101 Must-See Cosmic Objects: The Whirlwind Galaxy

M51, the fabulous Whirlpool Galaxy of Canes Venatici, is the largest (in apparent diameter) and brightest spiral in the night sky. It sports the iconic face of what astronomers call a “grand design spiral galaxy” – a galaxy that displays a clearly defined, well-organized spiral structure unfolding in an orderly fashion from a clear core.

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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