Draconian Discoveries, Disparities and Doppelgängers

By Guy Mackie, Okanagan Centre (guy.m@shaw.ca)

While browsing through the Messier List of the RASC Observer’s Handbook I noted one object with a total of three question marks in its reference information. How could my natural astronomical sense of inquiry not be piqued? Alan Dyer hints at part of the story behind the question marks in his "The Messier Catalogue" introduction found in the Observer’s Handbook. A more detailed explanation of this object’s uncertain identity can be found in the book Messier’s Nebulae and Star Clusters by Glyn Jones. I am speaking here of the "clerical error"that is M102, in the constellation Draco the Dragon.

The tale of M102’s identity crisis is a labyrinth of misprints, miscalculations, mistakes, and miscommunication, beginning with the last minute inclusion of observations by Pierre Mechain which Messier did not have time to check before publication of his final Catalogue. Glyn Jones recounts the detective work done by Admiral Smyth, Harlow Shapley, J. L. E. Dreyer and Helen Hogg, trying in hindsight to find the most likely candidate for the original observation. A lingering doubt about the true identity of M102 remains to this day and similar mysteries cloak M47, M48, and M91. The story of the lost identity of M102 makes for entertaining research and sheds light on the methods and human fallibilities which bring to life the personalities of some of the central players in the history of astronomical discovery. Alan Dyer identifies NGC 5866 as M102 for our RASC Messier List since it is the historical best fit, and I hope that while observing NGC 5866 you will consider some other historical mysteries that share the stage with M102.

As James McRae, Okanagan Centre Librarian, would say, "You will find M102 near the hockey stick of Draco" – where most people see triangles James sees only hockey sticks! In this case the hockey stick is the three stars Eta, Theta and Iota Draco that mark where Draco turns the corner east of the handle of Ursa Major. M102 is about four degrees southwest of Iota. To make a quick one-stop hop, start from Iota and go towards the first star (Eta) in Ursa Major’s handle following roughly the same line and same distance as between Eta and Theta Draco. It’s easy once you are familiar with it.

The 9.9-magnitude M102 is nestled closely between two faint stars (11.25 and 12.20 magnitude) making a positive identification straightforward. Using my twelve and a half inch Starmaster reflector and a 10-mm Radian eyepiece to yield 158× I see a bright elongated core area surrounded by a bright oval disk that I see as 5′ × 2.5′. Averted vision reveals a tiny nucleus that twinkles intermittently.

Using M102 as a jump off point there are many other rewarding discoveries for you to make in the Dragon’s domain. It is a short journey from M102 to the showpiece (!!) Finest NGC Object NGC 5907. A medium power view of M102 will have the 7.6 magnitude star SAO 29401 just over 10′ southwest of the M102 core. Slide your scope 50′ northeast of M102 using the line of SAO 29401 and M102 as a pointer. Here you will find the 7.9-magnitude star SAO 29440 that is the first in a crooked line of three stars that point, as though it were their sole purpose, directly at 10.3-magnitude NGC 5907. Aptly referred to as the Knife’s Edge, the Splinter or the Needle, this edge-on 12′ × 2′ spiral lives up to all nomenclature hyperbole. Allow some time for your eye to adjust in order to appreciate the full extent of the elongated disk as it stretches out much further than you might initially think. The elongated core will silhouette a subtle dust lane under good seeing conditions.

I use the disk of NGC 5907 as a pointer guide and with a slight drift towards M102, move 40′ south of the southern tip of the disk to find what I have coined as the Dragon’s Diminutive Doppelgängers or NGC 5908 and NGC 5905. When I observe this face-on/edge-on pair of galaxies, separated by only 12′. I am reminded of the comparative spiral galaxy view angles of M81 and M82. It is the relative view and the size-to-separation distance that makes the two galaxy pairs doppelgängers in my imaginative opinion. Using a 10-mm Radian eyepiece to yield 158×, nearly edge-on NGC 5908 has an elongated streak of a core area surrounded by a faint 11.8-magnitude 3′ × 1′ elongated disk. Face-on spiral NGC 5905 glows weakly at 11.7-magnitude with a dim round disk of about 3′ diameter brightening slightly to a small core area. Both galaxies hold in direct vision at 125× through James McRae’s eight-inch SCT from the clear and steady mountaintop skies at the 2002 Mount Kobau Star Party.

In 1844 Admiral Smyth described NGC5879 as "probably the object seen by Mechain," referring with reservations to another possible M102 candidate. Again I use the disk of NGC 5907 as a pointer guide to find NGC 5879. At a distance of 58′ northwest of the north tip of the disk of NGC 5907 it is located at almost a mirror of the positions of NGC 5905 and NGC 5908. Using a 10-mm Radian eyepiece to yield 158× I see NGC 5879 as being very similar in shape and structure to M102 but at a magnitude of 11.6 it is considerably dimmer: a small, elongated core surrounded by a 4′ × 1′ faint disk.

A most amazing sight awaits stargazers with larger apertures on the other side of the hockey stick. I was first shown the "triple-stack-variety-pack" of NGC 5982, 5985 and 5981 in Wallace Helter’s ten-inch Orion Dobsonian at the 1999 Mount Kobau Star Party. The straight line and even spacing of these three galaxies, one elliptical, one face-on spiral and one edge-on spiral, characterize this group. The face-on NGC 5985 is the largest and most easterly of the group. Using 158× it is an elongated blur: a dim 11.1 magnitude 4′ × 2′ disk surrounds a small core region. At the mid-point of this chain is the elliptical galaxy NGC 5982, the meat in our galactic sandwich, which at 11.1-magnitude is visually the brightest of the trio. Using 158× the 2′ diameter disk of NGC 5982 has a star-like nucleus, which is set in a bright core area. Anchoring the western end, and always the first to drift out of the field of view in an un-driven reflector, is the dim slash of NGC 5981. In my twelve and a half inch reflector this 13.0-magnitude edge-on galaxy is a dim elongated 2′ × 0.5′ streak with a slightly brighter slash of a core area. Wallace Helter’s ten-inch will reveal all three galaxies in this 16′ long chain, but edge-on NGC 5981 fades to "averted imagination" in James McRae’s eight-inch SCT.

Good luck, keep your head up… and keep your stick on the ice!

Biography:

Guy Mackie is a past president of the Okanagan Centre of the Royal Astronomical Society of Canada. He currently is Director of Telescopes for the Okanagan Centre. He enjoys observing "Old Light" with his 12.5-inch telescope and then sketching and describing what he sees. He wishes to thank Alan Dyer for suggesting resource materials for this article; his thoughtful and generous guidance was very much appreciated.

Bibliography:

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[1] From the Cambridge International Dictionary of English: doppelgänger, noun, the spirit of a living person which has exactly the same physical appearance as them: seeing your doppelgänger is said to be a sign that your death is imminent.