101 Must-See Cosmic Objects: The Whirlwind Galaxy
Charles Messier discovered M51 on October 13, 1773; the following January he recorded it as a “very faint nebula without any stars”. In Knowledge of the Times of 1784, Messier added 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, which interacts with it.
It was not until 1845 that William Parsons, Earl of Rosse, detected the nebula’s “spiral convulsions” with his 72-inch speculum mirror reflector at Birr Castle in Ireland, making M51 the first galaxy to have a spiral structure. This revelation led to the belief that Rosse had discovered a forming solar system – a notion that was not shattered until 1923, when astronomers learned the true nature of the mysterious spiral nebulae.
The galaxy pair M51-NGC 5195 is 27 million light-years away. M51 is the larger of the two, measuring almost 90,000 light-years across and shining with a brightness of about 10 billion suns. NGC 5195 is a small disk galaxy about 55,000 light-years across. It most likely made its closest pass through M51 around 70 million years ago and is now moving away from us at a speed 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 a circular glow of magnitude 8 (11′ by 7′), and NGC 5195 appears as a 6′ “node” just under 5′ north of M51’s core.
M51’s spiral structure teases the eye through telescopes less than 8 inches in aperture. Larger telescopes bring out the arms, which appear to encircle the core. With patience, these branches break down into finer patches of star-forming regions. Telescopes 10 inches or larger will also clearly show the dusty bridge across the face of NGC 5195 – a telltale sign that the smaller galaxy is moving away from the larger one.
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