The Stars in the Plough
& Their Observed Order of Magnitude
By Vaughan Cooper
Further to the article in the March '84 edition of MIRA 6, titled 'For the Beginner', I carried out the suggested exercise of estimating the brightness of the stars in the Plough, in order of descending magnitude.
As a result of ay first three observations, which I compared to the answers in the following May edition, along with Webb's and Norton's, a number of interesting inconsistencies have come to light.
Firstly I wish to bring your attention to Norton's Star Atlas (based on Bayer's 1603 star catalogue, where he is credited in most popular text books for developing the system of designating the brightest star in each constellation α, the second brightest β, and so on through the Greek alphabet.) For some reason the stars in the Plough do not follow this system, and there are other instances of this. I can only suggest that for some reason Bayer decided to catalogue the stars in the Plough in order of Right Ascension. Can anybody give a reason for this? (perhaps because it is difficult to estimate their order of brightness - Ed.)
Bayer's designations top
Cov. & Warks. 'MIRA' listed order of brightness middle
Webb's listed order of brightness bottom
My three observations
My first three observations seem fairly consistent with each other. However, they do not compare very well with Cov. & Warks. and Webb's and do not tie up very well either. I have no satisfactory explanation for any of this.
While making the observations, I had the feeling that Dubhe was a red or orange-red star. Having later confirmed this I began to suspect that perhaps Dubhe (like many other red stars) was variable, explaining the main discrepancy in the observations. Unfortunately I failed to find any reference to Dubhe being variable. I am afraid I omitted to record the times of the observations, (I treated them as a simple exercise in estimating relative brightnesses of stars), so if Dubhe is variable I cannot relate my observations of its brightness to anyone else's. From this I have learnt a salutary lesson in making notes of observations. Even the most mundane observations day turn out to be important.
I would be pleased to hear from anyone who could add any comments to this article.
A Short Appreciation by Ted Nicholls
The retirement of Mr. Arculus as College Principal and President of the Society, last August, marked the end of an era lasting 22 years. During this time the Society had many ups and downs, but could always count on the help and support of Mr. Arculus through the good times when membership was large, or the lean times when the membership was small, and, nearly, but not quite, reached the stage where the majority of members were on the Committee.
Previous Principals who had been Presidents of the Society had only had a rather tenuous connection, (their names appeared on the letter headings and Society notepaper). When I was secretary I found that he always took a friendly interest in the welfare of the Society whenever I happened to meet him, and subsequently both Alan Hancocks and Rob Moseley found the same friendly attitude.
Occasions that I particularly recall were when the Warwickshire Society and ourselves amalgamated, when the Cooke refractor went to Coombe and when there was trouble with the Students Union. He also gave permission for us to use the College print dept. to produce this magazine. Happily although he has now retired, he will not be leaving the district and intends to keep an active interest in the Society.
By Rob Moseley
Over the past weeks everyone will surely have noticed Venus, hanging like a lamp over the evening twilight. Such a gorgeous naked eye sight, Venus is often dismissed as telescopically uninteresting - a point of view with which I strongly disagree.
Certainly one problem associated with observing Venus during its winter evening elongations is the very practical one of being free to get to the telescope early enough in the afternoon. Ideally, of course, the planet should be observed in broad daylight or as soon after sunset as possible. But even 20 minutes after the sun goes down can be too late - and many would argue that when Venus becomes readily visible to the naked eye telescopic observation will be a waste of time. This is all due to the great brilliance of the disc, which when contrasted against a dark back-ground sky will totally obliterate any of the subtle shadings.
Some beginners seem to be put off the task of locating Venus in broad daylight, believing that setting circles are necessary. This is not the case. Venus is so bright that near its greatest elongations any pair of binoculars will serve to show it quite readily, the smallest finder scope will also suffice. Make sure the finder is properly aligned to the optical axis of the main instrument and there will be no problem - except one snag which is often overlooked. In a smallish telescope an unfocussed image of Venus will be quite difficult to recognise in the field of view, and unless the Moon happens to be in the sky you will have nothing to focus on. I have encountered just this problem using an 8" Cooke refractor, so never consider yourself unusual in encountering difficulties in the subtle art of telescopy. The solution is a simple one. The next time you use the telescope on the night sky make a mark on the draw-tube at the position of focus for your lowest power eyepiece - it will then be in automatic focus for daylight work.
The main concern of the Venus observer is to monitor the behaviour of the fugitive disc markings and to determine the date of dichotomy (ie the exact half phase). This never accords with theoretical calculations, and no accepted explanation for this mystery (known as the "Schroter Effect") has ever been forthcoming. Colour filters add to the value of the above observations, but should only be undertaken by advanced observers.
Due to the problems of observing in the early afternoon my observations of Venus over this elongation have only just begun in earnest, and so I have largely ignored my usual study of the "Schroter Effect". Using the 6 ½" Cooke refractor at the College Observatory on Jan. 22nd I saw the terminator as sensibly straight — two days earlier that predicted. I include two recent drawings of the planet. Notice the difference in the appearance of the horns of the crescent. They seem to be much sharper in the daylight view of Feb. 16th. A dark shading was glimpsed bordering the S cusp on the 14th but not confirmed on the 16th. On both dates the cusps appeared bright.
This year provides a nice compensation for missing the excitement of dichotomy - an opportunity to follow the planet without interruption from its winter evening elongation, through inferior conjunction, to the start of its summer morning elongation. This is because at inferior conjunction (ie its position between the Earth and Sun) this year Venus will pass 6 degrees North of the Sun. With great care and a well mounted equatorial telescope the planet should be visible as an enormous slender crescent, and perhaps even a complete ring of illuminated atmosphere at the moment of inferior conjunction. This is one of the rarest and most beautiful sights available to the amateur astronomer, and well worth putting in some extra effort for.
VENUS 1985 Feb 14 17.35 U.T. VENUS 1985 Feb 16 16.10 U.T.
Seeing III / IV Seeing III trans. good
6 ½" OG. x150 - x230 10" spec. x120 - x240
Observer R Moseley
I have made my own drawing of Venus, with a 4" reflector. Compare this with Rob's drawings, made with the Society's Cooke refractor. My drawing, was made a few days before Rob's but the phase has not changed noticeably - Ed.
VENUS 1985 Feb 11 16.30 U.T.
4" spec. x133
Observer R Barrett
Note bright cusps. The northern (bottom) cusp was brightest.
This apparently insignificant constellation, positioned below the tail of the Great Bear, is often overlooked by observers simply because it is small and lacks bright stars. For the experienced observer (with a fairly large telescope), however, the area is littered with galaxies. Burnham's lists 55 galaxies down to mag. 13. This is not to say that this constellation is only of interest to the more advanced observers. There about 7 galaxies bright enough to be seen with a 4" reflector (ie. mag. 10 and brighter), and 4 of these are Messier objects. Canes Venatici also contains one of the most magnificent globulars in the entire sky. In my own opinion it is certainly more impressive than M13, the famous globular cluster in Hercules. With a magnitude of 6, and a diameter of 18 minutes of arc it is easily seen in binoculars as a large fuzzy patch about 12° NW of Arcturus (the brightest star in the Northern hemisphere). The stars in the cluster have magnitudes from 11 to beyond the limits of detectability, which makes the cluster quite easily resolvable (I have suspected resolution into stars with my 4" reflector).
Canes Venatici from Norton's 2000 (Note the drawings used in MIRA 10 did not reproduce well and are not used here)
Of the galaxies in Canes Venatici the most famous is, of course, M51 - The Whirlpool Galaxy. It is located 3½° SW of Eta Ursa Majoris. Owing to its large size (10.0' x 5,5') it is far more difficult object than its 8.7 integrated magnitude would suggest. Low powers and wide fields are a definite advantage when observing this object. (I have seen it clearly in 10x50 bins. but find it rather elusive in my telescope). The chart below shows stars down to mag. 8.
Another bright galaxy in Canes Ven. is M94. This forms an isosceles triangle with ALPHA and BETA Canum. It lies about 1½" North of a line joining these stars. M94 has a very bright middle, which is only 30" across. It therefore appears almost stellar in small telescopes. Around this central region is a relatively faint disk extending out to a radius of 60". The chart shows the best way to find M.94 by star-hopping from BETA Canum.
M63 is a galaxy of magnitude 9.8 positioned mid—way between α and the end of the tail of the Plough. It has a very bright central region 6" in diameter, surrounded by a dish of tightly coiled,and quite bright spiral arms out to a radius of 50", which is surrounded by another faint set of spiral arms. Thus the galaxy looks like a star surrounded by a halo of nebulosity.
NGC 4258 is an additional Messier object (M106). It is an Sb type spiral near the border with Ursa Major. Although mag. 9, it is a difficult object because of its large size (giving it a low surface brightness). M106 is somewhat elongated as it is nearly edge on to us.
Besides these interesting deep sky objects there ore two stars of particular note. The first of these is ALPHA Canum, which is an excellent double star, resolvable even with snail telescopes. The two components are of mags. 3 and 5½, with separation 19.6". There is little or no colour contrast, both stars being white. (Webb recorded the colour of the fainter
star as 'pale copper' which agrees with its spectral type FO. The primary is of type AO).
The other star which is worth looking at is Y. It is shown at the top left of the map for M106. Y is a semi—regular variable (period 158 days) with range 5.0 — 6.4. The unusual feature of this star is its extreme red colour. With spectrum N3 it is one of the reddest naked—eye stars. Although in a blank region of sky about ⅓ of the way between ALPHA CV and DELTA UMA it can easily be identified by its colour. This star was given the title 'La Superba' in reference to its spectrum.
Below is a list of all galaxies in Canes Venatici brighter than mag. 11. It is important to remember, though, that the size of an object is just as important as its integrated magnitude in determining real brightness, since a large object will have its light spread out over a larger area, so its surface brightness will be lower.
NGC M mag size type notes position
4214 - 10.5 7.0'x4.5' I? possibly early barred spiral 12131n3636
4244 - 10.7 13.0'x1.0' Sb edge-on galaxy, narrow streak 12150n3805
4258 106 9.0 9.5'x6.5' Sb see above 12165n4735
4449 - 10.5 4.2'x3.0' I 12258n4422
4490 - 10.1 5.0'x2.0' Sc somewhat elongated 12283n4155
4631 - 9.7 12.5'x1.2' Sc? bright edge-on spiral 12398n3249
4730 94 8.9 5.0'x3.5' Sb see above 12486n4123
5005 - 10.8 4.7'x1.6' Sb rather elongated, bright middle 13085n3719
5055 63 9.8 9.0'x4.0' Sb see above 13135n4217
5194 51 8.7 10.0'x5.5' Sc sec above 13278n4727