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Home , 67 Ophiuchi, 70 Ophiuchi, Alpha Ophiuchi, IC 4756, NGC 6633, Theta Serpentis

A corner of for binoculars by John Flannery, SDAS

PPEARING OVER the southeastern horizon as tinguish from foreground randomly scattered we slip into Summer is Ophiuchus, the across the field. “SerpentA Bearer”. The pattern is becoming well placed Keeping Cr 350 at the west (right) edge of the at the moment for observers that wish to plumb the 20x60s will enable the double S694 to fall within for the rich array of deep sky objects to be the same field of view at the eastern edge. Remember found within its boundaries. that while my binoculars have a 3° field, lower power A large dim constellation that abuts the Milky instruments will have a wider field of view. S694 is a Way, it has 13 stars above fourth magnitude with pair of almost equal magnitude (+6.9 and +7.1 Rasalhague, or , a second magnitude respectively) with a separation of 82 arcseconds. The marking the head of Æsculapius, the mythological two were easily split with both appearing dusky-grey in figure whom the constellation represents. Eight de- tint – probably because of the hazy sky at the time. grees roughly to the south of this star is Beta (+2.7), Next up is a non-existant object, or rather, a now- or Cheleb, marking the start point for our tour. defunct constellation. From a dark site you may spy a Just north of cream-coloured Beta is the loose open downward pointing triangular-shaped group of stars cluster IC 4665. My 20x60mm binoculars showed nu- with the naked eye about 8° east of Beta. These com- merous stars sprinkled across the field with Beta itself prise 67, 68 and with 66, slightly north of just at the lower edge of the 3° field of these instru- 67, ruining the symmetry. The group was once known ments. The centre of the cluster has a nice chain of as Taurus Poniatowski, which was formed in 1777 blue-white suns running across the field with a small in honour of Stanislaus Poniatowski, King of Poland. arc of stars just below this. My impression was of a tiny Johann Elert Bode (1747-1826) scraped together some celestial bridge spanning a tributary of the Milky Way. eighty stars as members of this “constellation” in his A small knot of stars at the southwestern edge of the monumental star atlas, Uranographia, which was pub- cluster was reminiscent of a comet – a pair of stars em- lished in 1801. bedded in a faint haze. 70 Ophiuchi is a celebrated star in its own right. Nudging the binoculars slightly southeast of Beta It was discovered to be a binary by Sir William Her- you come to Gamma Ophiuchi (+3.7) below which schel in 1779 but a telescope is required to resolve the lies the sparse Cr 350. The cluster was not star into its components unfortunately. It is also a rela- readily apparent in a pocket pair of 8x30s while the tively nearby star at a distance of only some 16.6 light- 20x60s revealed a few eighth-magnitude members of and this fact alone makes it noteworthy as one of the group. Indeed, it is so loose that it is difficult to dis- the few stars in the solar neighbourhood to be visible to

IC 4665 NGC 6572

NGC 6633 b Oph

66 IC 4756 73 “cluster” g Oph 74 67 70 q Ser (Cauda) 68 S 694 Cr 350 Mel 186 Ophiuchus

This chart, generated in SkyMap Pro, details the objects described in the text. It is centred on 18h 20m R.A. and 4°N with a limiting magnitude of +7. The “hatched” box to the right of 66 Oph shows roughly the location of Barnard’s Star.

— 5 — the naked eye from our latitudes. umn in Sky and Telescope magazine. The “cluster” is The whole field of Taurus Poniatowski is itself part north of 70 Oph and west of 73 Oph – anchoring an of a loose star cluster. Mel 186 measures a whopping open-angled triangle with both stars. An attractive tight 4° in diameter and is centred on . The chain of stars runs across the field to the northeast also. cluster needs low power instruments to reveal its true Have a look yourself some evening and see if you see nature and the 8x30s showed numerous sparks sprin- the group differently to what I did. kled liberally across the field with the eye being partic- Four degrees north of 73 Oph is the planetary neb- ularly drawn to a nice concentration of stars to the ula NGC 6572. I must admit that I did not search for northeast of 70 Oph. I’ll come back to this later be- the this particular night because light from the cause I initially set off to the northwestern edge of Mel waning gibbous moon was flooding across the sky by 186 to where a single star lay that is one of the most fa- the time I turned by attention to searching for it. Phil mous stars in the sky. Harrington’s book “Touring the Universe Through This is Barnard’s Star, a dim magnitude +9·5 Binoculars” gives it a photographic magnitude of +9 red dwarf that has the highest (or appar- with a diameter of eight arcseconds. It should therefore ent motion across the sky) of any charted star in the appear as a dim nebulous spot in larger binoculars. heavens. The American astronomer E.E. Barnard first NGC 6633 is on the border of Ophiuchus and Ser- noticed its nature in 1916 when comparing plates made pens (Cauda). Through a borrowed pair of 10x50s the in 1894 and 1916. He found that its annual motion is cluster was resolved into numerous points of light all 10.29 arcseconds in a direction almost due north – entangled in a misty haze – hinting at other members of meaning that it covers a distance equivalent to the di- the cluster that lay below visibility. The cluster is visi- ameter of the Full Moon in only 175 years! ble to the naked eye from a very dark site. Some au- Barnard’s Star is also the second closest star to the thors have often commented on how is it that Charles Solar System (if you take Alpha Centauri as a single sys- Messier overlooked NGC 6633 during the compilation tem) and there have been suggestions that a dim com- of his catalogue of “nebulous” objects – though it was panion, or large planet, may be in attendance. noted by Philippe Loys de Cheseaux in 1746. Although I had printed a map of the location of the Crossing a short distance into Serpens we come to star from SkyMap Pro, I found the best charts for locat- another fine binocular cluster: IC 4756. The whole ing the star were in the small pocket “Stars” book from group spans about one degree of sky and again, the the Collins Gem series. The book is one of the best 8x30s were much better at revealing the cluster. The portable atlases I have ever used though I have come richness of the Milky Way in this region also rewards across some discrepancies between the new and earlier observers casually sweeping along its length. editions where brightish stars on some charts were left The final port of call on our tour is Theta Ser- out to make room for text or labels! pentis, or Alya, a fourth magnitude triple The Gem chart shows an “arrowhead” in for small telescopes but also resolvable in binoculars. th th the field and the star is located a short distance southeast The B and C companions are of 5 and 8 magnitude of this. I was surprised at how easy Barnard’s Star was in respectively with the fainter C component being 414 the 20x60s. In a way, I have not really pushed the arcseconds distant. The two brighter members of the capabilities of these instruments before so it was a nice system lie 22 arcseconds apart and are a good test of bonus to pick up this stellar neighbour. your observing skill – both appeared whitish in colour. Heading eastward back across Mel 186, I returned While may mark the end of this to the small clump of stars near 70 Oph that I men- particular tour, it shouldn’t mark the end of your ad- tioned earlier. This was more apparent in the 20x60s ventures in binocular astronomy. Some patience and than what I had noticed before in the 8x30s and it sent preparation will be needed to find all the objects men- me scurrying to the atlases – but nothing was marked tioned – try not to be too disappointed if you fail to here! Had I found a new cluster? Unfortunately, no, as spot them all in one night – and your impressions may Uranometria Volume 1 charts the stars as being unre- not exactly match mine. lated — although they are in the Mel 186 field. In time, you will learn the technique of star- My notes from the night do record though that hopping and how to judge the field of view in your there was a tight knot of three stars with two fainter binoculars. You may move on to bigger and better members very close by and the whole grouping en- things – like purchasing a telescope – but for the mo- veloped with a slight haze. It is a fine example of how ment, why not dust off those binoculars and discover different instruments will show the sky in a variety of new gems of the night sky. ways. This was a point often written about by the late Got a favourite “star hop” you’d like to share? Drop Walter Scott Houston in his “Deep Sky Wonders” col- us a line at any of the editorial e-mail addresses.

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