A 2024 revision to an earlier image, which I felt had greenish tint. I have also taken the opportunity to re-process the calibrated raw images to take advantage of recently developed tools in PixInsight. The rest of the description is unchanged from the original post.

M51 is a pair of galaxies in the constellation Canes Venatici, the Hunting Dogs. It comprises the face-on spiral galaxy NGC5194 and its smaller companion, NGC5195.

Discovered in 1773 by Charles Messier, M51 was first observed in detail in 1845 by William Parsons, the 3rd Earl of Rosse, with his 72″ reflecting telescope at Birr Castle in Ireland. Lord Rosse’s sketches clearly revealed the spiral structure, though at the time it was not realized that these ‘nebulæ’ were galaxies in their own right at vast distances.

M51 is around 30 million light years away and is about 7 arc-minutes across on the sky, corresponding to a physical diameter of around 100,000 light years, only slightly smaller than the Milky Way.

The larger galaxy is a tightly-wound spiral (type SA(s)b), and the structure of its spiral arms is attributed to interactions with its smaller companion over the course of several hundred million years as they orbit each other under the influence of gravity. In fact the brownish structures that we see in front of the smaller galaxy are part of the larger galaxy’s northern spiral arm (north is to the upper left corner in this image).

We can also see numerous bright blue stars and red H-II regions in the spiral arms. The blue stars have been born (relatively) recently from compression waves passing through the galaxy’s spiral arms which in turn triggers the gravitational collapse of gas clouds and the formation of hot young stars.

H-II regions are clouds of glowing ionized hydrogen. Having been ionized by the ultra-violet radiation from hot young blue stars, the red glow results from the recapture of an electron by the hydrogen nucleus (simply a proton). The red colour of the emitted photon corresponds to the second-innermost energy level in the atom at a wavelength of 656.3 nm – the so-called Hydrogen-alpha spectral line in the Balmer series.

Faint tidal tails can also be seen where gas and dust have been pulled from the two galaxies.

To the left of NGC5195 is a very small and faint distant galaxy, IC4277. Its magnitude is 15.69 and while no distance information was found in the literature, it is likely of the order of 300 million light years away.