Sky Object of the Month – June 2018

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Sky Object of the Month – June 2018
(Courtesy LVAS Observer’s Challenge*)
Messier 51 – Face-on Spiral Galaxy in Canes Venatici (Mag. 8.4; Size 11.2’ X 6.9’)

If you’ve been actively engaged in backyard astronomy for any length of time, you must certainly have trained your telescope in Messier 51. One of the brightest of Messier Catalog galaxies (it was discovered by the French comet hunter in 1773), it can even be glimpsed with binoculars from dark sky locations. M51 is actually two galaxies in one – an interacting system comprised of a face-on spiral (M51a [NGC 5194]) and a smaller galaxy of indeterminate type (M51b [NGC 5195]).

Through a small aperture scope (or medium sized instrument under hazy or slightly light-polluted skies), this galactic pair bears the “double nebula” appearance that Messier noted. The spiral nature of M51 was discovered by William Parsons, the 3rd Earl of Rosse, who viewed it with a 72-inch reflecting telescope (the “Leviathan of Parsonstown”) in 1845. Compare his drawing (below left) with a recent photographic image made by Amateur Telescope Makers of Boston member Mario Motta (below right).

The speculum metal mirror of Parsons’ 72-inch scope reflected a fraction of the light that the aluminum-coated glass mirrors of modern-day reflectors do. You won’t need a 72-inch scope to capture the spiral arms of M51 or the bridge of light that connects the two. What is the smallest aperture that will reveal both?

M51 is easily located 3½ degrees southwest of the 2nd magnitude star eta (η) Ursae Majoris. A quick search method is to make a low power sweep of the area one-fourth of the way between eta UMa and Cor Caroli (alpha [α] Canum Venaticorum). This magnificent spiral is about 25 million light years away and is about half the size of the Milky Way.

Glenn Chaple for the LVAS Mario Motta, MD

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