Date 13th. December 1986
Time 21h to 21-45h UT
Instrument 6in. Reflector x60 & 120
Observer Vaughan Cooper
Deep Sky Notes
M 11 (NGC 6705) "The Wild Duck" Cluster
A showpiece object of the Summer skies, described by D'Arrest in Latin hyberbole as "magnifica innumerabilium stellarum coacervatio".
This beautiful clot of stars, lying just north of the Scutum star cloud provides much interest to the visual observer, and illustrates the subtle changes in appearance in varying apertures common to many galactic clusters.
The cluster lies at a distance of 6000 light years, with a linear diameter of 21 light years, with 600 members brighter than mag 15. Its nickname comes from Admiral Smyth, who compared it to a flight of wild ducks. However, this appearance depends very much on the aperture and magnification employed.
Smyth used a 6 inch refractor for his observations. I first beheld M 11 during the summer of 1968 using a 4 inch reflector at a power of x30. What I saw reminded me of a comet. It was a small fuzzy fan-shaped object, Smyth's description puzzled me.
Over the past three years I have observed the cluster with a number of different telescopes.
On 1987 June 20th, I tested the Society's new 5 ½ inch F/3.6 Celestron Schidt/Newtonian "Comet Catcher" at x28. This superb rich field instrument gives a field of 1 ½ degrees. My impression was very similar to my original view: "The cluster appears small and fan-shaped. An 8th mag. star lies at the apex, pointing towards two 9th mags. It is not resolved, but mottled in texture - the star-poor areas clearly showing."
A few nights later (July 1st) I looked again, this time using a 5 inch F/5 refractor, giving a 1 degree field at x37. "The prettiest view of the cluster I have ever had. Resolved into a peppering of faint stars, seen on a perfect scale with in the field,"
But what of Smyth's "flight of ducks"? With low powers structure is certainly visible, caused by the star poor areas, but with increased aperture and magnification we get this; "1984 June 18th. 10 inch reflector, x120. Astounded by this marvellous sight. Smyth's description immediately recognised for the first time. To my eyes there are three distinct "wedges" or "Chevrons" of stars flying to towards the two prominent 9th mag stars on the Sf periphery of the cluster. The general effect is thus —
"The whole region is peppered with a stardust of 10 - 13th magus, but the areas between the wedges are relatively star poor. A splendid sight;"
But the effect disappears. Using the 18 inch reflector at Conder Brow observatory even the star poor begin to fill with fainter cluster members, and the cluster, though exquisite loses its "Wild Duck" appearance.
Unknown Worlds Part Two
By Rob Moseley
Prior to the Voyager encounter with Uranus, even such fundamentals as the rotation period were in doubt. By studies of the previously unknown magnetic field Voyager has shown this to be 17 hrs 18 mins, with an uncertainty of 3 mins. This rapid rotation, as with Jupiter and Saturn tends to flatten the poles of the planet to the order of between one twelfth and one fourteenth of the equatorial diameter. For a Uranian, looking at the sky is quite uninteresting. The Sun appears almost fixed among the background stars and takes 84 years to circle the ecliptic. And due to the peculiar tilt of Uranus' axis some very strange seasons occur. The tilt is 98°, which means that Uranus "rolls" along its orbit. First the northern hemisphere, then the southern hemisphere is plunged into darkness for 21 years, with 21 years of light between. The San appears to travel from north to south and back again in 84 years. There is a legend that the axis of the Earth was originally straight, but was pushed off axis 23 degrees by the sins of Man - we must assume any Uranians to be a pretty rum lot!
The Sun itself appears nineteen times smaller to Uranian eyes, therefore Uranus receives 368 times less light than we do. Nevertheless, the Sun is by no means a dim object even at this great distance. Although its angular diameter is only 1'40" it has a light equal to 1584 full moons! Unless Uranian astronomers were very clever mathematicians they would be completely unaware of our own world, let alone Venus or Mercury. The Earth could never appear more than 3° away from the Sun and would be almost completely lost in its glare. Mars and Jupiter would only be vaguely visible and Saturn nothing more than a small morning or evening star. Neptune would be a dim night star — and Pluto completely invisible without a telescope.
The most prominent objects in the Uranian skies would be the five moons, all shining brighter than we see Venus. Before Voyager these moons were expected to be rather dull cratered affairs and one of the greatest surprises of the mission was the close-up views of their surfaces. The greatest mystery was provided by Kuiper's moon, Miranda. It can only be described as "weird" showing a chaotic terrain at present defying geological explanation. There is certainly nothing else quite like it in the Solar System.
What can we, as amateurs, see of Uranus? Like Herschel we can gaze through the smallest of telescopes at the tiny dot of its disc, which never gets larger than 4 arc seconds. The two Herschelian moons, Titania and Uberon are visible in a good 8 inch reflector under ideal conditions, but the others are well beyond our capabilities. Percival Lowell, using a 24 inch refractor saw faint equatorial bands on the blue-green disc. It is claimed that a 10 inch instrument can reveal these but close examination with the Pie du Midi 24 inch refractor in the 1960s showed nothing but strong limb darkening. This is confirmed by Voyager. A dark polar hood was detected, along with bright equatorial bands - but dark and bright are rather strong terms, as we are dealing with albedo variations or only 1 or 2%. These features are certainly invisible in earthbound telescopes, in the past Uranus has sometimes seen suspected of variability, and Voyager data has so far given no clue on this matter. It would have to be connected in some way with physical intensity changes on the planet's surface, either raising or lowering the albedo. From what we now know for certain about the atmosphere it is hard to see now this could take place. Nevertheless, the question is still open and an interesting field of research beckons for the amateur armed with nothing more than a pair of binoculars. Brightness variations were also found for Titania and Oberon in 1926 and 1947 by Steavenson, observing with hie 30 inch reflector at Cambridge.
Leaving Uranus and travelling out into the dark periphery of the Solar System we come to a world well described as the twin of that planet. Slightly smaller, slightly more massive, Neptune never comes closer to the Sun than 2,675 million miles, in the discovery of Neptune we see one of the greatest triumphs of mathematics and the repudiation of contemporary doubts in the universal validity of Newton's Laws of Gravitation.
The doubts arose thus. When Uranus was discovered and recognised as a planet mathematicians immediately set to worK to determine its future path among the stars. Unfortunately Uranus stubbornly refused to obey Newton. Slowly, as the years passed, the actual position of the planet differed increasingly from the calculated position. By 1845 Uranus was deviating by 2 minutes of arc - an intolerable situation for mathematical astronomy to find itself in. But long before 1845 astronomers had beea stirred to action over the embarrassing problem. Bouvard, working in Paris around 1820, was the first to notice the discrepancy, and as it steadily increased several eminent astronomers came to the conclusion that this could only be due to the influence of an unknown planet orbiting beyond Uranus. But how to find it? It seemed an impossibly difficult task, like a mathematical detective case they had the victim but had to apprehend the culprit.
Mädler, Bessel, Valz and Arago all agreed that a trans-Uranian planet was responsible. Bessel actually started calculations but died soon after. Then Arago advised a young mathematical genius called Leverrier to take up the task. He began by verifying the earlier tables of Bouvard. Then he considered the perturbing effects of Saturn on Uranus. Then Jupiter on Uranus. He took into account the 19 pre-discovery observations and used 179 made between 1781 and 1845... and found that the errors could indeed only be explained by effects caused by an unknown body. By the end of 1845 he was able to say, "I have demonstrated that there is a formal inconsistency between the observations of Uranus and the hypothesis that this planet is subject only to the actions of the Sun and the other planets acting according to the principle of Universal Gravitation. We can never succeed in representing, by this hypothesis, the observed motions." So he too postulated the existence of an unknown planet beyond Uranus, and on the basis of Bode's "Law" recalculated the orbit taking the stranger into account. It was a monumentally difficult task - but it worked.
On August 31st 1846 Leverrier announced to the French Academy of Science that the new planet would be found at the heliocentric longitude of 326° which placed it 5° to the east of Delta Capricornii.
The French astronomers were completely lethargic. Not one bothered to check Leverrier's position. However, Leverrier was forward enough to write to Johann Galle whom he knew was working as an assistant at the Berlin Observatory. His letter arrived on the 23rd September and Galle immediately implored Encke, the Director, to allow him to search. Encke reluctantly agreed, considering the whole affair with suspicion. Galle was helped by D'Arrest, an even more junior assistant. If it had not been for him the discovery might well have been delayed, for when Galle was confronted by the myriad stars in the 8 inch refractor he was inclined to give up quite soon. D'Arrest urged him on, then remembered that the 21 hour section of a new star chart by Breniker had just been printed, though not yet published. He rushed to an untidy drawer and fetched it. With this chart their way was clear, and within two hours they noticed a discrepancy. They called Encke, who decided that they must wait for the next night to see if the star involved had moved. We can imagine with what excitement they awaited the following evening. Fortunately it was clear. The 8th magnitude point of light had moved!
They had discovered the new planet, and the sensation was quickly announced - with D'Arrest's name omitted from the credits.
Hearing of the discovery on October 1st a certain Professor Challis in England looked in hie notebook and with thunderstruck horror saw that he too had observed the planet two months earlier.
This was no mere coincidence, for not one but two searches were being made for the planet, resulting from the work of two mathematics each oblivious to the activities of the other. On the 10th September, only two weeks before Galle's discovery, there was an air of expectancy in British astronomy, for Sir John Herschel addressed the British Association thus:
"The past year has given us the new asteroid Astraea - it has done more, it has given us the probable prospect of another. We see it as Columbus saw America from the shores of Spain. Its movements have been felt trembling along the far reaching line of our analysis, with a certainty hardly inferior to ocular demonstatlon."
It is a sad statement for nothing else but its date. For it could have been made a year earlier if anyone had bothered to listen to John Couch Adams.
John Couch Adams
Two men are responsible for the blame posterity has laid upon then. Professor James Challis, Director of Cambridge Observatory and Sir George Airy, the Astronomer Royal. Adams was a young student who should have been heard judging by his almost unnatural genius in mathematics. In his final tripos examination at Cambridge he collected an unheard of 4,000 marks - his nearest rival, Bashford, could only manage 2,000... and he went on to become a professor of mathematics! Adams started work on the Uranus problem while still at university, Challis (his tutor) did not exactly encourage his efforts but eventually advised him to show his calculations to Airy.
By Vaughan Cooper
The study of double stars has been a rather neglected field of observation by both amateur and professional astronomers partly because of this neglect, the observations, if accurately produced are very valuable in checking on the present position of the two stars, as many years may have elapsed since the last measurements were made.
However a great deal of satisfaction may be derived from just merely seeking out and examine the the different colours and testing ones telescope optics and your acuity of eyesight.
I hope the following list covering some very easy doubles to start the raw beginner off with along with a few difficult stars to split for the more experienced observer with larger telescopes.
As the list hasn't been compiled from first hand experience with my telescope, do have a go at observing these stars and report back to me for publication in the December issue on what you've seen or failed to see.
39 Draconis 4.7 - 7.7 mag. — 3.1" sep. White and reddish, also a 7.1 mag. star lies nearby all at 17 light years away.
ϒ Delphini 4.0 - 5.0 mag. — 10.4" sep. Yellow and emerald at 110 light years away. In the same field of view appears Σ2725 consisting of 7th. and 8th. mag. stars with a separation of of 4.2.
β Cygni (Albireo) 3.0 - 5.3 mag — 35" sep. Mid yellow and intense blue, regarded as one of the loveliest doubles in the sky and a fine object in any small telescope.
δ Cygni 3.0 - 6.5 mag. — 2.1"sep. A stiff test for any small telescope because of the brilliant primary, may have more success if observed in a twilight sky instead of total darkness. The secondary is considered a long period variable of 321 years lying at 160 light years away.
Σ2780 Cephei 6.0 - 7.0 mag. — 1.1" sep. Test for any 4in. telescope.
β Cephei 3.3 - 8.0 mag. — 14" sep. Greenish white and blue, one of the members of this pair is a prototype of the β Canis Majoris class of pulsating stars.
κ Cephei 4.0 - 8.0 mag. — 7.4" sep. White and blue.
61 Cygni 5.6 - 6.3 mag. — 28" sep. The celebrated star which was the first to have it's distance measured.
16 Cygni 5.1 - 5.3 mag. — 38" sep. A yellow star situated in a superb field.
α Lyra (Vega) 0.0 - 9.0 mag. — 60" sep. The companion is not a true binary only a optical line of sight however the companion is hard to see against the blaze of Vega. (I would be interested to know the smallest telescope in the society to split this pair.)
ε1 ε2 Lyra The Double Double The two components have been known to be split with the unaided eye being of magnitude 4.0— 4.5, separation 208. Whilst any 3in. telescope will show each primary is itself a double, of the two, ε1 is supposed to be the easier to divide.
ε1 Lyra 4.6 - 6.3 mag. — 2.9" sep.
ε2 Lyra 4.3 - 5.9 mag. — 4.4" sep.
ζ Lyra 4.3 - 5.9 mag. — 4"'sep. A wide easy double again another optical double the primary a blue white star lying at 880 light years away and the companion a red giant star lying at 72 light years away, the light of which varies erratically between mags. 4.5 and 6.5.
η Cass. 3.7 - 7.4 mag. — 11" sep. Yellowish and purple at 191 light years away.
ι Cass. 4.2 - 7.1 mag. and 8.1 mag. 2.4" and 7.4" sep. A splendid triple star of yellow, blue and blue. A telescope of at least 4ins. and a high magnification will be required to see the 7.1 against the 4.2 primary.
η Persi 4.0 - 8.5 mag. — 2.8" sep. Intense yellow and deep blue, a fine contrast. The primary is a super giant star at 820 light years away.
ϒ And. 2.2 - 5.0 mag. — 9.8" sep. A grand yellow and blue double, the fainter star is again double with a companion of 6.2 mag. separated by 0.7 requiring a 10in. telescope with a high magnification and a steady night to see it.
ζ Persi 2.7 - 9.3 mag. — 12" sep. The primary white, a very luminous giant star with large telescopes two other companions can be seen.
Σ552 Persi 6.3 - 6.5 mag. — 9.O" sep. Both white a splendid pair.
Σ533 Persi 6.0 - 7.5 mag. — 20" sep. Reddish and bluish set in a grand field.
ι Tri 5.0 - 6.4 mag. — 3.9" sep. Yellow and blue a fine contrast.
α Hercules 3.0 - 6.1 mag. — 4.4" sep. α is a variable golden yellow star about 600 times the size of the Sun making it one of the largest stars known, lying at 540 light years away. The greenish companion makes it a fine pair.
κ Hercules 5.0 - 6.0 mag. — 29" sep. Yellow and bronze in a fine field lying at 280 light years, easily seen in small telescopes.