Charles Messier discovered M51 on Oct. 13, 1773; the following January, he recorded it as a “very faint nebula without any stars.” In the 1784 Connaissance des Temps, Messier appended a reference to Pierre Méchain’s observation that M51, in fact, appeared to be a double galaxy with two nuclei. We now know that M51’s companion is a diffuse disk galaxy, NGC 5195, that is interacting with it.
Not until 1845 did William Parsons, Earl of Rosse, detect the nebula’s “spiral convulsions” with his 72-inch speculum mirror reflector at Birr Castle in Ireland, making M51 the first galaxy shown to have spiral structure. This revelation led to the belief that Rosse had discovered a solar system in formation — a notion that was not shattered until 1923, when astronomers learned the true nature of the mysterious spiral nebulae.
The M51-NGC 5195 pair of galaxies lies 27 million light-years away. M51 is the grander of the two, measuring nearly 90,000 light-years across and shining with a luminosity of about 10 billion suns. NGC 5195 is a small disk galaxy some 55,000 light-years across. It most likely made its closest pass by M51 some 70 million years ago and is now receding from us at a rate of 290 miles per second (467 kilometers per second).
To find these fascinating galaxies, look about 2° south-southwest of 24 Canum Venaticorum. M51 is an 8th-magnitude circular glow (11′ by 7′), and NGC 5195 appears as a 6′ “knot” a little less than 5′ north of M51’s nucleus.
M51’s spiral structure teases the eye through telescopes smaller than 8 inches in aperture. Larger telescopes bring out the arms, which appear to encircle the nucleus. With patience, those branches break down into finer patches of star-forming regions. Telescopes 10 inches or larger will also clearly show the dusty bridge slicing across NGC 5195’s face — a telling sign that the smaller galaxy is receding from the larger.
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