Follow the handle of the Plough in Ursa Major away from the body of the Bear until you get to the handle’s last bright star. Then, just nudge your telescope a little to the south and west and you will find this stunning pair of interacting galaxies.

The Whirlpool Galaxy lies around twenty three million light years away from Earth. Charles Messier first observed the galaxy in 1773 and noted it as the 51^st object in his famous catalogue and so it is known as Messier 51 or M51. In 1845, the Whirlpool Galaxy became the first galaxy ever to have its spiral structure observed, by the 3rd Earl of Rosse, William Parsons, using his 72-inch reflecting telescope at Birr Castle in Ireland, which was the largest optical telescope in the world at the time.

The Whirlpool Galaxy is not alone, it is orbited by the small galaxy M51b, discovered in 1787 by Messier’s friend Pierre Méchain, which you can clearly see to the left of the main galaxy. It is thought that the gravitational influence of this smaller galaxy has made the spiral arms in the Whirlpool so prominent, as the two slowly orbit and interact with each other. The mass of M51 is about 160 billion times the solar mass (3.2 x 10⁴¹ kg) and it is receding from the Earth at about 460 km/s.

It is believed that a black hole, surrounded by a ring of dust, exists at the heart of M51 with the centre part currently undergoing a period of enhanced star formation. Unusually, there have been three observed supernovae in the Whirlpool in the last twenty five years; in 1994, 2005 and 2011. The reasons for this high rate are still unknown.

This version of the image is a reprocess of the 2015 data. It brings out more of the blue colour in the spiral arms and the pinky spots that are star forming regions.