42 ASTRONOMY • JANUARY 2022
56 The Whirlpool Galaxy
M51, the fabulous Whirlpool Galaxy in Canes Venatici, is the largest
(in apparent diameter) and brightest face-on spiral in the night sky.
It sports the iconic visage of what astronomers call a “grand-design
spiral galaxy” — one that displays clearly defined and well-organized
spiral structure unwinding in orderly fashion from a clear core.
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. — S.J.O.