Sky at Night Magazine - - THE VIRGO GALAXIES -

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

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