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Discovered by Pierre Méchain in 1781.

Messier 108 (M108, NGC 3556) is a nice edge-on spiral galaxy situated nearthe conspicuous star Beta Ursa Majoris, and in one field of view withtheOwl Nebula, M97.

According toCharles Messier's hand-written preliminary and unpublished version ofhis catalog, M108, similar toM109, was discovered byPierre Méchain shortly afterM97 (which he had found February 16, 1781): Méchain discovered M108 3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. Messierlisted this object, M108, under number "98" in his preliminary manuscript version of his catalog, without giving a position.According toOwen 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 inPierre Méchain's letterof May 6, 1783, which supports the suspicion that he probably wanted to addthem to a later edition of Messier's catalog.The object M108 was finallyadded 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, andcataloged 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 obscurationalong the major axis, with few H II regions and young star clusters exposedagainst 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 modern data (e.g., NED), M108is about 33 million light years distant (formerly, we had used Brent Tully'svalue of 45 million light years), and a member of theUrsa 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.

Two supernovae have been observed when they occurred in M108:

• Type II supernova 1969B on Jan 23, 1969, discovered by Wild and Balazs, 116 arc secs W, 20 S odf the galaxy's center; it reached mag 13.9.
• Supernova 2023dbc occurred in M108 69.7 arc second W and 2.4 N, and was found on March 13, 2023 by Koichi Itagaki; it brightened to mag 16.7.

M108 is quite easy for the amateur, easier than the published values of itsbrightness (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 fieldand "deep spatial depth" photos together with theOwl nebula M97, which is only about 48' to the SE.

NGC Online data for M108

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Hartmut Frommert
Christine Kronberg
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Last Modification: January 7, 2024

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