'((3 6.< TOUR

Some elu­sive clus­ters lurk in Ophi­u­cus but they are worth search­ing out

Sky at Night Magazine - - THE SKY GUIDE -

Tick the box when you’ve seen each one 1 NGC 6426

This month’s ob­jects can be found in a strip of sky pass­ing across the mid­dle of the large con­stel­la­tion of Ophi­uchus, the Ser­pent Bearer. The first ob­ject of in­ter­est on our list is the most northerly in this month’s tour, lo­cated 1.5° to the south and slightly east of mag. +2.8 Ce­bal­rai (Beta (`) Ophi­uchi). NGC 6426 is a faint, mag. +11.1 glob­u­lar clus­ter which best suits aper­tures over 200mm. Even then, this elu­sive clus­ter can be a strug­gle to spot, ap­pear­ing merely as a gen­tly glow­ing patch against the dark back­ground sky. Mat­ters don’t im­prove much through a 300mm scope, with the clus­ter ap­pear­ing as a weak smudge of light with un­even bright­ness and no sig­nif­i­cant res­o­lu­tion to speak of. SEEN IT


Ex­tend the line from Ce­bal­rai through NGC 6426 and keep go­ing for the same dis­tance again to ar­rive at an­other rather ill-de­fined ob­ject, the open clus­ter Collinder 350. This is an­other tricky tar­get be­cause it falls apart un­der mag­ni­fi­ca­tion ow­ing to its dif­fuse nature. There are about 15 view­able stars as­so­ci­ated with the clus­ter, and – as sug­gested – the best way you’re go­ing to get to see these is by us­ing a very low power. A large num­ber of back­ground stars in this area also helps Collinder 350 cam­ou­flage it­self into the back­ground. The clus­ter has a large ap­par­ent di­am­e­ter of around half-a-de­gree, roughly equiv­a­lent to the ap­par­ent size of the full Moon. SEEN IT

3 M14

Our next tar­get this month is the far more promis­ing mag. +7.6 glob­u­lar clus­ter, M14. Lo­cated at a dis­tance of 30,300 lightyears, this clus­ter ap­pears off to the east of the gi­ant box shape that forms the body of the ser­pent bearer. One way to find M14 is to place the mag. +3.2 star Eta ( ) Ophi­uchi cen­trally in the field of view of ad­low-power eye­piece and then swing the tele­scope west in RA by 00h 44m (11°). A 150mm tele­scope at high power re­veals the clus­ter to have a dis­tinctly mot­tled tex­ture. Through larger aper­tures you can be­gin to dis­cern its elon­gated shape with a few of the mem­ber stars start­ing to be re­solved at pow­ers over 200x. SEEN IT

4 NGC 6366

Our next stop­ping-off point is the glob­u­lar clus­ter NGC 6366, which at mag. +8.9 lacks the bril­liance of M14. It lies 16 ar­cmin­utes east of the mag. +4.5 star, HIP 85365. The faint clus­ter also has a 9th mag­ni­tude star lo­cated just

to the west. A 150mm tele­scope re­veals an 11th mag­ni­tude pair of stars to the south-south­west of the clus­ter’s core. Small in­stru­ments aren’t likely to pick up much in the way of res­o­lu­tion when it comes to NGC 6366, in­stead re­veal­ing the clus­ter as a gen­tle glow. A 250mm scope at 250x power starts to re­solve mem­bers against what looks like a more mot­tled back­ground glow. Larger in­stru­ments con­tinue this pro­gres­sion, cre­at­ing the im­pres­sion of a smat­ter­ing of evenly spread faint stars against a mot­tled back­ground al­most 10 ar­cmin­utes across. SEEN IT

5 M10

Next we head west­ward for our penul­ti­mate tar­get, an­other glob­u­lar clus­ter known as M10, which lies at a dis­tance of 14,300 lightyears from Earth. This par­tic­u­lar clus­ter sits a de­gree west of 30 Ophi­uchi, a mag. +4.8 star lo­cated south­west of the cen­tre of the gi­ant box shape rep­re­sent­ing the Ser­pent Bearer’s body. Un­like most of our pre­vi­ous tar­gets on this tour, mag. +6.7 M10 re­solves fairly well through even a 150mm scope, with many of the stars ap­pear­ing su­per­im­posed over the glow­ing clus­ter back­ground. Larger in­stru­ments only im­prove the view, re­veal­ing an elon­ga­tion in the clus­ter’s core. Through a 200mm in­stru­ment, M10 ap­pears with an ap­par­ent di­am­e­ter of 10 ar­cmin­utes, with the well-de­fined core oc­cu­py­ing ap­prox­i­mately half this size. SEEN IT

6 M12

The end point for this month’s jour­ney is yet an­other glob­u­lar clus­ter and for­tu­nately for us, it’s one that – at mag. +6.8 – finds it­self at the brighter end of the spec­trum. M12 lies 3.3° north­west of M10, the two ob­jects of­ten be­ing cited as a pair. It’s an easy small tele­scope ob­ject which ap­pears less con­densed than M10. Around 70 stars are re­solved through a 150mm scope at 200x mag­ni­fi­ca­tion. Larger aper­tures vastly in­crease the num­ber of in­di­vid­ual stars that you can see, with the num­ber in­creas­ing into the hun­dreds through a 250mm in­stru­ment. The out­ly­ing stars are quite un­evenly spaced in M12 com­pared to those around M10. M12’s core also ap­pears markedly smaller than M10’s. SEEN IT

M14 is about 100 lightyears across, and has an es­ti­mated age of 13 bil­lion years

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