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M101 and Surrounding Galaxies

 

M101 and surrounding galaxies

 

M101 in Ursa Major is a beautiful, large, face-on spiral galaxy of the type Sc, meaning it has extended spiral arms and a very small central bulge. Like in other large nearby spiral galaxies, several knots of emission nebulosity can be recorded on deep photographs. The galaxy is quite near, at a distance of only 18 million light years, and is the center of a small group of galaxies with 23 known members, mostly dwarf galaxies. This is a neighbor group to the Local Group of galaxies, of which our Milkyway Galaxy is a member. Click on the thumbnail image below to get identifications on the galaxies surrounding M101 in the above picture:

Of the NGC galaxies, NGC 5474 is a peculiar HII galaxy of the type Sd, NGC 5477 and NGC 5486 are irregular spirals of the type Sm, NGC 5422, NGC 5473 and NGC 5485 are lenticular S0 galaxies seen at different angles and NGC 5484 is an elliptical galaxy of the type E2. NGC 5368 is a faint spiral galaxy of the type Sab. Only NGC 5474 and NGC 5477 are members of the M101 group, the others are background objects.

M101, Wide Field, Digital Camera Newtonian photograph.


Exposure Data


Observing Hints

M101 is an impressing face-on spiral galaxy in photographs. Of course objects like that attract visual observers. However, the spiral arms, which are probably the main attraction of M101, are definitely not easy game.

But first M101 has to be located. It is situated a bit apart of prominent stars, but there are good guides. By elongating the line from Megrez (Delta UMa) to Alioth (Epsilon UMa) and Mizar (Zeta UMa) in the tail of the bear, M101 can be found at another time the distance between Megrez and Alioth, measured from Mizar. Another possibility is putting the Mizar/Alcor pair into a wide-angle eyepiece a bit north of the center and moving 5.5 degrees to the west, M101 should then appear in the field-of-view.

The galaxy is visible in a 7x50 finderscope as fuzzy patch of light, but it should be a clear night, of at least 6th magnitude. The best time for observing M101 is late spring, when the galaxy approaches the zenith.

It takes a quality 4" refractor and low power to show the galaxy as disk with small brighter core and several "knots" of light in the spiral arms, these are HII regions. The spiral arms themselves are not discernable in a 4" refractor. Some of the HII regions do have their own NGC designations: NGC 5447, 5455, 5457, 5461, 5462 and 5471 are included in the Uranometria sky charts. These take higher magnification, but then the disk will disappear, i.e. becomes too dim.

With increasing aperture the knots and disk of the galaxy become easier observable, but the spiral pattern is reserved for big light-buckets, considerably greater than 10". Under good conditions, an 18" will show the "fire wheel" with 4 arms quite impressive, suggesting that slightly smaller scope also have a chance, but averted vision will be necessary. But, no illusions, the appearance never equals the well-known photographs. Only the innermost and brightest parts of the spiral arms are observable, but you can search for the widely spread HII-regions. The sighting of the spiral arms is heavily dependant on sky brightness, if the sky is not very dark, the pattern appears dim even in an 18" scope, and will be hard if not impossible to see with smaller apertures. Therefore it is not easy do define a lower limit for the observability of the spiral pattern.

If you have the chance to see M101 in a big quality telescope under superb sky conditions, grab it, it is surely worth your time...


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© 2010 Walter Koprolin