[13] William Parsons, 3rd Earl of Rosse, employing a 72-inch (1.8 m) reflecting telescope at Birr Castle, Ireland, found that the Whirlpool possessed a spiral structure, the first "nebula" to be known to have one.

The Whirlpool Galaxy was discovered on October 13, 1773, by Charles Messier while searching for objects that might confuse comet hunters.

Deep in the constellation Canes Venatici, M51 is often found by finding the easternmost star of the Big Dipper, Alkaid, and going 3.5° southwest.

Its declination is, rounded, +47°, making it circumpolar (never setting) for observers above the 43rd parallel north;[a] it reaches a high altitude throughout this hemisphere making it an accessible object from the early hours in November through to the end of May, after which observation is more coincidental in modest latitudes with the risen sun (due to the Sun approaching to and receding from its right ascension, specifically figuring in Gemini, just to the north).

With larger (>300 mm) instruments under dark sky conditions, the various spiral bands are apparent with HII regions visible, and M51 can be seen to be attached to M51B.

As is usual for galaxies, the true extent of its structure can only be gathered from inspecting photographs; long exposures reveal a large nebula extending beyond the visible circular appearance.

In 1984, thanks to the high-speed detector—the so-called image-photon-counting-system (IPCS)—developed jointly by the CNRS Laboratoire d'Astronomie Spatiald (L.A.S.-CNRS) and the Observatoire de Haute Provence (O.H.P.)

3.60m Cassegrain focus on the summit of Mauna Kea in Hawaii, Hua et al. detected the double component of the very nucleus of the Whirlpool Galaxy.

The largest of these features is the so-called Northwest plume, which extends out to 43 kiloparsecs (140,000 light-years) from the galaxy's center.

The study remarks that a simulation that takes into account only one passage of NGC 5195 into the Whirlpool Galaxy will fail to produce an analogue to the Northeast tail.

It was classified as type Ic, indicating that its progenitor star was very massive and had already shed much of its mass, and its brightness peaked at apparent magnitude 12.91.

[30] In June 2005 the type II supernova SN 2005cs was observed in the Whirlpool Galaxy, peaking at apparent magnitude 14.

[33] This supernova, designated SN 2011dh, showed a spectrum much bluer than average, with P Cygni profiles, which indicate rapidly expanding material, in its hydrogen-Balmer lines.

No object could be seen at the position of the transient in archival Hubble images, indicating that the progenitor star was heavily obstructed by interstellar dust.