This journey across Virgo’s ethereal realm of galaxies has taken us across some 60° of the sky and we end our exploration of this extraordinary region with one of the most beautiful galaxies anywhere on the celestial sphere. M51, otherwise known as the Whirlpool Galaxy, has captivated astronomers for centuries and continues to intrigue both amateurs and professionals today. M51 was being scrutinised by astronomers long before its true nature – as a galaxy in its own right and not just another glowing nebula within the Milky Way – was really known.
William Parsons, the third Earl of Rosse, famously sketched M51 in 1845 using the enormous Leviathan of Parsonstown, a 72-inch reflecting telescope housed at Birr Castle in Ireland. His exquisite drawing clearly depicts the sweeping form of the Whirlpool – and its neighbour, the galaxy NGC 5195 – that’s instantly recognisable in the astro images taken with today’s photographic equipment.
Our perspective of M51, looking down on the galaxy’s disc, affords us a superb view of the physics unfolding there. Within the disc, density waves have formed spiral arms, which are home to vast numbers of hot, relatively young, blue stars. Photographs of the galaxy reveal another striking feature of these arms: numerous crimson patches of light scattered throughout M51’s disc. This feature is one that, just like the hot young stars, is testament to the star formation occurring there. These crimson patches are regions where the radiation of infant and newborn stars is exciting their surrounding maternal nebulae, causing the gas clouds to shine with the characteristic ruby hue of glowing hydrogen.
These dramatic flourishes of star formation aren’t the only dynamism on display with the Whirlpool Galaxy either. NGC 5195 is interacting with M51 and long-exposure images of the pair show extensive swathes of stars – known as tidal streams – near the galaxies that have been drawn out during this gravitational dance.
The swirling spiral of M51 has been fascinating stargazers for centuries
William Parsons revealed the spiral structure of M51 with his Leviathan telescope