DSO of the Month :
Messier 51
AKA: Whirlpool Galaxy; NGC 5194 and 5195
Position: 13 hr 29 min 53 sec +47 degrees 11 min 43 sec
Due south at 22.57 (BST) on 15 May 2022
Messier 51 and NGC 5195
Image: Les Brand, HAS member. Used with permission.
In the DSO of the Month for April 2022, I mentioned the difficulties of seeing galaxies in a light-polluted area with a small telescope. Beginners are often, perhaps always, disappointed that galaxies do not appear in the telescope like the images they have seen in books. Few galaxies are more iconic than M51 and few are perhaps more disappointing when seen with the eye in a telescope in a light polluted area. Yet because it is so well-known I feel obliged to include it in this series. M51 was discovered by Charles Messier himself on 13 October 1773 and was duly listed in his catalogue. The adjourning galaxy was seen by Messier’s collaborator Pierre Méchain eight years later and it was eventually listed in the New General Catalogue separately as NGC 5195.For many years, they were just smudges in the sky and were not considered to be anything special. It was only when the 3rd Earl of Rosse looked at them through his new 72 inch telescope in 1845 that anyone realised it had a swirly appearance. In fact Messier 51 was the first spiral galaxy to be recognised as such, although for nearly ninety years afterwards, these spiral galaxies were thought to be inside our Milky Way. It is remarkable how imaging has advanced so much since 1845 that a small telescope with a camera can now show M51 more clearly than the Earl of Rosse’s “Leviathan of Parsonstown” (compare the image below with the image at the top). The development of radio astronomy was necessary before it became clear that NGC 5194 and its small neighbour NGC 5195 were connected. Strictly speaking Messier 51 only refers to the large spiral galaxy, but the designation is sometimes used for both galaxies which are then referred to as M51a and M51b. The name Whirlpool Galaxy for the main galaxy was first widely used in the 1940s but it is not clear who give it the name, possibly Dinsmore Alter at the Griffiths Observatory.
Despite its impressive appearance, M51 is a small galaxy. Lying a relatively close 26 million light years away, it measures 76,000 light years across compared with 185,000 light years for the Milky Way and it only has 10% of its mass. Its marked spiral appearance is a result of its companion passing through its disc about 500 million years ago, thus enhancing star formation. NGC 5195 is a dwarf galaxy which has become distorted because of its interaction with the larger galaxy and it will eventually be consumed by its neighbour.
While it is comparatively easy to make stunning images of M51 in a small telescope in Havering, the visual appearance of the galaxy in the same telescope at least in light-polluted Havering is disappointing. All I hav e ever seen is a vague circular blur, which looks red if you are using a light pollution filter, but by all means try to observe it. It is fairly easy to locate the Whirlpool Galaxy. Although M51 is very close to the Plough, ity is just over the border in Canes Venatici. Go to the end of the Plough, namely magnitude 1.9 Alkaid (Eta Ursae Majoris) and then go 3.5 degrees to the south-east roughly in the direction of magnitude 2.9 Cor Caroli (Alpha Canum Venaticorum). M51 has an apparent size of 11 arcminutes by 7 arcminutes and a magnitude of 8.4, and given it is both small and fairly bright, it is suprising how difficult it is to observe with the eye – light pollution has a lot to answer for!
Sketch of M51 made by the Earl of Rosse in 1845
rotated to match the image at the top
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