New pictures from the Hubble telescope are giving astronomers a detailed view 
of the Whirlpool galaxy's spiral arms and dust clouds, which are the birth sites 
of massive and luminous stars. This galaxy, also called M51 or NGC 5194, is having 
a close encounter with a nearby companion galaxy, NGC 5195, just off the upper 
edge of this image. The companion's gravitational influence is triggering star 
formation in the Whirlpool, as seen by the numerous clusters of bright, young 
stars [highlighted in red].

This Hubble photo is giving astronomers an unprecedented view of the intricate 
structure of the Whirlpool's spiral arms and dust clouds. Astronomers are seeing 
dust "spurs" along the spiral arms that branch out almost perpendicular to the 
main spiral arms. The regularity and large number of these spurs suggests that 
astronomers may have to rethink the popular theory that a spiral galaxy has two 
arms. The new image also reveals a dust disk in the center of the galaxy, which 
may feed a black hole.

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The famous Whirlpool galaxy Messier 51 (M51, NGC 5194) is one of the most 
conspicuous, and probably the most wellknown spiral galaxy in the sky.

M51 was one of Charles Messier's original discoveries: He discovered it on October 
13, 1773, when observing a comet, and described it as a "very faint nebula, without 
stars" which is difficult to see. Its companion, NGC 5195, was discovered on 
March 21, 1781 by his friend Pierre Méchain, so that it is mentioned in Messier's 
1781 catalog: `It is double, each has a bright center, which are separated 4'35". 
The two "atmospheres" touch each other, the one is even fainter than the other.' 
In addition to the description, in his personal copy of the catalog, Messier has 
added a small drawing, or sketch of the two "nebulae," M51 and NGC 5195, one of the 
rare cases a detailed drawing by Charles Messier of one of his objects is preserved. 
NGC 5195 was assigned an own catalog number by William Herschel: H I.186. 
Occasionally, there is some confusion what is meant with the designation M51: The 
pair (justified by Messier's mention of both "nebulae") or the larger galaxy, 
NGC 5194. If the pair is meant, NGC 5194 is sometimes called "M51A", and NGC 5195 
is then "M51B".

M51 is the dominating member of a small group of galaxies, which also contains M63 
and a number of fainter galaxies. As it is about 37 million light years distant and 
so conspicuous, it is actually a big and luminous galaxy. The value of M51's (and the 
whole group's) distance is still not very well known. Our value, of 37 Mly, is based 
on photometric methods and e.g. given by Kenneth Glyn Jones. Some authors give 
significantly lower values (less than 20 Mly), but a recent (2001) STScI Press 
Release gave 31 million light years.

This galaxy was the first one where the spiral structure was discovered, in spring 
1845 by Lord Rosse, who made a very careful and acurate painting. Therefore, M51 is 
sometimes referenced as Rosse's Galaxy or Lord Rosse's "Question Mark" - he is cited 
with this name (see, e.g., NED).

According to our present understanding, the pronounced spiral structure is a result 
of M51's current encounter with its neighbor, NGC 5195 (the fainter one in Messier's 
description). 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. Halton Arp has included M51 as No. 85 in his Catalogue of Peculiar 
Galaxies as "Spiral with Large High-Surface-Brightness Companion".

For the amateur, M51 is easy and a showpiece if the sky is dark, but is quite 
sensitive for light pollution which easily makes it fade in the background. Under 
very good conditions, even suggestions of its spiral arms can be glanced with 
telescopes starting from 4-inch. Low magnification is best for viewing this pair.