M51, also known as the Whirlpool Galaxy, is a grand design spiral galaxy located in the constellation of Canes Venatici. It's one of the most famous galaxies in the sky, appearing face-on when viewed from Earth. At magnitude +8.4, it's relatively bright and visible in binoculars, especially from dark sites. M51 has a much smaller dwarf companion, known as NGC 5195, and together they form the finest and most studied example of an interacting galaxy pair in the sky.
M51 was one of Charles Messier original discoveries on October 13, 1773. His friend Pierre Méchain discovered NGC 5195 on March 20, 1781. Messier described M51, as a faint nebula without stars, that was difficult to see. In his catalogue of 1781, he put M51 and NGC 5195 in the same note and therefore some confusion exists. Was he referring to M51 as the larger galaxy, or did he mean the pair itself. If it was the pair, then the main galaxy should really be referred to as M51A, with NGC 5195 separately known as M51B.
Canes Venatici is a small northern constellation of faint stars that was created by Johannes Hevelius in the 17th century. Apart from its brightest member, Cor Caroli (α CVn - mag. +2.9), it contains no stars better than 4th magnitude. However, finding M51 isn't difficult as it positioned only a few degrees from the handle of the seven stars, that form the famous Plough or Big Dipper asterism of Ursa Major. To pinpoint M51, first identify Alkaid (η UMa - mag +1.9) the end star of the handle of the Plough. Located 3 degrees directly west of Alkaid is 24 UMa (mag. +4.5). Positioned a degree northeast of 24 UMa is a mag. +6.5 star. Now imagine a line connecting this star with 24 UMa and continue it southwards for a further two degrees. This leads to a triangle of stars of magnitudes +7.1, +7.1 and +7.5. All three are easily visible in binoculars, with M51 located just west of the southernmost star.
M51 is best seen from the Northern Hemisphere during the months of March, April or May. From latitudes greater than 42N, it's circumpolar and therefore never sets.
The Whirlpool galaxy is visible through 10x50 binoculars and finderscopes as a smudge or patch of light, not unlike an out of focus star. An 80mm (3.1-inch) telescope reveals a diffuse patch of light with a brighter core, although not much more. When viewed through a 200mm (8-inch) scope, M51 has a well-defined bright core, surrounded by a large, faint diffuse halo. The galaxy's surface appears mottled, with some evidence of faint dark dust lanes and the spiral structure. Satellite galaxy NGC 5195 is also visible, although it's difficult to detect a connection between the two. When viewed through larger instruments of the order of 300mm (12-inch), numerous spiral bands are apparent with large HII regions of gas also visible. The attaching band of light between M51 and NGC 5195 can also be seen. The true beauty of this galaxy pair is seen in long exposure images.
Despite been one of the most studied galaxies of all, the distance to M51 is uncertain. Recent estimates place it at 23 million light-years, but other values between 15 and 35 million light-years have been quoted. The figure of 23 million light-years is based on a mag. +13.5 type II supernova (SN 2011dh) that was seen on May 31, 2011. To date, three supernovae have been observed (SN 1994I in April 1994, SN 2005cs in June 2005 and SN 2011dh in May 2011). It's also believed that a black hole, surrounded by a ring of dust, exists at the heart of M51 with the centre part currently undergoing a period of enhanced star formation.
The Whirlpool Galaxy has the distinction of being the first to have its spiral nature recognised. This was achieved in 1845 by the 3rd Earl of Rosse, William Parsons, using his 72-inch (1.83 m) reflecting telescope at Birr Castle in Ireland. Parson's enormous telescope was the largest in the World at the time.
M51 Data Table
|Object Type||Spiral galaxy|
|Distance (light-years)||23 Million|
|RA (J2000)||13h 29m 52s|
|DEC (J2000)||47d 11m 43s|
|Apparent Size (arc mins)||11.2 x 6.9|
|Number of Stars||>100 Billion|
|Notable Feature||Interacting with dwarf galaxy NGC 5195|