November 2010 Skynotes: Autumn Observing Notes

These notes are taken from Christopher Taylor's observation notes from an evening in November 2010, using the McIver Paton telescope, for a visiting undergraduate studying the history of astronomy.


Objects seen in 12½-inch reflector, Hanwell 10.xi.10, 18.30-20.30:


  (Mostly at x110 & x238, except M31, NGC404, perseus star-cluster and gamma Andromedae, which viewed with x75 wide-angle)


Double stars

61 Cygni (first object) – 1st star-distance ever measured, F.W.Bessel, Konigsberg Obsy. 1838 – one of our nearest stellar neighbours – a v. long-period physical binary with orbit of 6 or 7x 100 years.

β Cygni (Albireo) – beaut. colour-contrast, one of the finest in the heavens – but probably not binary (no sign of orbital motion ever detected, no signific-ant shared proper motion, discordance of Hipparcos parallax measures & of radial velocities); the primary (orange *), however, is itself a close visual pair of 0.3 arcsec separation, certainly a true binary.

z Aquarii – a pair of nearly identical white stars, separated by about 2 arcsec and just clearly resolved at x238. This is a long-period binary with a 600-year orbit (±!), historically of great significance as one of the half-dozen v. first binaries announced in Herschel’s milestone paper of 1802.

γ Andromedae – another fine colour-contrast, like Albireo but ⅓ of the apparent separation. While γ And. AB have shown no detectable orbital motion since their pairness was discovered in the C18th  , they are almost certainly members of a binary of v.v. long period and B (the blue *) is itself a close 61-year binary first resolved by Otto Struve ( Struve II, aka ‘OΣ’) with the Pulkova 15-inch O.G. in 1842: this pair, OΣ38, is difficult even at its widest opening of 0.6arcsec or so, as it was last in the 1980’s, and it has now closed up way beyond any possibility of resolution by direct imaging with ordinary telescopes#(although writers in some of  the glossy mags. of popular astronomy don’t seem to have rumbled to that!). 

Open star clusters

h+χ Persei (the ‘Double Cluster’NGC 869+884, our last object). One of the oldest discoveries of such objects –  <150 B.C. – these are amongst the very youngest star clusters known, at a fraction of 1% of the Sun’s age. Distance can only be estimated rather indirectly & uncertainly but seems to be about 7000 L.Y., making each cluster about 70 L.Y. across; total mass probably about 5000 solars. Quite impressive even in binoculars.

Globular star clusters

Messier 2 Aquarii (M2, the lower of these two, our 1st globular). Main-sequence fitting of the cluster’s H.-R. diagram suggests a distance of around 50,000L.Y., making it one of the most distant bright globulars and implying a diameter of about 150 L.Y.; at that distance the Sun would be at least 100 times too faint to see in the 12½-inch!   M2 contains at least 100,000 stars.

M 15 Pegasi (the brighter & richer of the two). Dist. probably~ 35000L.Y., diameter about 120-130. M15 is v. unusual in containing a planetary nebula, Pease 1;it also has a central X-ray source (black hole?)

  These ‘halo’ globulars are amongst the most ancient celestial systems known, as old as, or even older than, the Galaxy itself, i.e. twice the Sun’s age or more. They are v. important in the C20th  history of science in connection with the early work of Harlow Shapley.

Planetary nebulae

M 27 Vulpeculae (‘The Dumb-bell’ neb., the large, fuzzy one). There are no methods of measurement for distances of planetaries in any way comparable with those for stars & star-clusters, so the absurdly precise figures often given in astronomy books§should be treated with robust scepticism. However, M27 is probably one of the nearer planetaries – hence its large apparent size – at  somewhere between 600 & 1000 L.Y.  The nebula’s tiny central star has a surface temperature over 20 times that of the Sun and consequently has a luminosity about 100xsolar, when its huge ultra-violet emission is included.

M 57 Lyrae (‘The Ring Nebula’, and seen exactly so in the 12½-inch), perhaps the most famous of all the planetaries, although certainly not the brightest. Distance may be ~ 1500 L.Y.  There are only a very few planetaries among the hundreds known which show a clear ring-structure like this, and M57 is the most conspicuous of these. The claim is sometimes made that the central star was discovered visually by one F.von Hahn in Germany about the end of the C18th but I am deeply sceptical of this: it is definitely not visible in the 12½-inch working at x660 on the best nights to a limit of mag.15½, it has been reported invisible in the Mt. Wilson 60-inch (visually) and it was never seen by either Herschel or Rosse working with instruments far more powerful than anything which existed in Germany in 1800! However that may be, the star was in fact first brought to the general notice of astronomers by its photographic discovery in the late 1880’s. Try this with the 30-inch, perhaps?

External galaxies

M 31 Andromedae & its satellites M 32 & NGC 205 (M 32 is the small concentrated one, NGC205 the large, faint one). The ‘Andromeda Nebula’ is, of course, of all external galaxies the most significant in the history of astronomy, and the one that made Edwin Hubble famous; the reasons are fairly obvious. Curiously, there are continuing doubts over the exact distance of even this nearest extragalactic neighbour: since the 1960’s the generally-accepted figure has been 2.2 million L.Y. but some recent work, published even since the Hubble Space Telescope (HST) ‘Key Project’ on cosmic distances, has revised this to 2.9x106 .  Despite being so bright & familiar, M31is one of those objects which is critically dependent on the utmost sky-clarity & darkness to show its most interesting features, i.e. anything beyond the featureless central ‘hub’; on such nights – which unfortunately we did not have, good ‘though it was – the galaxy can be seen to span 3 degrees of sky and its overall elliptical outline is plain to view (in extremis, even with the unaided eye; I have seen this just twice in 40-odd years).

NGC 404 Andromedae (v.close to β Andromedae in same low-power field).

Although this has a reputation for being a difficult target visually due to the close proximity of the v.bright star, and it is only mag. 11.9, I am always surprised at how easy this is in the 12½-in. at x75 & x110, and find the striking contrast with the brilliant light of the star makes an attractive picture.

This dwarf elliptical galaxy actually has a blue-shift – it is approaching us at about 50 km./sec. – so is too close to the Milky Way for the cosmic expansion to dominate its own random motion; presumably,\, NGC404, like M31, is a member of the ‘Local Group’ and at a comparable distance.

(NGC 891 And. – a much more distant edge-on spiral – failed to find.)


Suggested further reading

For authoritative, fully referenced, short biogs. of important astronomers such as Herschel, Bessel and Hubble: The Dictionary of Scientific Biography C.C.Gillispie (Ed.), 16 vols.,N.Y. 1970-1980. A good general ref: The Cambridge Illustrated History of Astronomy M.Hoskin (Ed.), Cambridge 1997. The story of early attempts to measure stellar distances, up to Bessel, Struve I & Henderson, is well told in the populist Parallax:the race to measure the cosmos A.W.Hirshfeld, N.Y. 2001. Extracts from many of the original scientific papers: A Source Book in Astronomy H.Shapley & H.E. Howarth, N.Y. 1929 (up to 1900), and Source Book in Astronomy 1900-1950 H.Shapley, Harvard 1960.

One of the great curiosities of the historiography of astronomy is the complete absence of a full-scale scientific biography of William Herschel (such as has been done, e.g., for both Newton & Einstein), despite an extensive literature of Herscheliana spanning over a century and a half. Many lesser biographical essays have appeared, H.’s complete scientific papers were published in a magnificent Roy. Soc./ R.A.S. 2-volume collected edition in 1912 under the editorship of J.L.E.Dreyer (compiler of the ‘NGC’) but still no thorough, scholarly biography giving the complete picture of the man & his work in depth. Many fairly obvious historical questions remain unanswered, even w.r.t. H.’s ‘glory days’ 1781- 1810, or are subject to widespread conjecture & misinformation: it is more than high time that this greatest of all telescopic observers and surveyors of the cosmos was rendered his due by the right biographer – an open opportunity here!

  The DSB, at least, should be out on open shelves in the Radcliffe Science Library (i.e. Bodleian scientific), who must certainly have the Dreyer edn. of Herschel’s papers and most of the other items, as well.



                                                  Hanwell  12.xi.10



#  Note added 18.11.15: At their closest (‘periastron’), as over the last few years, the two stars of this system close up to no more than 0.05 arcseconds separation on the sky, about the extreme limit of what the Hubble Space Telescope can see resolved. On two nights of the finest definition (class I seeing in this aperture, a rare occurrence) in winter 2014 –15, the 12½-inch was pushed to its uttermost limits in a very close examination of OΣ38 at a power of x825: the binary appeared absolutely ‘round as a button’. This long experienced observer of subarcsecond double stars, who has made many successful observations of systems at 0.4 arcsec. and  below with the 12½-inch working at that power – the current record being 0.18 arcsec., about half the Dawes Limit for that size of telescope – and who has often resolved this binary in the past, is not expecting to detect any trace of OΣ38’s double nature for some years to come. Perhaps the most fascinating aspect of this extreme of visual double-star observing is watching the stars in such binaries actually move visibly, over a period of a decade or two.

§ a problem – scientific naivety or worse – which unfortunately bedevils a lot of recent astronomy writing.


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