|Right Ascension||11 : 11.5 (h:m)
|Declination||+55 : 40 (deg:m)
|Visual Brightness||10.0 (mag)
|Apparent Dimension||8x1 (arc min)
Discovered by Pierre Méchain in 1781.
Messier 108 (M108, NGC 3556) is a nice edge-on spiral galaxy situated near the conspicuous star Beta Ursa Majoris, and in one field of view with the Owl Nebula, M97.
According to Charles Messier's hand-written preliminary and unpublished version of his catalog, M108, similar to M109, was discovered by Pierre Méchain shortly after M97 (which he had found February 16, 1781): Méchain discovered M108 3 days after M97 on February 19, 1781, and M109 on March 12, 1781. Both objects were apparently also observed by Charles Messier when he measured the position of M97 (March 24, 1781), but apparently he didn't find occasion to obtain positions for these objects at that time. Messier listed this object, M108, under number "98" in his preliminary manuscript version of his catalog, without giving a position. According to Owen Gingerich, he measured an acurate position at a later time which he added by hand in his personal copy of the catalog. Both objects M108 and M109 are also mentioned in Pierre Méchain's letter of May 6, 1783, which supports the suspicion that he probably wanted to add them to a later edition of Messier's catalog. The object M108 was finally added to Messier's catalog by Owen Gingerich in 1953.
As the discovery of M108 had not been published, William Herschel independently rediscovered this object on April 17, 1789, and cataloged it as H V.46.
The nearly edge-on galaxy M108 appears to have no bulge and no pronounced core at all, it is just a detail-rich mottled disk with heavy obscuration along the major axis, with few H II regions and young star clusters exposed against the chaotic background -- in a word: "Very Dusty". There's little evidence for a well-defined spiral pattern in this Sc galaxy, which is receding at 772 km/sec. According to Brent Tully, it is about 45 million light years distant, and a member of the Ursa Major cloud, a loose agglomeration of galaxies (sometimes called the M109 group). Tully classifies this galaxy as SBcd, i.e. very late Sc, and with a bar; the present author can find no evidence for such a notion in the images he knows.
The type II supernova 1969B occurred in M108 and reached mag 13.9 on Jan 23, 1969.
M108 is quite easy for the amateur, easier than the published values of its brightness (exception: Don Machholz' estimate of mag 9.4) imply. Well matching in the opinion of the present author is John Mallas' description as a "silver-white beauty, saucer-shaped and very well defined" with a quite bright and irregular central region, surrounded by "light and dark nodules." It is a very elongated object with angular dimensions 8x1'. It is actually surprising how much detail can be seen in this galaxy with small instruments ! Color photos show an even more conspicuous appearance of this should-be showpiece, which often appears in wide field and "deep spatial depth" photos together with the Owl nebula M97, which is only about 48' to the SE.
Last Modification: August 30, 2007