April / May (Usually visible from northern latitudes)
The Messier Number 'M51' is sometimes used to refer to both the Whirlpool Galaxy and its companion NGC 5195 in combination. In that case, the Whirlpool itself is identified as 'M51A', and the companion as 'M51B'.
A prominent spiral galaxy in the northern constellation of Canes Venatici, the Whirlpool Galaxy lies close to the handle of the Plough or Big Dipper, falling southwestward of the Alkaid, the star that marks the eastern end of the Plough's handle. Historically, the Whirlpool was the first galaxy to be recognised as having a spiral form. Though it had been discovered by Charles Messier in 1773, its spiral structure was not identified until it was observed by Lord Rosse in 1845 (and so it is occasionally referred to as 'Rosse's Galaxy'), though Rosse imagined that he had found a 'spiralnebula' rather than a galaxy.
The Whirlpool Galaxy is not far from the Northern Galactic Pole in the sky, meaning that it is far 'above' the plane of our own Galaxy. The Whirlpool itself is much too far away to show on this diagram.
The Whirlpool Galaxy has a very pronounced spiral structure, made all the clearer because it is aligned 'face-on' as seen from Earth. The galaxy has two major arms, which wind around each other in a concentric spiral pattern and extend outward from the highly active and luminousnucleus. The galaxy lies some thirty million light years from the Milky Way, though estimates of its precise distance vary somewhat around this value. Because of this minor uncertainty, estimates of the galaxy'sdiameter also vary, but it appears to be approximately 100,000 light years across. On this basis, the main spiral form of the Whirlpool is comparable in size to the Milky Way Galaxy.
The galaxy is the most prominent member of the M51 Group, a small group of galaxies that also includes the Sunflower Galaxy, M63, which lies southward of the Whirlpool in the sky. Another member of this small group is the dwarf galaxyNGC 5195 (or M51B) which represents a small 'attendant' to the Whirlpool. It lies so close to the main galaxy that it distorts the Whirlpool's northern spiral arm, and the two galaxies interact directly with one another. It appears that this interaction has been even more significant in the past, with the dwarf galaxy passing through the galactic plane of the Whirlpool on at least one occasion, and possibly more.
A recent notable discovery within the Whirlpool Galaxy is a phenonenon designated M51-ULS-1. This is related to a strong X-ray source within the galaxy, produced by a stellar remnant such as a black hole interacting with a nearby star. This source shows a regular pattern of eclipses, which imply the existence of an orbiting object, most likely a small gas giantplanet. If this is correct, then it represents the first extragalactic planet to be discovered - that is, a planet in a galaxy beyond the Milky Way (and at a distance of some thirty million light years, this would be by far the most distant planet known).