27-09-2008 The Whirlpool Galaxy (Article 156)

[Simon Kenny]

The Whirlpool Galaxy (Article 156)

The Whirlpool Galaxy, discovered by Charles Messier, in 1773, is probably the best known spiral galaxy in the sky. It forms part of a small group of galaxies, located at a distance of approximately 30 million light years (the distance is not yet well determined), in the constellation of Canes Venatici. Messier was more interested in comet hunting and given the quality and light collecting ability of the instruments of that time, Messier described the fuzzy object as a "very faint nebula, without stars" which is difficult to see. The messier designation for the Whirlpool is M51. The Whirlpool Galaxy is the brightest galaxy in the M51Group that also includes M63 (the Sunflower Galaxy), NGC 5023 and NGC5229
It took Lord Rosse, using the Great Telescope at Birr, in the spring of 1845 to identify that the object had a spiral structure. It is sometimes referenced as Rosse’s Galaxy or Rosse’s Question Mark. It is now known that M51 is in a close encounter with a smaller galaxy known as
NGC 5195 or M51A.
In the past, it wasn't known for certain if the companion galaxy NGC 5195 was actually a true companion or rather some other galaxy passing at a distance. The advent of radio astronomy and subsequent radio images of M51 unequivocally demonstrated the reality of the interaction.
Recent studies and simulations bear out that M51's spiral structure was caused by NGC 5195 passing through the main disk of M51 about 500 to 600 million years ago. In this model,
NGC 5195 came from behind M51 through the disk towards the observer and made another disk crossing as recently as 50 to 100 Million years ago until it is where we observe it to be now, slightly behind M51.
The Hubble Space Telescope has given astronomers unprecendented views of the famous galaxy and 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.
The Whirlpool is easy to find and the amateur astronomer will be well rewarded with the amazing view of a spiral galaxy. Low magnification is best for viewing the encounter with the companion (NGC5195) galaxy, when there is little or no light pollution.
Don’t forget the Shannonside Astronomy Club Whirlpool Star Party is being held this weekend in Dooley’s Hotel, Birr from the Friday evening 26th to Sunday 28th September. Weather permitting, observing sessions will be held in the Castle Grounds.