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Astronomical Observatory: Cool Images

Astr112 Photography Projects, Spring 2006

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M108, Jonathan Vance


Located near the Owl Nebula in the Ursa Major Cloud, approximately 45 million light years from Earth, the spiral galaxy M108 was discovered twice and named four times. According to Charles Messier, M108 was first discovered by Pierre Mechain on February 19, 1781, three days after Mechain had discovered M97 (the Owl Nebula). Messier observed the object himself on March 24, 1781 and listed it in a draft of his catalog as "98" (name 1), since its discovery immediately followed that of M97. However, Messier did not record M108's position until after he had published the first version of his catalog, and then only by hand in his personal copy. As M108's discovery had not yet been published, William Herschel independently rediscovered it on April 17, 1789 and cataloged it as H V.46 (name 2). In 1953, Owen Gingerich finally added M108 to the official Messier catalog (name 3). M108 also goes by the designation NGC 3556 (name 4).

You could find M108 with your own telescope; it is located just outside the bowl of the Big Dipper. From our vantage M108 appears nearly edge-on with an angular size of 8.0 by 2.0 arcminutes. If we could see it face on, its disk would appear circular. Using the small angle formula we can calculate M108's linear size (i.e. the actual diameter of its disk) at approximately 100,000 light years, which is about the size of our own Milky Way spiral galaxy. As the image shows, and unlike the Milky Way, M108 has no bulge and no pronounced core. Its dust lanes are chaotic and clumpy, and its spiral arm structure is loose. M108 possesses relatively few H II regions and young star clusters and is instead, astronomers like to say, "very dusty." These features (no bulge, chaotic dust, clumpy arms) typically earn M108 classification as an Sc spiral galaxy. Astronomer Brent Tully classifies M108 as a barred galaxy, type SBcd. Most astronomers fail to see any bars, however, and the reader is invited to scrutinize the image herself. The three bright stars that appear in a horizontal row just below the center of the galaxy are not part of M108; they are in the camera's line of sight and are much closer to Earth than M108 is.


Messier Object Index <>

University of Alabama web page by Bill Keel <>

Right Ascension (J2000) 11:11:30
Declination (J2000) 55:40:00
Filters used clear(C)
Exposure time per filter 4 x 300 seconds in C
Date observed

March 20, 2006 (C)





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