Whirlpool Galaxy

Messier 51 comprises two interacting galaxies in the constellation Canes Venatici: the grand spiral NGC 5194, popularly known as the Whirlpool Galaxy, and its smaller companion NGC 5195. Charles Messier discovered the main galaxy on October 13, 1773, and Pierre Méchain noted the companion in 1781. At approximately 31 million light-years away and spanning over 65,000 light-years, NGC 5194 is a relatively nearby and luminous spiral. In 1845, the Third Earl of Rosse, observing with his great 72-inch reflecting telescope in Ireland, became the first person to detect spiral structure in a "nebula" — and M51 was the galaxy that revealed it, making Messier 51 one of the most historically significant objects in all of astronomy.

The spiral arms of NGC 5194 are among the most perfectly defined of any galaxy we can observe, and their extraordinary symmetry is likely sustained by the gravitational tidal pull of NGC 5195, whose close passage is thought to generate the density waves that compress gas and trigger star formation along the arms. The result is a galaxy ablaze with hot young O and B stars, star-forming regions, and luminous nebulae — making M51 considerably brighter than our own Milky Way despite being less massive. NGC 5195 passes behind one of NGC 5194's dust-filled arms, giving it a distinctly reddish cast where the arm's dust intervenes. The core of NGC 5194 harbors a bright ultraviolet source and one of the most compact, luminous radio sources known.

In binoculars M51 appears as a faint double smudge of light on a dark night; a 100 mm telescope shows the two galaxies clearly, and larger apertures begin to trace spiral structure in the main galaxy. This image was made by combining three CCD frames taken in red, green, and blue filters at the Kitt Peak National Observatory 0.9-meter telescope in 1991.