mag 6.8 HD 115400 lies at the southeast edge. 

NGC 5284 Star Cloud – Centaurus

RA 13 47 06.0   Dec -59 13 39   Size 20’

DSS image
NGV 5284. DSS image

John Herschel described NGC 5284 thus, “…cluster class VIII. L, v rich, loosely scattered; stars 7-8 .. 16th mag; it is an outlier of the milky way, but very rich and much insulated.”

I found it quite difficult to establish the boundaries of the star cloud as it and the whole field is the typical star-rich southern Milky Way – stars everywhere! But with a long and careful look, I discerned a loose scattering of a large number of mag 11.5-13 stars embedded in the rich haziness of unresolved starlight that appeared insulated enough to be called a separate object. A  string of stars oriented NW-SE appears to define the southern boundary. 

NGC 6421 Star Cloud – Scorpius

RA 17 45 14   Dec -33 41 12   Size 45’

DSS image
NGC 6421. DSS image

John Herschel described NGC 6421 thus: “A most remarkable, well insulated, semi-nebulous milky way patch of a branching rounded figure, within the limits of the field, quite insulated on the p, n and f side, and only connected on the south side by a narrow isthmus with a branch of the milky way, which runs meridionally to a great extent.  It forms a VI. class cluster of the utmost tenuity, barely resolvable, no resolved.”

A lovely star cloud; a pretty large and bright patch of nebulous starlight heavily speckled with small sparkers, and sharply defined by gorgeous dark nebulae on the eastern and western sides. There is a lovely little string of about 6 stars (roughly mag 9.5 – 11) along the southeastern edge of the star cloud.

NGC 6437 Star Cloud – Scorpius

RA 17 48 00.0   Dec -35 51 06   Size 40’

DSS image
NGC 6437. DSS image 

John Herschel described NGC 6437 thus: “A very decided, tolerably defined semi-nebulous mass in milky way, with abundance of vS st, forming altogether a telescopic magellanic cloud.  It fills about a field, and has branches and sinuses and is altogether a remarkable object”.

A gorgeous star cloud; a busy mass of stars in a rich and striking Milky Way field. The star cloud has masses of faint pin-prick stars toward the centre and brighter ones to the edges. The mass of stars is pretty well detached from the Milky Way field, except on the northwestern side where a hazy stream of stars extends to the north and disperses into a another rich mass of stars. It looks like a river delta carved out of starlight. Dark nebulae lie to the east, west and southwestern sides. Really pretty!

NGC 6415 Star Cloud – Scorpius

RA 17 42 54   Dec -35 08 18   Size 23’

DSS image
NGC 6415. DSS image

John Herschel described NGC 6415 thus: “…a great Nebulous projection of the milky way.”

This was a toughie, I saw a nice little grouping of eight bright stars, close together and standing out well against the fainter background stars. But it was difficult to separate a star cloud from the rich Milky Way field, and the whole area is glowing with unresolved starlight.

NGC 6455 Star Cloud – Scorpius

RA 17 51 48   Dec -35 10 30   Size 58’

DSS image
NGC 6455. DSS image

John Herschel described NGC 6455 thus: “…a very extensive nebulous clustering mass of the milky way. The stars of excessive smallness, and infinite in number.”

A lovely little star cloud is set in a very busy star field, and it is framed in dark nebulae that lie to the north-east, south-east and south-west. The star cloud appears as a frosty patch of both faint and fairly bright stars buried in nebulosity. There is a nice asterism of six stars forming a crooked line.

NGC 6480 – Scorpius

NGC 17 54 24.7   Dec -30 26 29   Size 17’

DSS image
NGC 6480. DSS image

John Herschel described NGC 6480 thus: “An extraordinary B nebulous portion of the Milky Way, on a black ground vL; an angle taken where there is a *12m”.

This is a lovely star cloud! The gorgeous dark nebula LDN 1788 creates a distinct V-shaped wedge that points east. The vertex of the V is in the middle of the dark nebula, and the west side of the star cloud just melts into the busy Milky Way field. The south side of the dark nebula forms a very pretty zigzag border.

Copyright © Susan Young 2017