Galaxy M51 was discovered by Charles Messier in 1773, but its outstanding spiral structure was first perceived by William Parsons (Earl of Rosse) in 1845, using his huge reflecting telescope, the Leviathan of Parsonstown. A large telescope is needed to see its intricate shape, but even small amateur telescopes reveal that this galaxy is not isolated, but has a small companion, the dwarf irregular galaxy NGC 5195.
Now it is clear that these two stellar systems are colliding and that the outstanding spiral shape of M51 is due, mainly, to the tidal forces unleashed during this process. Just by chance, we see the disk of M51 face on from Earth, what allows studying it in detail. At a distance of 23 millions of light-years, the apparent dimensions of M51 mean that that galaxy has to be quite similar to our own, yet somewhat smaller.
M51 and its companion are performing a cosmic dance that, during the last 500 million years has made NGC 5195 pass twice through the disk of M51. Now, the small galaxy is located slightly behind the disc of the Whirlpool, and moving away from us.
M51 and NGC 5195 – Click for 1600×1583 image
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