M51 - Whirlpool Galaxy

M51

The famous Whirlpool galaxy M51 was one of Messier's original discoveries of 1773. It is not a single object, but a pair of two galaxies in close contact with each other. The beautiful face-on spiral galaxy is designated NGC 5194, the smaller companion galaxy to the north NGC 5195.

M51 is the dominating member of a small group of galaxies. As it is about 37 million light years distant but still very conspicuous, it is actually a large and luminous galaxy. It is often handled as a prototypical example of a type Sc galaxy with large, open spiral arms and a small central bulge. The spiral arms are dominated by young blue stars, dust (seen as dark lanes in the spiral arms) and HII-regions (seen as isolated bright spots in the spiral pattern).

This galaxy was the first one where the spiral structure was discovered, in 1845 by Lord Rosse, who created several scatches of this galaxy, among those also some very careful and quite accurate paintings. According to the present theory, the pronounced spiral structure is the result of NGC 5194's close encounter with its less luminous neighbor, NGC 5195. Due to this interaction, the gas in the galaxy was disturbed and compressed in some regions, resulting in the formation of new young stars. As is common in galactic encounters, spiral structure is preferably induced in the more massive galaxy.

There are several more distant galaxies in the above image visible as small nebulous specks, the brightest of these is IC 4278 located to left of NGC 5195, near the left edge.

Exposure Data

Instrument: Intes Micro 8" f/6 Maksutov-Newton
F/stop: 6.0
Exposure: L: 44 min, R: 31 min, G: 25 min, B: 34 min
CCD: L: ST-7E, RGB: ST-402
Date: L: May 1, 2005; RGB: April 25, 2006 and May 3, 2006
Exposure start: L: 0:10 MESZ, RGB: 0:17 MESZ / 0:48 MESZ
Location: O2O Observatory, Mistelbach, Lower Austria
Conditions: L: Visual limiting magnitude 5.3 mag, seeing 3, temperature 10°C, no wind, moderate humidity; RGB: 5.3 mag / 5.0 mag, mediocre seeing, no wind.
Autoguider: ST-7E internal chip
Enhancement: CCDSoft, Adobe Photoshop, SGBNR, Paint Shop Pro - by Howdii

Observing Hints

M51 is possibly the most often observed galaxy in the northern sky. Nearly every starting astronomer has probably seen a picture of this galaxy, of course a photograph taken by a big observatory telescope. The "extension", NGC 5195, is of interest and of course everyone wants to see the spiral arms.

But first, M51 has to be located. It is situated about 4 degrees southwest of Benetnash (Eta UMa). It can be seen in a 7x50 finderscope as diffuse patch of light, but it is not very conspicuous. Therefore it is of advantage to memorize the stellar pattern in the environment of M51 to be able to locate it easily. With a little fantasy, the stars around M51 look like a "house", just like children draw it, where M51 is "residing" in the upper floor. (The gable points towards Benetnash.)

Beginners are often disappointed after locating M51 in the eyepiece. Small and medium-sized telescopes do not show more than two brighter cores and a bit of nebulosity around them. Despite that, M51 is THE galaxy where spiral arms can be most easily perceived; but let's be realistic: The observability of the spiral pattern is strongly dependant on sky conditions. If the conditions are not right, seeing the spiral arms is difficult even in a large telescope.

Being more specific, not only a dark sky is necessary, but also a good measure of observational skill. Choosing the right magnification is a key to success, as the image should not become too dark, but detail perception should be maximized. Giving an exact number is impossible as the correct magnification is strongly dependant on the given observing conditions; but smaller scopes will generally need more power than larger ones. Being skilled in averted vision is a necessary requirement. That way, it is possible to catch first hints of the spiral structure in a quality 4-inch telescope, but that is easier with 6 inches of aperture, even the connecting bridge between M51 and NGC 5195 will become discernible. But 6" are the lower limit for that feature, and only under the very best conditions. The task becomes easier with an 8-inch scope, where the "fun" really starts. Stepping up to 12 inch, M51 becomes quite detailed, and the spiral structure much easier to discern than with smaller scopes. Of course, the perception of detail in the galaxy's spiral arms improves the larger the aperture you use, but it never quite looks like the many photographs of the galaxy seen in astronomy textbooks.

M51 is one of those objects where observational experience and technique make a real difference, skilled observers will perceive much more structure in the Whirlpool Galaxy using the same scope than beginners.

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